Familial factors do not confound the association between birth weight and childhood asthma

Association of cord plasma metabolites with birth weight: results from metabolomic and lipidomic studies of discovery and validation cohorts

OBJECTIVE

Birth weight is a good predictor of fetal intrauterine growth and long-term health, and several studies have evaluated the relationship between metabolites and birth weight. The aim of this study was to investigate the association of cord blood metabolomics and lipidomics with birth weight, using a two-stage discovery and validation approach.

METHODS

Firstly, a pseudotargeted metabolomics approach was applied to detect metabolites in 504 cord blood samples in the discovery set enrolled from the Wuhan Healthy Baby Cohort, China. Metabolome-wide association scan analysis and pathway enrichment were applied to identify metabolites and metabolic pathways that were significantly associated with birth weight adjusted for gestational age Z-score (BW Z-score). Logistic regression models were used to analyze the association of metabolites in the most significantly associated pathways with small-for-gestational age (SGA) at delivery and low birth weight (LBW). Subsequently, 350 cord blood samples in a validation cohort were subjected to targeted analysis to validate the metabolites identified by screening in the discovery cohort.

RESULTS

In the discovery set, of 2566 metabolites detected, 2418 metabolites were retained for subsequent analysis after data preprocessing. Of these, 513 metabolites were significantly associated with BW Z-score (P-value adjusted for false discovery rate (PFDR) < 0.05), of which 298 Kyoto Encyclopedia of Genes and Genomes (KEGG)-annotated metabolites were included in the pathway analysis. The primary bile acid biosynthesis pathway was the most relevant metabolic pathway associated with BW Z-score. Elevated cord plasma primary bile acids were associated with lower BW Z-score and higher risk of SGA or LBW in the discovery and validation cohorts. In the validation set, a 2-fold increase in taurochenodeoxycholic acid (TCDCA) and in taurocholic acid (TCA) was associated with a decrease in BW Z-score (estimated β coefficient, -0.10 (95% CI, -0.20 to 0.00) and -0.18 (95% CI, -0.31 to -0.04), respectively), after adjusting for covariates. In addition, a 2-fold increase in cord plasma TCDCA and of cord plasma TCA was associated with an increased risk of SGA (adjusted odds ratio (OR), 1.52 (95% CI, 1.00-2.30) and 1.77 (95% CI, 1.05-2.98), respectively). The adjusted OR for LBW, for a 2-fold increase in TCDCA and TCA concentration, were 2.39 (95% CI, 1.00-5.71) and 3.21 (95% CI, 0.96-10.74), respectively.

CONCLUSIONS

These results indicate a significant association of elevated primary bile acids, particularly TCDCA and TCA, in cord blood with lower BW Z-score, as well as increased risk of SGA and LBW. Abnormalities of primary bile acid metabolism may play an important role in restricted fetal development. © 2024 International Society of Ultrasound in Obstetrics and Gynecology.

Long-term implications of fetal growth restriction

Fetal growth restriction (FGR), or intrauterine growth restriction (IUGR), is a complication of pregnancy where the fetus does not achieve its genetic growth potential. FGR is characterized by a pathological retardation of intrauterine growth velocity in the curve of intrauterine growth. However, the FGR definition is still debated, and there is a lack of a uniform definition in the literature. True IUGR, compared to constitutional smallness, is a pathological condition in which the placenta fails to deliver an adequate supply of oxygen and nutrients to the developing fetus. Infants with IUGR, compared to appropriately grown gestational age infants, have a significantly higher risk of mortality and neonatal complications with long-term consequences. Several studies have demonstrated how suboptimal fetal growth leads to long-lasting physiological alterations for the developing fetus as well as for the newborn and adult in the future. The long-term effects of fetal growth retardation may be adaptations to poor oxygen and nutrient supply that are effective in the fetal period but deleterious in the long term through structural or functional alterations. Epidemiologic studies showed that FGR could be a contributing factor for adult chronic diseases including cardiovascular disease, metabolic syndrome, diabetes, respiratory diseases and impaired lung function, and chronic kidney disease. In this review we discussed pathophysiologic mechanisms of FGR-related complications and potential preventive measures for FGR.

Growth pattern during early infancy, body mass index during childhood and childhood asthma

BACKGROUND

There is a lack of longitudinal studies of associations between growth from infancy to childhood and asthma development.

OBJECTIVE

The objective of the study was to investigate the effects of weight change during infancy, body mass index (BMI) and the interaction of these factors on the risk of childhood asthma.

METHODS

We enrolled children born in 2008 and 2009 at full-term and with normal birth weight. The weight change in infancy was grouped into slow, on-track and rapid. BMI status in childhood was stratified into low, normal and high groups and used as a time-varying variable. The outcome was asthma, defined as two or more diagnoses of asthma separated by at least 1 year after 2 years of age. The risk of asthma was assessed using Cox proportional hazard regression, with adjustment for sex, residence area at birth, economic status and feeding types in infancy.

RESULTS

Of 917,707 children born in Korea in 2008 and 2009, 271,871 were eligible for analysis. The risk of asthma was greater in groups with low birth weight (aHR 1.06, 95% CI 1.04 to 1.08), rapid body weight change during early infancy (aHR 1.08, 95% CI 1.07 to 1.10) and high BMI during childhood (aHR 1.06, 95% CI 1.04-1.08). The interaction of weight change during early infancy with BMI during childhood was significant for asthma (p < .01). Rapid weight gain in infancy was associated with lower risk of asthma in those with low BMI during childhood; had no association with asthma in those with normal BMI during childhood; and was associated increased asthma risk in those with high BMI during childhood-aHR 1.26 (95% CI 1.19 to 1.33) and aHR 1.33 (95% CI 1.12 to 1.56) compared with on-track and slow infant weight gain, respectively.

CONCLUSION

Low birth weight, high BMI during childhood and, in those with high childhood BMI, rapid weight gain during early infancy are associated with increased risk of childhood asthma.

Investigating the influence of breastfeeding on asthma in children under 12 years old in the UK Biobank

Background

Childhood-onset asthma (COA) has become a major and growing problem worldwide and imposes a heavy socioeconomic burden on individuals and families; therefore, understanding the influence of early-life experiences such as breastfeeding on COA is of great importance for early prevention.

Objectives

To investigate the impact of breastfeeding on asthma in children under 12 years of age and explore its role at two different stages of age in the UK Biobank cohort.

Methods

A total of 7,157 COA cases and 158,253 controls were obtained, with information regarding breastfeeding, COA, and other important variables available through questionnaires. The relationship between breastfeeding and COA were examined with the logistic regression while adjusting for available covariates. In addition, a sibling analysis was performed on 398 pairs of siblings to explain unmeasured family factors, and a genetic risk score analysis was performed to control for genetic confounding impact. Finally, a power evaluation was conducted in the sibling data.

Results

In the full cohort, it was identified that breastfeeding had a protective effect on COA (the adjusted odds ratio (OR)=0.875, 95% confidence intervals (CIs): 0.831~0.922; P=5.75×10^-7). The impact was slightly pronounced in children aged 6-12 years (OR=0.852, 95%CIs: 0.794~0.914, P=7.41×10^-6) compared to those aged under six years (OR=0.904, 95%CIs: 0.837~0.975, P=9.39×10^-3), although such difference was not substantial (P=0.266). However, in the sibling cohort these protective effects were no longer significant largely due to inadequate samples as it was demonstrated that the power was only 23.8% for all children in the sibling cohort under our current setting. The protective effect of breastfeeding on COA was nearly unchanged after incorporating the genetic risk score into both the full and sibling cohorts.

Conclusions

Our study offered supportive evidence for the protective effect of breastfeeding against asthma in children less than 12 years of age; however, sibling studies with larger samples were warranted to further validate the robustness our results against unmeasured family confounders. Our findings had the potential to encourage mothers to initiate and prolong breastfeeding.

Genome-Wide Interaction Study of Late-Onset Asthma With Seven Environmental Factors Using a Structured Linear Mixed Model in Europeans

Asthma is among the most common chronic diseases worldwide, creating a substantial healthcare burden. In late-onset asthma, there are wide global differences in asthma prevalence and low genetic heritability. It has been suggested as evidence for genetic susceptibility to asthma triggered by exposure to multiple environmental factors. Very few genome-wide interaction studies have identified gene-environment (G×E) interaction loci for asthma in adults. We evaluated genetic loci for late-onset asthma showing G×E interactions with multiple environmental factors, including alcohol intake, body mass index, insomnia, physical activity, mental status, sedentary behavior, and socioeconomic status. In gene-by-single environment interactions, we found no genome-wide significant single-nucleotide polymorphisms. However, in the gene-by-multi-environment interaction study, we identified three novel and genome-wide significant single-nucleotide polymorphisms: rs117996675, rs345749, and rs17704680. Bayes factor analysis suggested that for rs117996675 and rs17704680, body mass index is the most relevant environmental factor; for rs345749, insomnia and alcohol intake frequency are the most relevant factors in the G×E interactions of late-onset asthma. Functional annotations implicate the role of these three novel loci in regulating the immune system. In addition, the annotation for rs117996675 supports the body mass index as the most relevant environmental factor, as evidenced by the Bayes factor value. Our findings help to understand the role of the immune system in asthma and the role of environmental factors in late-onset asthma through G×E interactions. Ultimately, the enhanced understanding of asthma would contribute to better precision treatment depending on personal genetic and environmental information.

Parental socioeconomic status and asthma in children: Using a population-based cohort and family design

BACKGROUND

The observed association between the parental socioeconomic status (SES, measured as education/income) and asthma or wheezing in offspring may be explained by confounding of unmeasured factors (shared genes and family environment). We aimed to study the association between parental SES and asthma/wheeze using cousin comparison.

METHOD

Data were collected on individuals born in Sweden 2001-2013. Parental SES (education and income) was gathered from Statistics Sweden. Asthma/wheeze was identified using national health registers. The association between parental SES at birth and incident asthma/wheeze was estimated using Cox regression also comparing differently exposed cousins. The association between parental SES at 5 years and current asthma was estimated using logistic regression.

RESULTS

Included were 955,371 individuals. Mothers with compulsory school only (lowest education group) compared with those with further education (highest education group) was associated with incident asthma/wheeze below 1 year of age HRadj = 1.45 (1.38-1.52) and over 1 year of age HRadj = 1.17 (1.13-1.20). The corresponding estimates for the lowest income group were HRadj = 1.61 (1.54-1.69) and HRadj = 0.94 (0.92-0.97), respectively. In maternal cousin comparisons, the associations for asthma/wheeze over 1 year of age was HRadj = 1.21 (1.05-1.40) for compulsory school only and HRadj = 0.94 (0.84-1.07) for the lowest income group. The ORadj for current asthma at 5 years was 1.05 (1.00-1.11) for mother's compulsory school only and 0.98 (0.94-1.02) for mother's lowest income group. Results for estimates were similar for father's SES.

CONCLUSION

We confirm an association between low parental SES (measured as education) and asthma/wheeze. Cousin comparison suggests that this association is not wholly due to confounding of unknown familial factors, therefore supporting a causal relationship. The relationship between parental income and asthma/wheeze is less clear. This study is important for understanding risk factors for asthma/wheeze and for future prevention strategies. Further research is warranted to investigate the possible mechanisms for association between parental education and asthma/wheeze.

Pre-eclampsia and risk of early-childhood asthma: a register study with sibling comparison and an exploration of intermediate variables

BACKGROUND

We aimed to study whether pre-eclampsia is associated with childhood asthma, allergic and non-allergic asthma, accounting for family factors and intermediate variables.

METHODS

The study population comprised 779 711 children born in 2005-2012, identified from Swedish national health registers (n = 14 823/7410 exposed to mild/moderate and severe pre-eclampsia, respectively). We used Cox regression to estimate the associations of mild/moderate and severe pre-eclampsia with incident asthma, before and after age 2 years. Cox regressions were controlled for familial factors using sibling comparisons, then stratified on high and low risk for intermediate variables: caesarean section, prematurity and small for gestational age. We used logistic regression for allergic and non-allergic prevalent asthma at 6 years as a measure of more established asthma.

RESULTS

The incidence of asthma in children was 7.7% (n = 60 239). The associations varied from adjusted hazard ratio (adjHR) 1.11, 95% confidence interval (CI): 1.00, 1.24 for mild/moderate pre-eclampsia and asthma at >2 years age, to adjHR 1.78, 95% CI: 1.64, 1.95 for severe pre-eclampsia and asthma at <2 years age. Sibling comparisons attenuated most estimates except for the association between severe pre-eclampsia and asthma at <2 years age (adjHR 1.45, 95% CI: 1.10, 1.90), which also remained when stratifying for the risk of intermediates. Mild/moderate and severe pre-eclampsia were associated with prevalent non-allergic (but not allergic) asthma at 6 years, with adjusted odds ratio (adjOR) 1.17, 95% CI: 1.00, 1.36 and adjOR 1.51, 95% CI: 1.23, 1.84, respectively.

CONCLUSIONS

We found evidence that severe, but not mild/moderate, pre-eclampsia is associated with asthma regardless of familial factors and confounders.

Fetal Growth Restriction and Asthma: Is the Damage Done?

Trajectories of airway remodeling and functional impairment in asthma are consistent with the notion that airway pathology precedes or coincides with the onset of asthma symptoms and may be present at birth. An association between intrauterine growth restriction (IUGR) and asthma development has also been established, and there is value in understanding the underlying mechanism. This review considers airway pathophysiology as a consequence of IUGR that increases susceptibility to asthma.

Low birth weight as a predictor of adverse health outcomes during adulthood in twins: a systematic review and meta-analysis

BACKGROUND

Low birth weight might affect adverse health outcomes during a lifetime. Our study analyzes the association between low birth weight and negative health outcomes during adulthood in twin populations.

METHODS

Searches were conducted using databases inclusive of MEDLINE, CINAHL, Web of Science, and EBSCO. Observational studies on twins with low birth weight and adverse health outcomes during adulthood were included. Two reviewers independently screened the papers, and a third reviewer resolved the conflicts between the two reviewers. Following abstract and title screening, full-texts were screened to obtain eligibility. Eligible full-text articles were then assessed for quality using a modified Downs and Black checklist. Studies with a score within one standard deviation of the mean were included in the analysis. A fixed-effect model was used for analysis.

RESULTS

3987 studies were screened describing low birth weight as a risk factor for adverse health outcomes during adulthood for all twelve-body systems (circulatory, digestive, endocrine, lymphatic, muscular, nervous, reproductive, respiratory, skeletal, urinary, and integumentary systems). One hundred fourteen articles made it through full-text screening, and 14 of those articles were assessed for quality. Five papers were selected to perform two meta-analyses for two outcomes: asthma and cerebral palsy. For asthma, the meta-analyses of three studies suggested a higher odds of low birth weight twins developing asthma (OR 1.33, 95% CI 1.24-1.44, I2 = 77%). Meta-analysis for cerebral palsy included two studies and suggested a 4.88 times higher odds of low birth weight twins developing cerebral palsy compared to normal birth weight twins (OR 4.88, 95% CI 2.34-10.19, I2 = 79%). We could not find enough studies for other adverse health outcomes to pool data for a Forest plot.

CONCLUSIONS

The odds of low birth weight were found to be high in both asthma and cerebral palsy. There are not enough studies of similar nature (study types, similar body systems) to ensure a meaningful meta-analysis. We recommend that future research considers following up on twins to obtain data about adverse health outcomes during their adult lives.

Genetic Meta-Analysis of Twin Birth Weight Shows High Genetic Correlation with Singleton Birth Weight

Birth weight (BW) is an important predictor of newborn survival and health and has associations with many adult health outcomes, including cardio-metabolic disorders, autoimmune diseases, and mental health. On average, twins have a lower BW than singletons as a result of a different pattern of fetal growth and shorter gestational duration. Therefore, investigations into the genetics of BW often exclude data from twins, leading to a reduction in sample size and remaining ambiguities concerning the genetic contribution to BW in twins. In this study, we carried out a genome-wide association meta-analysis of BW in 42 212 twin individuals and found a positive correlation of beta values (Pearson's r = 0.66, 95% confidence interval [CI]: 0.47-0.77) with 150 previously reported genome-wide significant variants for singleton BW. We identified strong positive genetic correlations between BW in twins and numerous anthropometric traits, most notably with BW in singletons (genetic correlation [rg] = 0.92, 95% CI: 0.66-1.18). Genetic correlations of BW in twins with a series of health-related traits closely resembled those previously observed for BW in singletons. Polygenic scores constructed from a genome-wide association study on BW in UK Biobank demonstrated strong predictive power in a target sample of Dutch twins and singletons. Together, our results indicate that a similar genetic architecture underlies BW in twins and singletons and that future genome-wide studies might benefit from including data from large twin registers.

Adverse pregnancy outcomes and risk of later allergic rhinitis-Nationwide Swedish cohort study

BACKGROUND

Perinatal conditions may be associated with future allergic disease; however, data are conflicting and incomplete for childhood allergic rhinitis (AR). The aim of this study was to examine pregnancy outcome (cesarean delivery, preterm birth, low birthweight) and offspring AR as defined by national registers.

METHODS

Nationwide longitudinal cohort study using prospectively recorded register data from 1 059 600 singleton livebirths born in Sweden in 2001-2012. Cox regression adjusted for infant sex and maternal factors (age at delivery, country of birth, parity, smoking, body mass index, and asthma/pulmonary disease) estimated hazard ratios (HRs) for AR during childhood.

RESULTS

During the study period 2001-2013, 22 386 (2.11%) children were diagnosed with AR. AR was more common in infants born through cesarean delivery (2.34%) than in those born vaginally (2.10%) (HR = 1.12; 95% confidence interval [95% CI] = 1.08-1.16). This was equivalent to one extra case of AR in 383 children followed up in our study. AR was also associated with moderately preterm birth (≥32-36 weeks of gestation: HR = 1.12, 95% CI = 1.04-1.20), large for gestational age (HR = 1.05, 95% CI = 1.01-1.10), and low (<7) 5-minute Apgar score (HR = 1.15, 95% CI = 1.02-1.30). Similar risk estimates were obtained when we restricted the outcome to ≥2 hospital-based records of AR. No association was observed between very preterm birth, post-term birth, low birthweight, or small for gestational age and AR.

CONCLUSION

Our study indicates an association between pregnancy outcomes and childhood AR, although observed effect sizes were generally modest.

Season of birth, childhood asthma and allergy in a nationwide cohort-Mediation through lower respiratory infections

BACKGROUND

Previous studies have suggested an association between season of birth and risk of childhood asthma and allergic disease. The association may be modified by birth year and region, or mediated by respiratory tract infections.

OBJECTIVE

We aimed to estimate the association between season of birth and risk of childhood asthma/wheeze or allergic rhinoconjunctivitis in a population-based setting, and the mediating effect of lower respiratory infections.

METHODS

Two population-based cohorts were identified from the nationwide Swedish Medical Birth, Patient and Prescribed Drug Registers. The association between birth month/season and asthma/wheeze incidence was analysed using Cox proportional regression in the younger cohort born 2005-2010 (n = 582 494) and asthma/allergic rhinoconjunctivitis prevalence during the 7th year of life using log-binomial models in the older cohort born 2001-2004 (n = 367 583). Interactions were formally tested. Mediation analyses to address the effect of lower respiratory infections were performed in the older cohort using the R package "medflex."

RESULTS

Children born during fall and winter had an increased risk of asthma/wheeze after 2 years of age in the younger cohort: hazard ratio 1.24 (95% confidence interval, CI 1.17, 1.33) for winter and risk of prevalent asthma during their 7th year of life in the older cohort; prevalence ratio (PR) 1.12 (95% CI 1.08, 1.16) for winter. These estimates were partly mediated by lower respiratory infections; the indirect effect for winter compared with summer was PR 1.03 (95% CI 1.03, 1.04). The association was similar for allergic rhinoconjunctivitis in the 7th year of life, but not mediated by respiratory infections.

CONCLUSION

We found that the association between season of birth and risk of childhood asthma/wheeze, but not allergic rhinoconjunctivitis, is partly mediated through lower respiratory infections.

CLINICAL RELEVANCE

This has important implications for patient care, such as asthma management programmes to notify timing of seasonality for viral respiratory tract infections.

Infant Feeding, Vitamin D and IgE Sensitization to Food Allergens at 6 Years in a Longitudinal Icelandic Cohort

Nordic Nutrition Recommendations (NNR) recommend exclusive breastfeeding until 6 months, partial breastfeeding until 1 year or longer and irrespective of breastfeeding, avoiding solid foods before 4 months. Strong evidence was found for benefits of breastfeeding regarding growth and infections but limited/inconclusive evidence regarding atopic disease and asthma. Vitamin D is of special interest in the Nordic diet. The aim of this prospective study was to compare infant feeding and vitamin D between immunoglobulin E (IgE) sensitized (n = 14) and non-sensitized (n = 130) children at 6 years. Information on diet and vitamin D supplement use were collected with dietary recall (<5 months), 1-d food records (5 and 6 months) and 3-d weighed food records (12 months and 6 years). Serum-specific IgE-antibodies against milk, egg, cod, wheat, soy and peanut (cut-off specific IgE ≥ 0.35 kUA/L) were measured at 6 years and serum 25-hydroxyvitamin D at 12 months and 6 years. At 4 months, 57% of IgE sensitized vs. 23% of non-sensitized children (p < 0.01) had received solid food. At 12 months, IgE sensitized children had a lower intake of vitamin D (median (25th, 75th percentiles): 3.9 μg/d (3.2, 7.2) vs. 8.1 μg/d (4.4, 12.3), p = 0.03) and at 6 years, fewer used vitamin D supplements regularly (23% vs. 56%, p = 0.03). Introduction of solid foods prior to 4 months increased the odds of IgE-sensitization, OR = 4.9 (95%, CI = 1.4–16.6) and vitamin D supplement at 6 years decreased the odds of IgE-sensitization, OR = 0.2 (95%, CI = 0.1–0.98), adjusting for maternal smoking. These observations support the NNR in their recommendation against introducing complementary solid foods before the age of 4 months. Furthermore, they support encouraging vitamin D intake for young children at northern latitudes.

Birth weight is not causally associated with adult asthma: results from instrumental variable analyses

The association between lower birth weight and childhood asthma is well established. However, it remains unclear whether the influence of lower birth weight on asthma can persist into adulthood. We conducted a Mendelian randomization analysis to assess the causal relationship of birth weight (~140,000 individuals) on the risk of adult asthma (~62,000 individuals). We estimated the causal effect of birth weight to be 1.00 (95% CI 0.98~1.03, p = 0.737) using the genetic risk score method. We did not observe nonlinear relationship or gender difference for the estimated causal effect. With the inverse-variance weighted method, the causal effect of birth weight on adult asthma was estimated to be 1.02 (95% CI 0.84~1.24, p = 0.813). Additionally, the iMAP method provides no additional genome-wide evidence supporting the causal effects of birth weight on adult asthma. Our results were robust against various sensitivity analyses, and MR-PRESSO and MR-Egger regression showed that no instrument outliers and no horizontal pleiotropy were likely to bias the results. Overall, our study provides no evidence for the fetal origins of diseases hypothesis for adult asthma, implying that the impact of birth weight on asthma in years of children and adolescents does not persist into adult and previous findings may be biased by confounders.

Clinical and Pathologic Factors Predicting Future Asthma in Wheezing Children. A Longitudinal Study

Wheeze is a common symptom in infants, but not all wheezers develop asthma. Indeed, up to 50% of wheezing children outgrow their symptoms by school age. How to predict if early wheeze will become asthma is still a matter of vivid debate. In this work, we sought to assess the clinical and pathological factors that might predict the future development of asthma in children. Eighty children (mean age 3.8 ± 1 yr) who underwent a clinically indicated bronchoscopy were followed prospectively for a median of 5 years. At baseline, clinical characteristics with a particular focus on wheezing and its presentation (episodic or multitrigger) were collected, and structural and inflammatory changes were quantified in bronchial biopsies. Follow-up data were available for 74 of the 80 children. Children who presented with multitrigger wheeze were more likely to have asthma at follow-up than those with episodic wheeze (P = 0.04) or without wheeze (P < 0.0001). Children with asthma also had lower birth weights (P = 0.02), a lower prevalence of breastfeeding (P = 0.02), and a trend for increased IgE (P = 0.07) at baseline than those with no asthma. Basement membrane thickness and airway eosinophils at baseline were increased in children who developed asthma at follow-up (P = 0.001 and P = 0.026, respectively). Multivariate analysis showed that among all clinical and pathological factors, multitrigger wheezing, basement membrane thickening, and reduced birth weight were predictive of future asthma development. We conclude that multitrigger wheeze and reduced birth weight are clinical predictors of asthma development. Basement membrane thickening in early childhood is closely associated with asthma development, highlighting the importance of airway remodeling in early life as a risk factor for future asthma.

Genetic Architectures of Childhood- and Adult-Onset Asthma Are Partly Distinct

The extent to which genetic risk factors are shared between childhood-onset (COA) and adult-onset (AOA) asthma has not been estimated. On the basis of data from the UK Biobank study (n = 447,628), we found that the variance in disease liability explained by common variants is higher for COA (onset at ages between 0 and 19 years; h²_g = 25.6%) than for AOA (onset at ages between 20 and 60 years; h²_g = 10.6%). The genetic correlation (r_g) between COA and AOA was 0.67. Variation in age of onset among COA-affected individuals had a low heritability (h²_g = 5%), which we confirmed in independent studies and also among AOA-affected individuals. To identify subtype-specific genetic associations, we performed a genome-wide association study (GWAS) in the UK Biobank for COA (13,962 affected individuals) and a separate GWAS for AOA (26,582 affected individuals) by using a common set of 300,671 controls for both studies. We identified 123 independent associations for COA and 56 for AOA (37 overlapped); of these, 98 and 34, respectively, were reproducible in an independent study (n = 262,767). Collectively, 28 associations were not previously reported. For 96 COA-associated variants, including five variants that represent COA-specific risk factors, the risk allele was more common in COA- than in AOA-affected individuals. Conversely, we identified three variants that are stronger risk factors for AOA. Variants associated with obesity and smoking had a stronger contribution to the risk of AOA than to the risk of COA. Lastly, we identified 109 likely target genes of the associated variants, primarily on the basis of correlated expression quantitative trait loci (up to n = 31,684). GWAS informed by age of onset can identify subtype-specific risk variants, which can help us understand differences in pathophysiology between COA and AOA and so can be informative for drug development.

Asthma, Rhinoconjunctivitis, Eczema, and the Association with Perinatal Anthropometric Factors in Vietnamese Children

Few studies have investigated possible causative and protective factors associated with allergic diseases in resource-limited countries, Southeast Asia. We estimated the current prevalence of asthma, rhinoconjunctivitis, and eczema among 6-year-old children, and identified anthropometric factors associated with asthma, rhinoconjunctivitis and eczema, in South-Central Vietnam. A birth cohort study recruited 1,999 children born at a provincial hospital in Nha Trang, Vietnam between May 2009 and May 2010. A 6-year follow-up survey was conducted where clinical, familial, and environmental information was collected by interviewing caregivers using a standardized form based on the International Study of Asthma and Allergies in Childhood, Phase Three Core and Environmental Questionnaire for 6–7-year-old children. The odds ratios of asthma, rhinoconjunctivitis, and eczema for anthropometric factors were estimated using logistic regression analysis. In total, 1202 children participated in the follow-up survey. The proportions of children who had current asthma, rhinoconjunctivitis, and eczema were 5.1% (95% confidence interval [CI] 3.9–6.5%), 11.5% (9.7–13.4%), and 6.7% (5.3–8.2%), respectively. Low birthweight (adjusted odds ratio 5.12, 95% CI 1.92–13.64) was independently associated with increased risk of eczema. Further studies are necessary to understand the involved mechanism.

Factors associated with asthma expression in adolescents

Objective:

To evaluate risk factors associated with asthma symptoms in adolescents in the 13- to 14-year age bracket.

Methods:

This was a cross-sectional study involving adolescents enrolled in randomly selected public schools in the city of Belo Horizonte, Brazil, and conducted with the use of the International Study of Asthma and Allergies in Childhood (ISAAC) questionnaire and its supplementary module for risk factor assessment. The ISAAC questionnaire was completed by the students themselves, whereas the supplementary questionnaire was completed by their parents or legal guardians. Variables showing p ≤ 0.25 in the univariate analysis were included in the multivariate analysis. Stepwise regression with backward elimination was used for variable selection.

Results:

We evaluated 375 adolescents, 124 (33.1%) of whom had asthma symptoms. The final multivariate analysis model revealed that asthma symptoms were associated with birth weight < 2,500 g (p < 0.001), day care center or nursery attendance (p < 0.002), maternal history of asthma (p < 0.001), contact with animals during the first year of life (p < 0.027), current contact with animals outside the home (dogs, cats, or farm animals; p < 0.005), and more than 20 cigarettes per day smoked by parents or other household members (p < 0.02).

Conclusions:

Exposure to animals in and outside the home is associated with asthma symptoms, as is environmental tobacco smoke exposure. Families, health professionals, and administrators of health care facilities should take that into account in order to prevent asthma and reduce asthma morbidity.

Cesarean delivery, preterm birth, and risk of food allergy: Nationwide Swedish cohort study of more than 1 million children

BACKGROUND

Little is known about early-life risk factors for food allergy in children.

OBJECTIVES

We examined the association between perinatal characteristics and future risk of food allergy in offspring.

METHODS

This nationwide Swedish cohort study of 1,086,378 children born in Sweden in 2001-2012 used prospectively recorded data from health care registers. Using Cox regression, we estimated hazard ratios (HRs) with 95% CIs for the association between perinatal characteristics (eg, cesarean delivery and preterm birth) and food allergy as defined by diagnoses in the National Patient Register, adjusting for infant sex and maternal factors (age at delivery, country of birth, parity, smoking, body mass index, and asthma/pulmonary disease).

RESULTS

During the 13-year follow-up, 26,732 (2.5%) children were given a diagnosis of food allergy. Food allergy was positively associated with cesarean delivery (HR, 1.21; 95% CI, 1.18-1.25), large for gestational age (HR, 1.15; 95% CI, 1.10-1.19), and low 5-minute Apgar score (HR, 1.22; 95% CI, 1.10-1.36) but negatively associated with very preterm birth (<32 weeks of gestation: HR, 0.74; 95% CI, 0.56-0.98). No association was found between food allergy and moderately preterm birth, low birth weight, or small for gestational age. Risk estimates were similar when the outcome was restricted to 2 records of diagnosed food allergy. In 1,000 children undergoing cesarean delivery, an extra 5 developed food allergy compared with the reference group, suggesting that 17% of food allergy in children born by means of cesarean delivery can be explained by this exposure (attributable fraction).

CONCLUSIONS

Cesarean delivery was associated with increased risk of food allergy, whereas very preterm birth decreased risk.

The STOPPA Twin Study Explains the Exhaled Nitric Oxide and Asthma Link by Genetics and Sensitization

BACKGROUND

The link between asthma and exhaled nitric oxide (FENO) is not completely understood. The aim of this study was to estimate the association between FENO and asthma, taking genetics, sensitization, and inhaled corticosteroids (ICS) into account.

METHODS

A total of 681 twins (53% monozygotic [MZ] and 47% dizygotic [DZ]) from the population-based STOPPA study (mean age 12.6 years) were recruited and information on FENO (parts per billion), parental report of current asthma, sensitization to airborne allergens (Phadiatop; IgE ≥0.35 kUA/l), and ICS-treatment was collected. We estimated the association between FENO and asthma, sensitization, and ICS in all twins and within pairs (DZ and MZ) to address shared genetic and environmental factors. Linear regression of log-transformed FENO was used and results presented as exponentiated regression coefficients (exp[β]), with 95% confidence interval (CI).

RESULTS

We found an association between asthma and FENO in all twins, exp(β) 1.31 [1.11, 1.54]. In within-pairs analysis, the association was stronger within DZ pairs discordant for FENO, exp(β) 1.50 [1.19, 1.89], compared to MZ pairs, exp(β) 1.07 [0.84, 1.37], p = .049. There was no difference in FENO in non-sensitized children with asthma, compared to children with neither asthma nor sensitization, exp(β) 0.89 [0.77, 1.03]. However, increased FENO was associated with sensitization, exp(β) 1.48 [1.30, 1.69], and with sensitization together with asthma, exp(β) 1.98 [1.57, 2.51], in all twins and within DZ pairs discordant for FENO, but not in MZ pairs. The FENO asthma association remained in DZ pairs without regular ICS-treatment.

CONCLUSIONS

The association between FENO and asthma is explained by genetics and sensitization.

Prenatal maternal psychosocial stress and offspring's asthma and allergic disease: A systematic review and meta-analysis

BACKGROUND

Prenatal maternal stress may influence offspring's atopic risk through sustained cortisol secretion resulting from activation of the hypothalamic-pituitary axis (HPA), leading to Th2-biased cell differentiation in the foetus. We undertook a systematic review and meta-analysis investigating the relationship between prenatal maternal psychosocial stress and risk of asthma and allergy in the offspring.

METHODS

We searched 11 electronic databases from 1960 to 2016, searched the grey literature and contacted experts in the field. Type of stress indicator included mood disorders, anxiety, exposure to violence, bereavement and socio-economic problems occurring during pregnancy, both objectively and subjectively measured. We included all possible asthma and IgE-mediated allergy outcomes. We conducted random-effects meta-analyses to synthesize the data.

RESULTS

We identified 9779 papers of which 30 studies (enrolling >6 million participants) satisfied inclusion criteria. The quality of 25 studies was moderate, 4 were strong, and one was weak. Maternal exposure to any type of stressors was associated with an increased risk of offspring atopic eczema/dermatitis (OR 1.34, 95% CI 1.22-1.47), allergic rhinitis (OR 1.30, 95% CI 1.04-1.62), wheeze (OR 1.34, 95% CI 1.16-1.54) and asthma (OR 1.15, 95% CI 1.04-1.27). Exposure to anxiety and depression had strongest effect compared to other stressors. Exposure during the third trimester had the greatest impact compared to first and second trimesters. The increased risk was stronger for early-onset and persistent than for late-onset wheeze. Bereavement of a child (HR 1.28, 95% CI 1.10-1.48) or a spouse (HR 1.40, 95% CI 1.03-1.90) increased the risk of offspring asthma.

CONCLUSIONS

Exposure to prenatal maternal psychosocial stress was associated with increased risk, albeit modestly, of asthma and allergy in the offspring. The pronounced risk during the third trimester may represent cumulative stress exposure throughout pregnancy rather than trimester-specific effect. Our findings may represent a causal effect or a result of inherent biases in studies, particularly residual confounding.

Relationship between birth weight or fetal growth rate and postnatal allergy: a systematic review protocol

REVIEW QUESTION/OBJECTIVE

The objective of this systematic review is to synthesize the best available evidence on the relationship between size at birth or fetal growth and postnatal allergy. Specifically, this review aims to assess evidence regarding relationships between absolute birth weight at term, birth weight corrected for gestational age, expressed as relative to population or customized growth data, or fetal growth measures and physician-diagnosed or parent- and self-reported postnatal clinical allergic disease (eczema/atopic dermatitis, hay fever/rhinitis, allergic asthma or anaphylaxis).The specific review question is: what is the association between the absolute birth weight at full-term or birth weight relative to population or customized data and corrected for gestational age or direct measures of fetal growth, and physician-diagnosed or parent- and self-reported clinical allergic disease (eczema/atopic dermatitis, hay fever/rhinitis, allergic asthma or anaphylaxis)?

Is it who you are or where you live? Residential segregation and racial gaps in childhood asthma

Higher asthma rates are one of the more obvious ways that health inequalities between African American and other children are manifested beginning in early childhood. In 2010, black asthma rates were double non-black rates. Some but not all of this difference can be explained by factors such as a higher incidence of low birth weight (LBW) among blacks; however, even conditional on LBW, blacks have a higher incidence of asthma than others. Using a unique data set based on the health records of all children born in New Jersey between 2006 and 2010, we show that when we split the data by whether or not children live in a "black" zip code, this racial difference in the incidence of asthma among LBW children entirely disappears. All LBW children in these zip codes, regardless of race, have a higher incidence of asthma. Our results point to the importance of residential segregation and neighborhoods in explaining persistent racial health disparities.

Towards non-conventional methods of designing register-based epidemiological studies: An application to pediatric research

AIMS

Various epidemiological designs have been applied to investigate the causes and consequences of fetal growth restriction in register-based observational studies. This review seeks to provide an overview of several conventional designs, including cohort, case-control and more recently applied non-conventional designs such as family-based designs. We also discuss some practical points regarding the application and interpretation of family-based designs.

METHODS

Definitions of each design, the study population, the exposure and the outcome measures are briefly summarised. Examples of study designs are taken from the field of low birth-weight research for illustrative purposes. Also examined are relative advantages and disadvantages of each design in terms of assumptions, potential selection and information bias, confounding and generalisability. Kinship data linkage, statistical models and result interpretation are discussed specific to family-based designs.

RESULTS

When all information is retrieved from registers, there is no evident preference of the case-control design over the cohort design to estimate odds ratios. All conventional designs included in the review are prone to bias, particularly due to residual confounding. Family-based designs are able to reduce such bias and strengthen causal inference. In the field of low birth-weight research, family-based designs have been able to confirm a negative association not confounded by genetic or shared environmental factors between low birth weight and the risk of asthma.

CONCLUSIONS

We conclude that there is a broader need for family-based design in observational research as evidenced by the meaningful contributions to the understanding of the potential causal association between low birth weight and subsequent outcomes.

Pre-birth origins of allergy and asthma

Allergy is a chronic disease that can develop as early as infancy, suggesting that early life factors are important in its aetiology. Variable associations between size at birth, a crude marker of the fetal environment, and allergy have been reported in humans and require comprehensive review. Associations between birth weight and allergy are however confounded in humans, and we and others have therefore begun exploring the effects of early life events on allergy in experimental models. In particular, we are using ovine models to investigate whether and how a restricted environment before birth protects against allergy, whether methyl donor availability contributes to allergic protection in IUGR, and why maternal asthma during pregnancy is associated with increased risks of allergic disease in children. We found that experimental intrauterine growth restriction (IUGR) in sheep reduced cutaneous responses to antigens in progeny, despite normal or elevated IgE responses. Furthermore, maternal methyl donor supplementation in late pregnancy partially reversed effects of experimental IUGR, consistent with the proposal that epigenetic pathways underlie some but not all effects of IUGR on allergic susceptibility. Ovine experimental allergic asthma with exacerbations reduces relative fetal size in late gestation, with some changes in immune populations in fetal thymus suggestive of increased activation. Maternal allergic asthma in mice also predisposes progeny to allergy development. In conclusion, these findings in experimental models provide direct evidence that a perturbed environment before birth alters immune system development and postnatal function, and provide opportunities to investigate underlying mechanisms and develop and evaluate interventions.

Twins' risk of childhood asthma mediated by gestational age and birthweight

BACKGROUND

Children born with low gestational age (GA) or low birthweight (BW) are at increased risk of asthma. Twins as compared to singletons are on average more likely to be born with lower GA and BW and have been hypothesized to comprise a high-risk population for asthma. Many previous studies have not accounted for potential confounders or mediators.

OBJECTIVE

To investigate the association between twinship and childhood asthma or early life wheeze and identify potential mediators, such as GA/BW.

METHODS

The study population consisted of two cohorts including all children born in Sweden from 1 January 1993 to 1 June 2001 (n = 756,363 singletons, n = 22,478 twins) and 1 July 2005 to 31 December 2009 (n = 456,239 singletons, n = 12,872 twins). Asthma was defined using validated register-based outcomes of diagnosis or medication. The data were analysed using logistic (older cohort) and Cox regression (younger cohort). Adjusted models incorporated potential confounding or mediating factors including gestational age and birthweight.

RESULTS

In the younger cohort, the crude hazard ratio (HR) of asthma medication after 1.5 years of age was 1.12 (95% CI 1.01-1.23), and fully adjusted HR was 0.80, 95% CI 0.72-0.89. Crude HR of asthma diagnosis in the same age group was 1.14 (95% CI 0.99-1.30), fully adjusted 0.78 (0.68-0.98). Adjusted analyses in the older group yielded similar results.

CONCLUSIONS

Twins were at significantly higher unadjusted risk of asthma or early life wheeze compared to singletons in the younger, but not in the older cohort. Associations attenuated following adjustment for GA/BW, suggesting that GA/BW mediates the effect of twinship on asthma risk. After adjustments, twins were at lower risk of asthma outcomes, possibly due to unmeasured confounding.

In utero Programming of Allergic Susceptibility

BACKGROUND

Around 30-40% of the world's population will experience allergy, the most common and earliest-onset noncommunicable disease. With a steady rise in the incidence of allergic disease over recent decades, up to 18% of children will suffer a respiratory, food or skin allergy before their 18th birthday. There is compelling evidence that the risk of developing allergy is influenced by early life events and particularly in utero exposures.

METHODS

A comprehensive literature review was undertaken which outlines prenatal risk factors and potential mechanisms underlying the development of allergy in childhood.

RESULTS

Exposures including maternal cigarette smoking, preterm birth and Caesarean delivery are implicated in predisposing infants to the later development of allergy. In contrast, restricted growth in utero, a healthy maternal diet and a larger family size are protective, but the mechanisms here are unclear and require further investigation.

CONCLUSION

To ameliorate the allergy pandemic in young children, we must define prenatal mechanisms that alter the programming of the fetal immune system and also identify specific targets for antenatal interventions.

Impact of asthma medication and familial factors on the association between childhood asthma and attention-deficit/hyperactivity disorder: a combined twin- and register-based study: Epidemiology of Allergic Disease

BACKGROUND

Asthma and attention-deficit/hyperactivity disorder (ADHD) are prevalent in childhood and may cause functional impairment and stress in families. Previous research supports an association between asthma and ADHD in children, but several aspects of this relationship are unclear.

OBJECTIVE

Our aim was to study whether the association between asthma and ADHD is restricted to either the inattentive or the hyperactive/impulsive symptoms of ADHD, to explore the impact of asthma severity and asthma medication and the contribution of shared genetic and environmental risk factors on the asthma-ADHD relationship.

METHODS

Data on asthma, ADHD, zygosity and possible confounders were collected from parental questionnaires at 9 or 12 years on 20 072 twins through the Swedish Twin Register, linked to the Swedish Medical Birth Register, the National Patient Register and the Prescribed Drug Register. The association between asthma and ADHD, the impact of asthma severity and medication, was assessed by generalized estimating equations. Cross-twin-cross-trait correlations (CTCT) were estimated to explore the relative importance of genes and environment for the association.

RESULTS

Asthmatic children had a higher risk of also having ADHD [odds ratio (OR) 1.53, 95% confidence interval (CI): 1.16-2.02]. The association was not restricted to either of the two dimensions of ADHD. The magnitude of the association increased with asthma severity (OR 2.84, 95% CI: 1.86-4.35) for ≥ 4 asthma attacks in the last 12 months and was not affected by asthma treatment. The CTCTs possibly indicate that the genetic component in overlap of the disorders is weak.

CONCLUSIONS AND CLINICAL RELEVANCE

Childhood asthma, especially severe asthma, is associated with ADHD. Asthma medication seems not to increase the risk of ADHD. Clinicians should be aware of the potential of ADHD in asthma. Optimal asthma care needs to be integrated with effective evaluation and treatment of ADHD in children with co-existing disorders.

Birth weight and asthma incidence by asthma phenotype pattern in a racially diverse cohort followed through adolescence

OBJECTIVE

Low birth weight (LBW) has been shown to be an independent risk factor for asthma. We hypothesized that LBW would have its greatest impact on early onset disease.

METHODS

A racially diverse cohort of children born from 1983 to 1985 at two hospitals, one urban and one suburban in the same metropolitan area, and oversampled for babies weighing ≤2500 g, was identified retrospectively when the children were 6 years of age and followed periodically. At the age 17 years study visit, cohort members and their parent/guardians were separately interviewed face-to-face regarding the subject's history of asthma using the standardized ISAAC questionnaire. We measured the cumulative incidence of asthma from birth through adolescence defined by age of diagnosis and persistence/remittance.

RESULTS

Six-hundred and eighty teens (82.6% of the original cohort) were included in the analyses, 387 with LBW and 293 of normal birth weight. The prevalence of physician-diagnosed "Current Asthma" was associated with LBW (p = 0.003 for trend), with patterns stronger in males and whites. LBW was associated most strongly with Late Onset Persistent asthma (current asthma that was diagnosed after 8 years); p for trend 0.032. This trend was again most evident in males and whites. None of the asthma categories classified as "remittent" were statistically associated with LBW.

CONCLUSIONS

LBW was not associated with diagnosed asthma that remitted before age 17 years. LBW was associated with asthma diagnosis in mid-childhood that persisted through adolescence, suggesting that the asthmagenic effects of LBW can become evident post the early years of childhood and persist into adulthood.

The contribution of twin studies to the understanding of the aetiology of asthma and atopic diseases

The prevalence of asthma and other atopic diseases has increased markedly during the past decades and the reasons for this are not fully understood. Asthma is still increasing in many parts of the world, notably in developing countries, and this emphasizes the importance of continuing research aimed at studying the aetiological factors of the disease and the causes of its increase in prevalence. Twin studies enable investigations into the genetic and environmental causes of individual variation in multifactorial diseases such as asthma. Thorough insight into these causes is important as this will ultimately guide the development of preventive strategies and targeted therapies. This review explores the contribution of twin studies to the understanding of the aetiology of asthma and atopic diseases.

Cohort Profile: Swedish Twin Study on Prediction and Prevention of Asthma (STOPPA)

Asthma is a common childhood disease and several risk factors have been identified; however, the impact of genes and environment is not fully understood. The aim of the Swedish Twin study On Prediction and Prevention of Asthma (STOPPA) is to identify environmental (birth characteristics and early life) and genetic (including epigenetic) factors as determinants for asthmatic disease. Based on the Child and Adolescent Twin Study in Sweden (CATSS) (parental interview at 9 or 12 years, N ~23,900) and an asthma and/or wheezing algorithm, we identified a sample of monozygotic (MZ) and dizygotic (DZ) same-sexed twin pairs. The twin pairs were classified as asthma concordant (ACC), asthma discordant (ADC) and healthy concordant (HCC). A sample of 9- to 14-year-old twins and their parents were invited to participate in a clinical examination. Background characteristics were collected in questionnaires and obtained from the National Health Registers. A clinical examination was performed to test lung function and capacity (spirometry with reversibility test and exhaled nitric oxide) and collect blood (serology and DNA), urine (metabolites), feces (microbiota), and saliva (cortisol). In total, 376 twin pairs (752 individual twins) completed the study, response rate 52%. All participating twins answered the questionnaire and >90% participated in lung function testing, blood-, and saliva sampling. This article describes the design, recruitment, data collection, measures, and background characteristics, as well as ongoing and planned analyses in STOPPA. Potential gains of the study include the identification of biomarkers, the emergence of candidates for drug development, and new leads for prevention of asthma and allergic disease.

Asthma and height in twins: a cohort and within-pair analyses study

In singletons, asthma may be associated with shorter height and delayed growth during adolescence. Yet, these studies do not account for heritability of asthma, puberty/menarche, and height. We aimed to study the association between asthma and puberty in boys and menarche in girls, and height, in a cohort of twins and subsequently in same-sex twin pairs discordant for asthma. From a Swedish twin cohort, parent- and self-reported data on asthma, puberty/menarche, and height were collected. Pubertal staging was established via the Petersen index. Logistic and linear regression was used to estimate associations between asthma and puberty/menarche and height, respectively. For within-pair analyses in twins discordant for asthma, conditional logistic and linear regression were used. Data on 2,658 (49.1% boys) twins were included. Among boys, asthma prevalence was 8.2% at 8-9 years and 10.2% at 13-14 years. Corresponding numbers for girls were 4.2% and 4.9%, respectively. In the entire cohort, no statistically significant associations were found between current asthma and puberty/menarche. Boys with asthma were shorter than boys without asthma at 8-9 years (on average, 1.86 [0.17-3.56] cm, p = .03) and at 13-14 years (on average, 2.94 [0.98-4.91] cm, p = .003) but not at 19-20 years. No such associations were found for girls. Within same-sex twin pairs discordant for asthma, no statistically significant associations were found for either sex. Twin boys, but not girls, with asthma were shorter than those without asthma. Non-statistically significant estimates from within-pair analyses suggest the association is partly confounded by genetic or familial environmental factors.

Prematurity and prescription asthma medication from childhood to young adulthood: a Danish national cohort study

INTRODUCTION

Preterm birth is associated with increased risk of asthma-like symptoms and purchase of prescription asthma medication in childhood. We investigated whether this association persists into adulthood and whether it is affected by accounting for neonatal respiratory morbidity (acute respiratory disease and bronchopulmonary dysplasia).

METHODS

A national cohort of all infants born in Denmark in the period 1980-2009 was included in this register study. Data on purchase of asthma medication (combination of inhaled β-2 agonists and other drugs for obstructive airway disease) in 2010-2011 were obtained from the Danish National Prescription Registry. Associations between gestational age (GA), neonatal respiratory morbidity and a cross-sectional evaluation of asthma medication purchase were explored by multivariate logistic regressions.

RESULTS

A full dataset was obtained on 1,790,241 individuals, 84.6% of all infants born in the period. Odds-ratios (95% CI) for the association between GA and purchase of asthma medication during infancy were: 3.86 (2.46-6.04) in GA 23-27 weeks, 2.37 (1.84-3.04) in GA 28-31 weeks and 1.59 (1.43-1.77) in GA 32-36 weeks compared to term infants with GA 37-42 weeks. Associations weakened in older age groups and became insignificant in young adults born extremely and very preterm with odds-ratios: 1.41 (0.63-3.19) and 1.15 (0.83-1.60) in GA 23-27 and 28-31 respectively. When adjusting for neonatal respiratory morbidity, the associations weakened but persisted both in childhood and adolescence.

CONCLUSION

There was a strong dose-response association between gestational age and the purchase of prescription asthma medication in infancy and childhood. This association weakened during adolescence and was mostly non-significant in young adulthood. The increased risk of prescription asthma medication purchase in ex-preterm children could only partly be explained by neonatal respiratory morbidity.

Asthma and obesity in children: current evidence and potential systems biology approaches

Both obesity and asthma are highly prevalent, complex diseases modified by multiple factors. Genetic, developmental, lung mechanical, immunological and behavioural factors have all been suggested as playing a causal role between the two entities; however, their complex mechanistic interactions are still poorly understood and evidence of causality in children remains scant. Equally lacking is evidence of effective treatment strategies, despite the fact that imbalances at vulnerable phases in childhood can impact long-term health. This review is targeted at both clinicians frequently faced with the dilemma of how to investigate and treat the obese asthmatic child and researchers interested in the topic. Highlighting the breadth of the spectrum of factors involved, this review collates evidence regarding the investigation and treatment of asthma in obese children, particularly in comparison with current approaches in 'difficult-to-treat' childhood asthma. Finally, the authors propose hypotheses for future research from a systems-based perspective.

Birth weight and childhood wheezing disorders: a systematic review and meta-analysis

BACKGROUND

Previous observational studies have claimed that birth weight and childhood wheezing disorders are associated although the results remained inconsistent. One systematic review and two systematic reviews that included meta-analyses reported inconsistent results. We aimed to conduct a systematic review and meta-analysis to investigate this.

METHODS

An online search of published papers linking childhood asthma and wheezing disorders with birth weight up to February 2014 was carried out using EMBASE and Medline medical research databases. Summary ORs were estimated using random-effects models. Subgroup meta-analyses were performed to assess the robustness of risk associations and between-study heterogeneity.

RESULTS

A total of 37 studies comprising 1,71, 737 participants were included in our meta-analysis. The unadjusted summary ORs for risk of childhood wheezing disorders associated with low birth weight (<2.5 kg) were 1.60 (95% CI 1.39 to 1.85, p<0.001) and 1.37 (95% CI 1.05 to 1.79, p=0.02) when compared with ≥2.5 and 2.5-4.0 kg birthweight groups, respectively. The overall summary OR for high birth weight (>4 kg) as compared to the 2.5-4.0 kg birthweight group was 1.02 (95% CI 0.99 to 1.04, p=0.13). There was substantial heterogeneity in the unadjusted low birth weight risk estimates which was not accounted for by predefined study characteristics. There was no significant heterogeneity in the high birth weight risk estimates. There was some evidence of funnel plot asymmetry and small study effects in the low birth weight (2.5 vs ≥2.5 kg and <2.5 vs 2.5-4 kg) OR estimates.

CONCLUSIONS

Our results suggest that low birth (<2.5 kg) is an independent risk factor for wheezing disorders during childhood and adolescence although there was substantial heterogeneity among the risk estimates. However, we found no significant association of high birth weight with wheezing disorders.

The interaction between mother and fetus and the development of allergic asthma

The rising prevalence of asthma and atopic disease in industrialized countries in the last 50 years has raised important questions about how and why the disease develops in susceptible populations. Most asthma begins in childhood in association with allergic sensitization and the development of a TH2 phenotype. It is recognized that asthma arises in the context of a complex interaction between genetic factors and the evolving immune system of the infant and the environment to which it is exposed, which now includes its in utero exposure. Early life exposures that lead to allergen sensitization and airway damage, especially in the form of viral respiratory tract infections, may lead to disease induction that commence the process that leads in some to asthma. Asthma models and early life observations suggest that repeated exposure to allergens and viral infection perpetuate a state of chronic airway inflammation leading to a maladaptive innate immune response that fails to resolve, characterized by chronic airway inflammation, airway remodeling and airway hyperresponsiveness. This article will concentrate on the development of asthma in the context of early life and maternal influences, including the effect of asthma on both the fetus and the mother.

Exploring the origins of asthma: Lessons from twin studies

This thesis explores the contribution of twin studies, particularly those studies originating from the Danish Twin Registry, to the understanding of the aetiology of asthma. First, it is explored how twin studies have established the contribution of genetic and environmental factors to the variation in the susceptibility to asthma, and to the variation in several aspects of the clinical expression of the disease such as its age at onset, its symptomatology, its intermediate phenotypes, and its relationship with other atopic diseases. Next, it is explored how twin studies have corroborated theories explaining asthma's recent increase in prevalence, and last, how these fit with the explanations of the epidemiological trends in other common chronic diseases of modernity.

Birth weight, gestational age, fetal growth and childhood asthma hospitalization

Background

Childhood asthma may have a fetal origin through fetal growth and development of the immunocompetence or respiratory organs.

Objective

We examined to which extent short gestational age, low birth weight and fetal growth restriction were associated with an increased risk of asthma hospitalization in childhood.

Methods

We undertook a cohort study based on several national registers in Denmark, Sweden and Finland. We included all live singleton born children in Denmark during 1979-2005 (N = 1,538,093), in Sweden during 1973-2004 (N = 3,067,670), and a 90% random sample of singleton children born in Finland during 1987-2004 (N = 1,050,744). The children were followed from three years of age to first hospitalization for asthma, emigration, death, their 18th birthday, or the end of study (the end of 2008 in Denmark, and the end of 2007 in Sweden or Finland), whichever came first. We computed the pseudo-values for each observation and used them in a generalized estimating equation to estimate relative risks (RR) for asthma hospitalization.

Results

A total of 131,783 children were hospitalized for asthma during follow-up. The risk for asthma hospitalization consistently increased with lower birth weight and shorter gestational age. A 1000-g decrease in birth weight corresponded to a RR of 1.17 (95% confidence interval (CI) 1.15-1.18). A one-week decrease in gestational age corresponded to a RR of 1.05 (95% CI 1.04-1.06). Small for gestational age was associated with an increased risk of asthma hospitalization in term but not in preterm born children.

Conclusions

Fetal growth and gestational age may play a direct or indirect causal role in the development of childhood asthma.

Placental restriction of fetal growth reduces cutaneous responses to antigen after sensitization in sheep

Prenatal and early childhood exposures are implicated as causes of allergy, but the effects of intrauterine growth restriction on immune function and allergy are poorly defined. We therefore evaluated effects of experimental restriction of fetal growth on immune function and allergic sensitization in adolescent sheep. Immune function (circulating total red and white blood cells, neutrophils, lymphocytes, monocytes, eosinophils, and basophils, and the antibody response to Clostridial vaccination) and responses to house dust mite (HDM) allergen and ovalbumin (OVA) antigen sensitization (specific total Ig, IgG1, and IgE antibodies, and cutaneous hypersensitivity) were investigated in adolescent sheep from placentally restricted (PR, n = 23) and control (n = 40) pregnancies. Increases in circulating HDM-specific IgE (P = 0.007) and OVA-specific IgE (P = 0.038) were greater in PR than control progeny. PR did not alter total Ig, IgG1, or IgM responses to either antigen. PR increased OVA-specific but not HDM-specific IgA responses in females only (P = 0.023). Multiple birth increased Ig responses to OVA in a sex-specific manner. PR decreased the proportion of positive cutaneous hypersensitivity responders to OVA at 24 h (P = 0.030) but had no effect on cutaneous responses to HDM. Acute wheal responses to intradermal histamine correlated positively with birth weight in singletons (P = 0.023). Intrauterine growth restriction may suppress inflammatory responses in skin downstream of IgE induction, without impairment in antibody responses to a nonpolysaccharide vaccine. Discord between cutaneous and IgE responses following sensitization suggests new mechanisms for prenatal allergy programming.

Birth weight and subsequent risk of asthma: a systematic review and meta-analysis

BACKGROUND

Previous studies suggest that birth weight is related to later risk of asthma. However, few meta-analyses have investigated these associations. Therefore, we performed a meta-analysis with different classifications to further validate the relationship between birth weight and asthma.

METHODS

PubMed (1990-2013), ScienceDirect (1990-2013), EMBASE(1990-2013),EBSCO (1990-2013) and Springer (1990-2013) were searched for articles. The following MeSH terms were used: "birth weight", "fetal growth retardation", "intrauterine growth restriction", "asthma", "wheezing".

RESULTS

We included 18 studies with data from a total of over 90,000 children and adults. (1) Low birth weight (<2,500g) as compared with BW>2,500g and BW=2500-4000g was associated with increased risk of asthma (Children: OR, 1.28; 95% CI, 1.09-1.50, P<0.05; OR, 1.34; 95% CI, 1.13-1.60, P<0.05, Adults: OR, 1.25; 95% CI, 1.12-1.39, P<0.05; OR, 1.25; 95% CI, 1.12-1.40, P<0.05). (2) High birth weight (>4,000g) was not associated with the risk of asthma when BW<4,000g and BW=2500-4000g were used as the reference.

CONCLUSIONS

These results suggest that low birth weight (<2,500g) is associated with increased risk of asthma both in children and adults and may serve as a mediator between prenatal influences and later disease risk; but high birth weight (>4,000g) was not associated with increased risk of asthma.

Preterm birth and childhood wheezing disorders: a systematic review and meta-analysis

BACKGROUND

Accumulating evidence implicates early life factors in the aetiology of non-communicable diseases, including asthma/wheezing disorders. We undertook a systematic review investigating risks of asthma/wheezing disorders in children born preterm, including the increasing numbers who, as a result of advances in neonatal care, now survive very preterm birth.

METHODS AND FINDINGS

Two reviewers independently searched seven online databases for contemporaneous (1 January 1995-23 September 2013) epidemiological studies investigating the association between preterm birth and asthma/wheezing disorders. Additional studies were identified through reference and citation searches, and contacting international experts. Quality appraisal was undertaken using the Effective Public Health Practice Project instrument. We pooled unadjusted and adjusted effect estimates using random-effects meta-analysis, investigated "dose-response" associations, and undertook subgroup, sensitivity, and meta-regression analyses to assess the robustness of associations. We identified 42 eligible studies from six continents. Twelve were excluded for population overlap, leaving 30 unique studies involving 1,543,639 children. Preterm birth was associated with an increased risk of wheezing disorders in unadjusted (13.7% versus 8.3%; odds ratio [OR] 1.71, 95% CI 1.57-1.87; 26 studies including 1,500,916 children) and adjusted analyses (OR 1.46, 95% CI 1.29-1.65; 17 studies including 874,710 children). The risk was particularly high among children born very preterm (<32 wk gestation; unadjusted: OR 3.00, 95% CI 2.61-3.44; adjusted: OR 2.81, 95% CI 2.55-3.12). Findings were most pronounced for studies with low risk of bias and were consistent across sensitivity analyses. The estimated population-attributable risk of preterm birth for childhood wheezing disorders was ≥3.1%. Key limitations related to the paucity of data from low- and middle-income countries, and risk of residual confounding.

CONCLUSIONS

There is compelling evidence that preterm birth-particularly very preterm birth-increases the risk of asthma. Given the projected global increases in children surviving preterm births, research now needs to focus on understanding underlying mechanisms, and then to translate these insights into the development of preventive interventions.

REVIEW REGISTRATION

PROSPERO CRD42013004965.

Fetal growth restriction is worse than extreme prematurity for the developing lung

BACKGROUND

Perinatal lung growth is highly vulnerable to inflammation and intrauterine growth restriction (IUGR), two major risk factors for chronic lung disease (CLD) in preterm neonates. However, the balance between extremely low gestational age (ELGA) and IUGR in very preterm infants as risk factors for CLD and co-morbidities remains poorly explored.

OBJECTIVES

This single-center study aims to compare neonatal morbidity (including CLD) and mortality among ELGA infants with normal birth weight (ELGA-AGA), very preterm infants with IUGR <3rd percentile (VLGA-IUGR) and very preterm infants with a birth weight appropriate for gestational age (VLGA-AGA), matched with VLGA-IUGR infants.

METHODS

Selected characteristics of the perinatal and neonatal periods were recorded and retrospectively compared among the three groups. Infants with major congenital anomalies were excluded. The diagnosis of CLD was based on whether the infant was receiving supplemental oxygen and/or non-invasive ventilation at a postmenstrual age of 36 weeks.

RESULTS

We found that, despite a median difference of 3 weeks in gestational age at birth between VLGA-IUGR and ELGA-AGA infants, neonatal mortality was 35% higher in neonates who had experienced fetal growth restriction, and that VLGA- IUGR was five times more predictive of CLD than was ELGA-AGA. These differences persisted after adjustment for confounding factors such as antenatal steroids, gender and respiratory distress syndrome.

CONCLUSIONS

This study reports that VLGA-IUGR infants are at higher risk of neonatal mortality and CLD than both ELGA-AGA and VLGA-AGA infants.

Use of acid-suppressive drugs in pregnancy and the risk of childhood asthma: bidirectional crossover study using the general practice research database

BACKGROUND

Recent studies have reported an association between maternal use of gastric acid-suppressive drugs during pregnancy and asthma in the offspring, but the association could have been confounded by unmeasured risk factors.

OBJECTIVE

We assessed the association between the use of acid-suppressive drugs during pregnancy and the risk of developing childhood asthma using a bidirectional crossover design.

METHODS

Mother-infant matched sets in the UK General Practitioners Research Database were used to identify children with a drug-treated asthma diagnosis during the years 2006-2010 who were matched to a sibling without asthma as controls. Primary exposure was use of any anti-suppressive drug during pregnancy, and subgroup analyses were conducted according to drug class (e.g. proton pump inhibitors or histamine 2 receptor antagonists) and trimester. Conditional logistic regression was used to estimate odds ratios (OR) with their corresponding 95 % confidence intervals (CIs).

RESULTS

A total of 1,874 children with asthma and 1,874 control siblings were included in the analysis. The exposure rate among case and control pregnancies was 22 and 20 %, respectively. After adjustments for gender, birth order, mother's age and general practice visits, the exposure to any gastric-acid suppressive drug during pregnancy slightly increased the risk for developing asthma (OR 1.23, 95 % CI 1.01-1.51; p = 0.042). A trend towards increased risks was observed for those who used proton pump inhibitors and/or histamine 2 receptor antagonists (adjusted OR 1.72, 95 % CI 1.00-2.98; p = 0.048).

CONCLUSIONS

These findings lend support to the emerging evidence that exposure to acid-suppressive drugs during pregnancy is associated with childhood asthma. More basic research is now warranted to investigate the mechanisms.

A twin study of early-childhood asthma in Puerto Ricans

BACKGROUND

The relative contributions of genetics and environment to asthma in Hispanics or to asthma in children younger than 3 years are not well understood.

OBJECTIVE

To examine the relative contributions of genetics and environment to early-childhood asthma by performing a longitudinal twin study of asthma in Puerto Rican children ≤ 3 years old.

METHODS

678 twin infants from the Puerto Rico Neo-Natal Twin Registry were assessed for asthma at age 1 year, with follow-up data obtained for 624 twins at age 3 years. Zygosity was determined by DNA microsatellite profiling. Structural equation modeling was performed for three phenotypes at ages 1 and 3 years: physician-diagnosed asthma, asthma medication use in the past year, and ≥ 1 hospitalization for asthma in the past year. Models were additionally adjusted for early-life environmental tobacco smoke exposure, sex, and age.

RESULTS

The prevalences of physician-diagnosed asthma, asthma medication use, and hospitalization for asthma were 11.6%, 10.8%, 4.9% at age 1 year, and 34.1%, 40.1%, and 8.5% at 3 years, respectively. Shared environmental effects contributed to the majority of variance in susceptibility to physician-diagnosed asthma and asthma medication use in the first year of life (84%-86%), while genetic effects drove variance in all phenotypes (45%-65%) at age 3 years. Early-life environmental tobacco smoke, sex, and age contributed to variance in susceptibility.

CONCLUSION

Our longitudinal study in Puerto Rican twins demonstrates a changing contribution of shared environmental effects to liability for physician-diagnosed asthma and asthma medication use between ages 1 and 3 years. Early-life environmental tobacco smoke reduction could markedly reduce asthma morbidity in young Puerto Rican children.

Association of childhood attention-deficit/hyperactivity disorder with atopic diseases and skin infections? A matched case-control study using the General Practice Research Database

BACKGROUND

Data to support the hypothesis of a relationship between attention-deficit/hyperactivity disorder (ADHD) and allergies are conflicting.

OBJECTIVE

To assess whether children with ADHD are more likely to have a history of atopic disorders, skin infections, and medical prescriptions than children without ADHD.

METHODS

We conducted a nested case-control study among boys using the UK General Practice Research Database (GPRD). Cases were defined as children with first-time diagnosis of ADHD who were treated with methylphenidate. Four controls who had neither ADHD nor ADHD drug prescriptions in their medical records were matched to each case on age and general practice.

RESULTS

We identified 884 boys with a first-time diagnosis of drug-treated ADHD and 3,536 controls. The independent odds ratios adjusted for age and presence of low birth weight or preterm delivery were 1.4 (95% confidence interval [CI], 1.2-1.7; P < .001) for a medical history of asthma, 1.5 (95% CI, 1.3-1.9; P < .001) for impetigo, and 1.5 (95% CI, 1.3-1.7; P < .001) for any antihistamine drug prescriptions. Other exposures that were more common in cases than controls, though not independently, were cow's milk intolerance and any prescription from the drug categories antiasthmatics, respiratory corticosteroids, topical steroids, antibacterials, or antifungals.

CONCLUSION

Despite possible limitations inherent to observational studies, this study lends support to the emerging evidence that childhood ADHD is associated with atopic diseases and impetigo. Further interdisciplinary research is needed to understand the underlying mechanisms and to evaluate targeted preventive, diagnostic, and therapeutic interventions.

Lack of relationship between birth conditions and allergic disorders in Japanese children aged 3 years

BACKGROUND

The current cross-sectional study examined the associations between low birth weight (LBW), preterm birth, and small-for-gestational-age (SGA) and the prevalence of wheeze, asthma, and eczema in Japanese children aged 3 years.

METHODS

Study subjects were 2004 children. All data were obtained using a questionnaire. Outcomes were defined according to the criteria of the International Study of Asthma and Allergies in Childhood. Adjustment was made for sex, number of siblings, breastfeeding duration, paternal and maternal educational level, paternal and maternal history of allergic disorders, maternal smoking during pregnancy, secondhand smoke exposure at home, and gestational age at birth.

RESULTS

The prevalence of wheeze, asthma, and eczema in the previous 12 months were 22.1%, 9.0%, and 17.5%, respectively, and 8.4% were classified as LBW (<2500 g), 4.7% as preterm birth (<37 weeks), and 7.1% as SGA (<10th percentile). There were no significant associations between LBW, preterm birth, or SGA and the prevalence of wheeze, asthma, or eczema. A positive relationship between preterm birth and asthma was of borderline significance in children whose mothers had smoked during pregnancy (adjusted OR: 4.71 [95% CI: 0.97-21.39]), but not in those whose mothers had never smoked during pregnancy; the multiplicative interaction between preterm birth and maternal smoking during pregnancy with respect to asthma was significant (p = .04).

CONCLUSIONS

The current study failed to detect significant associations between birth conditions and allergic disorders. Nevertheless, we did find evidence for an interaction between preterm birth and maternal smoking during pregnancy affecting asthma.

Perinatal factors and respiratory health in children

BACKGROUND

There is conflicting evidence regarding the associations between anthropometric birth measures and asthma and lung function in children, particularly for apparently healthy infants born at term.

OBJECTIVE

Our objective was to elucidate these relationships paying particular attention to features of study design and analysis that may threaten the validity of previous studies in this field.

METHODS

We analysed data from a cohort of children with a family history of asthma who were recruited antenatally. Anthropometric birth measures and potential confounders were recorded at birth and within the first year of life. Lung function and asthma outcomes were measured at 8 years of age. Airway hyperresponsiveness (AHR) was measured by methacholine challenge. The potential for a reversal paradox, due to inclusion of covariates on the causal pathway, was investigated.

RESULTS

Four hundred and fifty (73% of the initial cohort) children were tested at age 8 years. Birth weight in the lowest tertile was associated with current asthma (OR 1.95, 95% CI 1.08, 3.54) and recent wheeze (OR 1.87, 95%CI 1.08, 3.24), but not with AHR (OR 1.37, 95% CI 0.68, 2.78). Birth weight was positively associated with lung function. Current height modified the relationship between birth length and lung function suggesting that post-natal growth has an effect on this relationship.

CONCLUSIONS

Low birth weight is associated with a greater risk of current asthma and lower lung function at 8 years in children with a family history of asthma. Current height should be treated as an effect modifier when investigating the fetal origins hypothesis.

Fetal growth and risk of childhood asthma and allergic disease

Introduction

Early genetic and environmental factors have been discussed as potential causes for the high prevalence of asthma and allergic disease in the western world, and knowledge on fetal growth and its consequence on future health and disease development is emerging.

Objective

This review article is an attempt to summarize research on fetal growth and risk of asthma and allergic disease. Current knowledge and novel findings will be reviewed and open research questions identified, to give basic scientists, immunologists and clinicians an overview of an emerging research field.

Methods

PubMed-search on pre-defined terms and cross-references.

Results

Several studies have shown a correlation between low birth weight and/or gestational age and asthma and high birth weight and/or gestational age and atopy. The exact mechanism is not yet clear but both environmental and genetic factors seem to contribute to fetal growth. Some of these factors are confounders that can be adjusted for, and twin studies have been very helpful in this context. Suggested mechanisms behind fetal growth are often linked to the feto-maternal circulation, including the development of placenta and umbilical cord. However, the causal link between fetal growth restriction and subsequent asthma and allergic disease remains unexplained. New research regarding the catch-up growth following growth restriction has posited an alternative theory that diseases later on in life result from rapid catch-up growth rather than intrauterine growth restriction per se. Several studies have found a correlation between a rapid weight gain after birth and development of asthma or wheezing in childhood.

Conclusion and clinical relevance

Asthma and allergic disease are multifactorial. Several mechanisms seem to influence their development. Additional studies are needed before we fully understand the causal links between fetal growth and development of asthma and allergic diseases.