Invited commentary: Sibship effects and a call for a comparative disease approach

Sex Differences in the Association of Sibship Size and Position in Sibship with Lipid Profile during Adolescence: A Cross-Sectional Study

BACKGROUND

Epidemiologic studies have reported associations of sibship size and position of the child in the sibship with multiple health outcomes, including adiposity and diabetes. However, little is known about sibling effects on lipids. Hence, this study sought to evaluate associations of the number of total, older, and younger siblings with lipid profile among adolescents.

METHODS

In a cross-sectional study among high school students aged 14 to 19 years, lipid levels were measured in capillary blood. Parents reported the number of siblings (total, older, and younger). Geometric means of lipids were calculated, and linear regression was used to estimate the ratio of geometric means (RoGM) and 95% confidence intervals (CI). Analyses were sex stratified.

RESULTS

Of the total study sample (n = 1,584), 758 (47.9%) were boys and 826 (52.1%) were girls, with median age of 16.0 years. Total cholesterol (TC) was lower by 8% (adjusted-RoGM = 0.92, 95% CI: 0.88-0.96) among boys with ≥3 older siblings compared to those with no older siblings. Similarly, boys with ≥3 younger sibling compared to those with no younger siblings had reduced TC by 7% (adjusted-RoGM = 0.93, 0.87-0.99). Moreover, an increased number of total siblings (≥4 vs. 0/1: adjusted-RoGM = 0.80, 0.67-97) and older siblings (≥3 vs. 0: adjusted-RoGM = 0.90, 0.82-0.98) were associated with reduced low-density lipoprotein cholesterol (LDL-C) among boys. Similarly, lower levels of triglycerides (TG) were seen among boys with ≥3 older siblings compared to those with no older siblings (adjusted-RoGM = 0.87, 0.78-0.96). A higher number of younger siblings was associated with increased high-density lipoprotein cholesterol (HDL-C) among boys (≥3 vs. 0: adjusted-RoGM = 1.08, 1.01-1.17). Sibship characteristics were not associated with lipids among girls.

CONCLUSIONS

Increased number of total, older, and younger siblings were associated with favorable lipid profiles among adolescent boys, but not girls. Mechanisms underlying these associations need further investigations.

Psoriasis Among Adolescents in Kuwait and the Role of Siblings, Breastfeeding, and Household Cat and Secondhand Smoke Exposure: A Cross-Sectional Study

INTRODUCTION

Globally, the epidemiology of psoriasis is poorly understood, and most countries lack essential epidemiologic data regarding disease burden and its determinants. This study sought to estimate the prevalence of psoriasis among adolescents in Kuwait and assess its association with different risk factors, including obesity, sibship size, breastfeeding, and exposure to household secondhand smoke (SHS) and pets.

METHODS

Schoolchildren aged 11-14 years (n = 3864) were enrolled in a cross-sectional study. Lifetime and current (past 12 months) prevalence of psoriasis were ascertained according to ever having a history of doctor-diagnosis plus current active lesion(s) and/or current use of treatment of psoriasis. Associations were assessed using Poisson regression with robust variance estimation, and adjusted prevalence ratios (aPR) and 95% confidence intervals (CI) were estimated.

RESULTS

The lifetime and current prevalence of psoriasis were estimated to be 3.6% (136/3806) and 1.1% (42/3806), respectively. Commonly reported anatomical sites affected by psoriasis included scalp (47.6%) and the extensor surface of the knees (50%) and elbows (38.1%). Household SHS exposure was associated with increased lifetime psoriasis (aPR = 1.41, 95% CI 1.07-1.98), and showed a trend for association with current psoriasis (1.77, 0.89-3.53). Similarly, cat-keeping during infancy was associated with lifetime psoriasis (1.96, 1.14-3.37), and demonstrated a trend for association with current psoriasis (1.49, 0.52-1.98). In contrast, breastfeeding was associated with a decreased lifetime psoriasis (0.62, 0.44-0.89), but was not associated with current psoriasis. Trend analyses showed that the prevalence of lifetime and current psoriasis increased with increasing numbers of total, older, and younger siblings.

CONCLUSIONS

Psoriasis affects a considerable proportion of schoolchildren in Kuwait. Interestingly, psoriasis prevalence was related to risk factors also found in allergic diseases, such as exposure to SHS, cat-keeping in infancy, breastfeeding, and sibship size, possibly suggesting a role of immune dysregulation.

Trimester-specific association between antibiotics exposure during pregnancy and childhood asthma or wheeze: the role of confounding

PURPOSE

We conducted the meta-analysis to respectively evaluate the risk of prenatal antibiotics use during specific trimesters (first, second, and third trimester) on childhood asthma or wheeze and to explore whether the association was biased by potential confounding.

METHODS

The quality of included articles was assessed according to Newcastle-Ottawa Quality Assessment Scale and the Strengthening the Reporting of Observational Studies in Epidemiology. A random effects model was used to calculate pooled risk ratios and corresponding 95% confidence interval (CI), and publication bias was tested by Egger statistical test.

RESULTS

Eight studies were included finally. We found a crude positive association of prenatal antibiotics use during each pregnancy trimester and risk of childhood asthma or wheeze with RRs of 1.28 (95% CI, 1.09-1.51) for the first trimester of pregnancy, 1.25 (95% CI, 1.02-1.52) for the second trimester, and 1.25 (95% CI, 1.05-1.49) for the third trimester. However, when considering potential factors of maternal infections and presence of siblings, the relationship for each trimester was insignificant.

CONCLUSIONS

This systemic review and meta-analysis proposed a crude positive association between prenatal antibiotic use in every specific trimester and risk of childhood asthma or wheeze. However, adjustment for confounders decreased the relative risk estimates, supporting the concept that these associations are, at least in part, because of confounding by indication.

Birth order and paediatric allergic disease: A nationwide longitudinal survey

BACKGROUND

Environmental factors seem to be related to the incidence of allergic disease. Children with a later birth order are often exposed to environments, where pathogens and endotoxins can be found, and thus have a higher risk of developing infectious diseases. Therefore, birth order is regarded as an indicator that reflects post-natal environment. However, longitudinal studies are limited on this subject. This study sought to elucidate the relationships between birth order and allergic disease.

METHODS

From a nationwide longitudinal study that followed children born in 2001 (n = 47 015), we selected doctors' visits for 3 types of allergic disease-bronchial asthma, food allergy and atopic dermatitis-from infancy to 12 years of age and conducted binomial log-linear regression analysis to evaluate the associations between birth order and these diseases. We adjusted for the child and parental factors and estimated risk ratio (RR) and 95% confidence interval (CI) for each outcome.

RESULTS

The associations between birth order and bronchial asthma were diverse; later birth order increased the risk in early childhood, but decreased the risks during school age. For example, the adjusted RR comparing third-born or higher and first-born children was 1.19 (95% CI, 1.05-1.35) between 30 and 42 months of age, but was 0.76 (95% CI, 0.65-0.89) between 10 and 11 years. Later birth order was generally protective for food allergy but increased the risk of atopic dermatitis.

CONCLUSION

The influence of birth order depended on the type of allergic disease and the childhood period. Childhood is unique in terms of physical and immunological development, and the immune response to the post-natal environment in childhood appears to be heterogeneous.

Siblings, asthma, rhinoconjunctivitis and eczema: a worldwide perspective from the International Study of Asthma and Allergies in Childhood

Background

Associations of larger families with lower prevalences of hay fever, eczema and objective markers of allergic sensitization have been found fairly consistently in affluent countries, but little is known about these relationships in less affluent countries.

Methods

Questionnaire data for 210 200 children aged 6–7 years from 31 countries, and 337 226 children aged 13–14 years from 52 countries, were collected by Phase Three of the International Study of Asthma and Allergies in Childhood (ISAAC). Associations of disease symptoms and labels of asthma, rhinoconjunctivitis and eczema were analysed by numbers of total, older and younger siblings, using mixed (multi-level) logistic regression models to adjust for individual covariates and at the centre level for region, language and national affluence.

Results

In both age groups, inverse trends (P < 0.0001) were observed for reported ‘hay fever ever’ and ‘eczema ever’ with increasing numbers of total siblings, and more specifically older siblings. These inverse associations were significantly (P < 0.005) stronger in more affluent countries. In contrast, symptoms of severe asthma and severe eczema were positively associated (P < 0.0001) with total sibship size in both age groups. These associations with disease severity were largely independent of position within the sibship and national GNI per capita.

Conclusions

These global findings on sibship size and childhood asthma, rhinoconjunctivitis and eczema suggest at least two distinct trends. Inverse associations with older siblings (observations which prompted the ‘hygiene hypothesis’ for allergic disease) are mainly a phenomenon of more affluent countries, whereas greater severity of symptoms in larger families is globally more widespread.

Early-life hygiene-related factors affect risk of central nervous system demyelination and asthma differentially

The increasing prevalence of immune-related diseases, including multiple sclerosis, may be partly explained by reduced microbial burden during childhood. Within a multi-centre case-control study population, we examined: (i) the co-morbid immune diseases profile of adults with a first clinical diagnosis of central nervous system demyelination (FCD) and (ii) sibship structure in relation to an autoimmune (FCD) and an allergic (asthma) disease. FCD cases (n = 282) were aged 18-59 years; controls (n = 558) were matched on age, sex and region. Measures include: history of doctor-diagnosed asthma; sibling profile (number; dates of birth); and regular childcare attendance. FCD cases did not differ from controls with regard to personal or family history of allergy, but had a greater likelihood of chronic fatigue syndrome [odds ratio (OR) = 3·11; 95% confidence interval (CI) 1·11, 8·71]. Having any younger siblings showed reduced odds of FCD (OR = 0·68; 95% CI: 0·49, 0·95) but not asthma (OR = 1·47; 95% CI: 0·91, 2·38). In contrast, an increasing number of older siblings was associated with reduced risk of asthma (P trend = 0·04) but not FCD (P trend = 0·66). Allergies were not over-represented among people presenting with FCD. Sibship characteristics influence both FCD and asthma risk but the underlying mechanisms differ, possibly due to the timing of the putative 'sibling effect'.

Early life exposures and the risk of adult glioma

Exposure to common infections in early life may stimulate immune development and reduce the risk for developing cancer. Birth order and family size are proxies for the timing of exposure to childhood infections with several studies showing a reduced risk of glioma associated with a higher order of birth (and presumed younger age at infection). The aim of this study was to examine whether birth order, family size, and other early life exposures are associated with the risk of glioma in adults using data collected in a large clinic-based US case-control study including 889 glioma cases and 903 community controls. A structured interviewer-administered questionnaire was used to collect information on family structure, childhood exposures and other potential risk factors. Logistic regression was used to calculate odds ratios (OR) and corresponding 95% confidence intervals (CI) for the association between early life factors and glioma risk. Persons having any siblings were at significantly lower risk for glioma when compared to those reporting no siblings (OR=0.64; 95% CI 0.44-0.93; p=0.020). Compared to first-borns, individuals with older siblings had a significantly lower risk (OR=0.75; 95% CI 0.61-0.91; p=0.004). Birth weight, having been breast fed in infancy, and season of birth were not associated with glioma risk. The current findings lend further support to a growing body of evidence that early exposure to childhood infections reduces the risk of glioma onset in children and adults.

Birth order, atopy, and symptoms of allergy and asthma among inner-city children attending Head Start in New York City

BACKGROUND

In past research, children with older siblings were more likely than others to wheeze at age 2 years, but less likely by age 6 years. Higher infection transmission and a down-regulated allergic immune response as a result of these infections, respectively, were suggested as the causes. However, in a study of children aged 0-3 years in a low-income urban community in New York City, USA, with high asthma prevalence, we observed no birth-order effect.

OBJECTIVE

To evaluate the association between birth order and atopy and respiratory symptoms in 4-year-old children attending Head Start programs in NYC.

METHODS

Respiratory symptoms were assessed by questionnaire for 1005 children (mean age 4.0 years) living in high asthma prevalence neighbourhoods. Serum was collected from a subgroup of the children (n=494) and specific IgE responses to dust mite, cockroach, mouse, and cat allergens were measured.

RESULTS

Prevalence of specific IgE (> or =0.35 IU/mL) did not differ significantly among first (35%), second (35%), and later-born children (28%) (P=0.23). Increasing birth order was associated with increasing prevalence of respiratory symptoms in the prior year, including wheeze (first 20%, second 27%, third or later 35%; P<0.001), being awakened at night by cough (28%, 33%, 38%; P=0.005), emergency department visits (14%, 17%, 21%; P=0.02) and hospitalizations for difficulty breathing (6.1%, 6.6%, 10%; P=0.04). The associations of birth order with respiratory symptoms were statistically significant only for the non-seroatopic children and those without an asthmatic parent.

CONCLUSIONS

Non-seroatopic children with older siblings were more likely than those without older siblings to have respiratory symptoms at age 4 years. Although the stability of these associations over time remains to be determined, the differences in findings between this study and our previous NYC birth cohort study suggest that patterns of asthma development may vary even among low-income populations within the same city.

The link between the epidemics of obesity and allergic diseases: does obesity induce decreased immune tolerance?

There is increasing epidemiological evidence that obesity increases the risk of asthma, atopic, and autoimmune diseases. We hypothesize that the increase in these diseases is caused, at least in part, by decreased immunological tolerance as a consequence of immunological changes induced by adipokines (e.g. leptin and adiponectin) and cytokines [e.g. interleukin 6 (IL6) and tumor necrosis factor alpha (TNFalpha)] secreted by white adipose tissue. The increasing body weight increases the levels of circulating IL6, leptin, and TNFalpha. IL6 and leptin down-regulate the activity of regulatory T-lymphocytes (Tregs). Additionally, adiponectin, which decreases with increasing obesity, down-regulates the secretion of IL10 from macrophages and adipocytes. These changes in IL6, leptin, and IL10 decrease the regulatory effect of Tregs resulting in decreased immunological tolerance to antigens. In pregnant women, these obesity-induced immunological changes might be transmitted to the fetus by epigenetic inheritance thereby increasing the risk of atopic disease. We propose that obesity results in immunological changes resulting in decreased immunological tolerance to antigens and skewing of the immune system towards a Th2 cytokine profile increasing the risk of allergy and other immune-mediated diseases. Furthermore, this hypothesis offers a unifying explanation for the observation that older siblings appear to confer protection against atopic diseases, preeclampsia, and certain autoimmune diseases. More studies are definitely needed to explore further the immunological effects of obesity and its possible effects on allergic disease.

Asthma prevalence, family size, and birth order

BACKGROUND

Asthma prevalence may be reduced in large families. The hygiene hypothesis suggests that older siblings protect their younger siblings from asthma through a modulating effect on the still-maturing immune system. If the hygiene hypothesis is correct, asthma prevalence should be inversely related to birth order. The objective of this study was to examine the relationship between asthma prevalence, and family size and birth order.

METHODS

The medical records of 531,116 Israeli military conscripts were reviewed. The association between number of children in the family and the prevalence of asthma, and between birth order and the prevalence of asthma was assessed. Odds ratios for asthma by birth order and family size, adjusted for each other, were calculated.

RESULTS

Asthma was diagnosed in 26,833 male subjects (8.6%) and 15,079 female subjects (6.9%). Asthma prevalence was inversely related to the number of children in the family (p < 0.001). Among subjects who were the only child in the family, the prevalence of asthma was 7.3%. The prevalence increased to 8.95% among subjects from families with three siblings, and then progressively decreased as the number of siblings increased, and reached a trough of 0.58% in conscripts from families of 15 to 20 siblings. Asthma prevalence was similar for all birth orders.

CONCLUSIONS

In families with four or more children, asthma prevalence is inversely related to the number of children in the family. Asthma prevalence is similar for all birth orders. The similar asthma prevalence for all birth orders challenges the hygiene hypothesis as the mechanism for the decreased asthma prevalence in large families.

Early life circumstances and atopic disorders in childhood

The prevalence of childhood atopic disorders has risen dramatically in the last decades of the past century. Risk factors for the development of these disorders have been studied extensively. This review focuses on the role of early life risk factors such as pre-natal development, perinatal circumstances, birth order and childhood vaccinations.

Does paternal antigen-induced secretion of interleukin-10 by T regulatory cells mediate the birth order effect?

Childhood allergy constitutes a significant burden of disease in the Western world. The prevalence of this condition is highest in first born children, an as yet unresolved phenomenon called the "birth order effect". The hygiene hypothesis attempted to explain this differential risk by stating that less exposure to microbial agents at an early age of first born children would result in reduced activation of the immune system and subsequent polarization towards a Th2 phenotype. However, no conclusive evidence for or against the hygiene hypothesis has been found so far. Another, not necessarily conflicting, theory states that the birth order effect is already established during prenatal life and that the fetal-maternal interaction changes during successive pregnancies. Combining this theory with research on preeclampsia, another disease originating in pregnancy and also related to birth order, could suggest clues about the mechanisms underlying the birth order effect. Recent research on preeclampsia showed that preeclamptic women have higher levels of pro-inflammatory IL-6 and lower paternal antigen-induced secretion of IL-10 compared to normal pregnant women, indicating a lower number or lower functional activity of T regulatory (Treg) cells. These Treg cells play an important role in maintaining tolerance to fetal antigens in pregnancy and they are found in high numbers at the maternal-fetal interface. We hypothesize that nulliparous women have a lower activity of Treg cells specific to paternal antigens compared to parous women, either in peripheral blood or in the decidua (maternal part of the fetal-maternal interface). If this hypothesis is true, this would mean that the allergen suppressor mechanism of Treg cells is sub-optimal in first born children. This would explain at least a part of the mechanism of the birth order effect and would give us directions for developing preventive measures to prevent possibly 30% of future childhood allergies.

An overview of the association between allergy and cancer

Numerous epidemiological studies have evaluated some aspect of the association between a history of allergy and cancer occurrence. In this article, an overview of the epidemiological evidence is presented with a discussion of a number of methodological issues important in this area of study. Literature searches were conducted using the MEDLINE database from 1966 through to August 2005 to identify articles that explored a personal history of allergic disorders as a risk factor for cancer. Although it is difficult to draw conclusions between allergy and cancer at many sites because of insufficient evidence or a lack of consistency both within and among studies completed to date, strong inverse associations have been reported for pancreatic cancer and glioma, whereas lung cancer was positively associated with asthma. Additional studies are needed to confirm these finding and to address the limitations of previous studies, including the validity and reliability of exposure measures and control for confounding. Further, large prospective studies using cancer incidence would be particularly useful, including studies using biological markers of allergic status to reduce potential misclassification and to confirm the results of previous studies based on self-report. There is also a need for further basic research to clarify a potential mechanism, should an association exist.