Cardiovascular Mortality in Twins and the Fetal Origins Hypothesis

Association of Trauma With Long-Term Risk of Death and Immune-Mediated or Cancer Disease in Same-Sex Twins

Importance

Immediate consequences of trauma include a rapid and immense activation of the immune system, whereas long-term outcomes include premature death, physical disability, and reduced workability.

Objective

To investigate if moderate to severe trauma is associated with long-term increased risk of death or immune-mediated or cancer disease.

Design, Setting, and Participants

This registry-based, matched, co-twin control cohort study linked the Danish Twin Registry and the Danish National Patient Registry to identify twin pairs in which 1 twin had been exposed to severe trauma and the other twin had not from 1994 to 2018. The co-twin control design allowed for matching on genetic and environmental factors shared within twin pairs.

Exposure

Twin pairs were included if 1 twin had been exposed to moderate to severe trauma and the other twin had not (ie, co-twin). Only twin pairs where both twins were alive 6 months after the trauma event were included.

Main Outcome and Measure

Twin pairs were followed up from 6 months after trauma until 1 twin experienced the primary composite outcome of death or 1 of 24 predefined immune-mediated or cancer diseases or end of follow-up. Cox proportional hazards regression was used for intrapair analyses of the association between trauma and the primary outcome.

Results

A total of 3776 twin pairs were included, and 2290 (61%) were disease free prior to outcome analysis and were eligible for the analysis of the primary outcome. The median (IQR) age was 36.4 (25.7-50.2) years. The median (IQR) follow-up time was 8.6 (3.8-14.5) years. Overall, 1268 twin pairs (55%) reached the primary outcome; the twin exposed to trauma was first to experience the outcome in 724 pairs (32%), whereas the co-twin was first in 544 pairs (24%). The hazard ratio for reaching the composite outcome was 1.33 (95% CI, 1.19-1.49) for twins exposed to trauma. Analyses of death or immune-mediated or cancer disease as separate outcomes provided hazard ratios of 1.91 (95% CI, 1.68-2.18) and 1.28 (95% CI, 1.14-1.44), respectively.

Conclusion and Relevance

In this study, twins exposed to moderate to severe trauma had significantly increased risk of death or immune-mediated or cancer disease several years after trauma compared with their co-twins.

Heritability of Curve Patterns in Oral Glucose Tolerance Test

Type 2 diabetes, which is caused by both genetic and environmental factors, may be diagnosed using the oral glucose tolerance test (OGTT). Recent studies demonstrated specific patterns in glucose curves during OGTT associated with cardiometabolic risk profiles. As the relative contribution of genetic and environmental influences on glucose curve patterns is unknown, we aimed to investigate the heritability of these patterns. We studied twins from the Danish GEMINAKAR cohort aged 18-67 years and free from diabetes at baseline during 1997-2000; glucose concentrations were measured three times during a 2-h OGTT. Heterogeneity of the glucose response during OGTT was examined with latent class mixed-effects models, evaluating goodness of fit by Bayes information criterion. The genetic influence on curve patterns was estimated using quantitative genetic modeling based on linear structural equations. Overall, 1455 twins (41% monozygotic) had valid glucose concentrations measured from the OGTT, and four latent classes with different glucose response patterns were identified. Statistical modeling demonstrated genetic influence for belonging to a specific class or not, with heritability estimated to be between 45% and 67%. During ∼12 years of follow-up, the four classes were each associated with different incidence of type 2 diabetes. Hence, glucose response curve patterns associated with type 2 diabetes risk appear to be moderately to highly heritable.

THE EXPOSOME IN HUMAN EVOLUTION: FROM DUST TO DIESEL

Global exposures to air pollution and cigarette smoke are novel in human evolutionary history and are associated with about 16 million premature deaths per year. We investigate the history of the human exposome for relationships between novel environmental toxins and genetic changes during human evolution in six phases. Phase I: With increased walking on savannas, early human ancestors inhaled crustal dust, fecal aerosols, and spores; carrion scavenging introduced new infectious pathogens. Phase II: Domestic fire exposed early Homo to novel toxins from smoke and cooking. Phases III and IV: Neolithic to preindustrial Homo sapiens incurred infectious pathogens from domestic animals and dense communities with limited sanitation. Phase V: Industrialization introduced novel toxins from fossil fuels, industrial chemicals, and tobacco at the same time infectious pathogens were diminishing. Thereby, pathogen-driven causes of mortality were replaced by chronic diseases driven by sterile inflammogens, exogenous and endogenous. Phase VI: Considers future health during global warming with increased air pollution and infections. We hypothesize that adaptation to some ancient toxins persists in genetic variations associated with inflammation and longevity.

Associations between birth characteristics and age-related cognitive impairment and dementia: A registry-based cohort study

BACKGROUND

There is evidence for long-lasting effects of birth characteristics on cognitive ability in childhood and adulthood. Further, low cognitive ability throughout the lifetime has been linked to age-related cognitive decline and dementia risk. However, little is known about the effects of birth characteristics on cognitive dysfunction late in life. Here we explore potential associations between birth characteristics (weight, head circumference, length, and gestational age), adjusted and not adjusted for gestational age, and cognitive impairment and dementia late in life.

METHODS AND FINDINGS

Data from twins in the Swedish Twin Registry born 1926-1960 were merged with information from the Swedish birth, patient, and cause of death registries, resulting in a sample of 35,191 individuals. A subsample of 4,000 twins aged 65 years and older also participated in a telephone cognitive screening in 1998-2002. Associations of birth characteristics with registry-based dementia diagnoses and on telephone-assessed cognitive impairment were investigated in the full sample and subsample, respectively. The full sample contained 907 (2.6%) individuals with a dementia diagnosis (an incidence rate of 5.9% per 100,000 person-years), 803 (2.4%) individuals born small for gestational age, and 929 (2.8%) individuals born with a small head for gestational age. The subsample contained 569 (14.2%) individuals with cognitive impairment. Low birth weight for gestational age and being born with a small head for gestational age were significant risk factors for cognitive dysfunction late in life, with an up to 2-fold risk increase (p < 0.001) compared to infants with normal growth and head size, even after controlling for familial factors, childhood socioeconomic status, and education in adulthood. In line with this, each additional 100 g birth weight and each additional millimeter head circumference significantly reduced the risk for dementia (hazard ratio 0.98, 95% confidence interval 0.97 to 0.99, p = 0.004) and cognitive impairment (odds ratio 0.99, 95% confidence interval 0.99 to 1.00, p = 0.004), respectively. Within-pair analyses of identical twins, though hampered by small sample size, suggested that the observed associations between birth characteristics and dementia are likely not due to underlying shared genetic or environmental etiology. A limitation of the present study is that registry-based dementia diagnoses likely miss some of the true dementia cases in the population. Further, a more precise measure of cognitive reserve early in life as well as a date of onset for the cognitive impairment measure in the subsample would have been favorable.

CONCLUSIONS

In this study, we found that infants of smaller birth size (i.e., low birth weight or small head circumference adjusted and unadjusted for gestational age) have a significantly higher risk of age-related cognitive dysfunction compared to those with normal growth, highlighting the importance of closely monitoring the cognitive development of such infants and evaluating the potential of early life interventions targeted at enhancing cognitive reserve.

Handgrip strength and its prognostic value for mortality in Moscow, Denmark, and England

Background

This study compares handgrip strength and its association with mortality across studies conducted in Moscow, Denmark, and England.

Materials

The data collected by the Study of Stress, Aging, and Health in Russia, the Study of Middle-Aged Danish Twins and the Longitudinal Study of Aging Danish Twins, and the English Longitudinal Study of Ageing was utilized.

Results

Among the male participants, the age-standardized grip strength was 2 kg and 1 kg lower in Russia than in Denmark and in England, respectively. The age-standardized grip strength among the female participants was 1.9 kg and 1.6 kg lower in Russia than in Denmark and in England, respectively. In Moscow, a one-kilogram increase in grip strength was associated with a 4% (hazard ratio [HR] = 0.96, 95% confidence interval [CI]: 0.94, 0.99) reduction in mortality among men and a 10% (HR = 0.90, 95%CI: 0.86, 0.94) among women. Meanwhile, a one-kilogram increase in grip strength was associated with a 6% (HR = 0.94, 95%CI: 0.93, 0.95) and an 8% (HR = 0.92, 95%CI: 0.90, 0.94) decrease in mortality among Danish men and women, respectively, and with a 2% (HR = 0.98, 95%CI: 0.97, 0.99) and a 3% (HR = 0.97, 95%CI: 0.95, 0.98) reduction in mortality among the English men and women, respectively.

Conclusion

The study suggests that, although absolute grip strength values appear to vary across the Muscovite, Danish, and English samples, the degree to which grip strength is predictive of mortality is comparable across national populations with diverse socioeconomic and health profiles and life expectancy levels.

Genetic analysis of cause of death in a mixture model of bivariate lifetime data

A mixture model in multivariate survival analysis is presented, whereby heterogeneity among subjects creates divergent paths for the individual’s risk of experiencing an event (i.e., disease), as well as for the associated length of survival. Dependence among competing risks is included and rendered testable. This method is an extension of the bivariate correlated gamma-frailty model. It is applied to a data set on Danish twins, for whom cause-specific mortality is known. The use of multivariate data solves the identifiability problem which is inherent in the competing risk model of univariate lifetimes. We analyse the influence of genetic and environmental factors on frailty. Using a sample of 1470 monozygotic (MZ) and 2730 dizygotic (DZ) female twin pairs, we apply five genetic models to the associated mortality data, focusing particularly on death from coronary heart disease (CHD). Using the best fitting model, the inheritance risk of death from CHD was 0.39 (standard error 0.13). The results from this model are compared with the results from earlier analysis that used the restricted model, where the independence of competing risks was assumed. Comparing both cases, it turns out, that heritability of frailty on mortality due to CHD change substantially. Despite the inclusion of dependence, analysis confirms the significant genetic component to an individual’s risk of mortality from CHD. Whether dependence or independence is assumed, the best model for analysis with regard to CHD mortality risks is a model assuming that additive factors are responsible for heritability in susceptibility to CHD. The paper ends with a discussion of limitations and possible further extensions to the model presented.

Cognitive Functioning after Surgery in Middle-aged and Elderly Danish Twins

BACKGROUND

Postoperative cognitive dysfunction is common, but it remains unclear whether there are long-term adverse cognitive effects of surgery combined with anesthesia. The authors examined the association between exposure to surgery and level of cognitive functioning in a sample of 8,503 middle-aged and elderly twins.

METHODS

Results from five cognitive tests were compared in twins exposed to surgery, classified as major, minor, hip and knee replacement, or other, with those of a reference group without surgery using linear regression adjusted for sex and age. Genetic and shared environmental confounding was addressed in intrapair analyses of 87 monozygotic and 124 dizygotic same-sexed twin pairs in whom one had a history of major surgery and the other did not.

RESULTS

Statistically significantly lower composite cognitive score was found in twins with at least one major surgery compared with the reference group (mean difference, -0.27; 95% CI, -0.48 to -0.06), corresponding to one tenth of an SD, that is, a negligible effect size. In the intrapair analysis, the surgery-exposed co-twin had the lower cognitive score in 49% (95% CI, 42 to 56%) of the pairs. None of the other groups differed from the reference group except the knee and hip replacement group that tended to have higher cognitive scores (mean difference, 0.35; 95% CI, -0.18 to 0.87).

CONCLUSIONS

A history of major surgery was associated with a negligibly lower level of cognitive functioning. The supplementary analyses suggest that preoperative cognitive functioning and underlying diseases were more important for cognitive functioning in mid- and late life than surgery and anesthesia.

Genetic and environmental factors in associations between infant growth and adult cardiometabolic risk profile in twins

BACKGROUND

Accelerated infant growth is associated with an altered, mostly adverse adult cardiometabolic risk profile. The importance of genetic and environmental factors to these associations is unclear.

OBJECTIVE

The objective was to examine the importance of genetic and environmental factors in the associations between infant growth and adult cardiometabolic risk factors (anthropometric characteristics, lipids, insulin sensitivity, leptin, blood pressure, and fibrinogen) in twins.

DESIGN

Cardiometabolic risk factors were assessed in 240 twin pairs (aged 18-34 y) from the East Flanders Prospective Twin Survey. Infant growth was defined as change in weight z score. We regressed intrapair differences in growth during 4 growth windows (0-1, 1-6, 6-12, and 12-24 mo) against intrapair differences in the risk factors in monozygotic and dizygotic twins separately.

RESULTS

Within monozygotic twin pairs only, associations between infant growth and most adult lipids, glucose, leptin, and blood pressure (eg, systolic blood pressure: b = 5.95 mm Hg per change in z score, P = 0.01 in monozygotic twins; b = -1.64, P = 0.82 in dizygotic twins from 12 to 24 mo) were found. Within dizygotic twin pairs only, associations between growth and triglycerides and fibrinogen (eg, fibrinogen: b = 0.07 ln mg/dL per change in z score, P = 0.31 in monozygotic twins; b = 0.79, P = 0.01 in dizygotic twins from 0 to 1 mo) were identified. Most associations showed a detrimental effect of accelerated growth, but beneficial associations were also identified (eg, total-to-high-density-lipoprotein cholesterol ratio: b = -0.22 per change in z score from 1 to 6 mo, P = 0.008 in monozygotic twins).

CONCLUSION

Our data showed that environmental factors play a role in the associations between infant growth and most adult lipids, glucose, leptin, and blood pressure, whereas genetic factors are involved regarding triglycerides and fibrinogen.

Multimodality imaging of subclinical aortic atherosclerosis: relation of aortic stiffness to calcification and plaque in female twins

Aortic stiffness, an important predictor of cardiovascular events, may relate to aortic calcification rather than noncalcified atherosclerotic plaque. The aim of this study was to determine the relation of aortic stiffness to aortic plaque and aortic calcification in asymptomatic postmenopausal women. One hundred female twins (mean age±standard deviation 64±7 years) underwent computed tomography and magnetic resonance imaging (black-blood sequence) of the aorta. The topographical relation of plaque on magnetic resonance images and calcification on computed tomography images was assessed on magnetic resonance/computed tomography fused images. Carotid-femoral pulse wave velocity was used as a measure of aortic stiffness. Aortic plaque was identified in 87% and calcification in 65% of subjects, both increased with age and were higher in the abdominal compared with thoracic aorta (P<0.0001). Plaque correlated with calcification (R=0.68; P<0.0001), but was also detected in 58% of women who had no calcification. Pulse wave velocity (adjusted for age and blood pressure) increased across quartiles of calcification (P<0.01) but not plaque score (P=0.56). Shared genetic factors accounted for >99% of the correlation (0.35) between PWV and calcification. In conclusion, there is a high prevalence of subclinical atherosclerosis within the aorta in asymptomatic middle-aged women. Aortic stiffening relates to aortic calcification, but not to atherosclerotic plaque burden, and the association of aortic stiffness with calcification is driven by common genes.

The Norwegian Twin Registry from a Public Health Perspective: A Research Update

We describe the importance of the Norwegian Twin Registry (NTR) for research in public health and provide examples from several programs of twin research at the Norwegian Institute of Public Health (NIPH), including the Nordic Twin Study of Cancer, our epigenetics platform, and our large program of research in mental health. The NTR has become an integral component of a national strategy for maximizing the research potential from Norwegian registries and biobank-based studies. The information provided herein builds upon and complements our recent report describing the establishment of the NTR and the cohorts comprising it. Although Norway has a long tradition in twin research, the centralization and administration of the twin data through a single register structure is fairly recent. The NTR was established in 2009 and currently includes 47,989 twins covering birth years 1895–1960 and 1967–1979; 31,440 of these twins have consented to participate in medical research (comprising 5,439 monozygotic pairs, 6,702 dizygotic same-sexed pairs, and 1,655 dizygotic opposite-sexed pairs). DNA from approximately 4,800 twins is banked at the NIPH biobank and new studies continuously add new data to the registry. The value of NTR data is greatly enhanced through record linkage possibilities offered by Norway's many nation-wide registries (medical, demographic, and socio-economic) and several studies are already taking advantage of these linkage opportunities for research.

Lower Marriage and Divorce Rates Among Twins Than Among Singletons in Danish Birth Cohorts 1940–1964

Few studies have examined differences of civil status of twins and singletons and the conclusions are contradictory. In the present study, based on a linkage between the Danish Twin Register, a random 5% sample of the total Danish population, and administrative register databases, the authors compare rates of marriage and divorce in a sample of 35,975 twins and 81,803 singletons born 1940–1964. Cox-regressions are used in order to control for potential confounders. We find that compared with singletons twins have significantly lower marriage rates: (males: 15–19 years: Hazard Ratio (HR) = 0.66 (95%CI: 0.58–0.76); 20–24 years: 0.85 (0.82–0.88); 25 years or more: 0.96 (0.93–0.98) and females: 15–19 years: 0.70 (0.67–0.75); 20–24 years: 0.83 (0.80–0.85); 25 years or more: 0.94 (0.91–0.97)). There is no difference in divorce rates for males, but a significantly lower divorce rate for female twins compared with singletons (HR=0.87, 95%CI: 0.83–0.90). These differences offset each other, thus 57% of both populations remain in their first marriage until censoring. The interpretation may be that since twins have a partner from birth, they do not have the same need for marriage as singletons but have more experience in maintaining a relationship if they do marry.

The Foetal Origins of Adult Disease: Interpreting the Evidence From Twin Studies

Twin studies have a contribution to make to the debate concerning the foetal origins of adult disease. Twins are growth retarded compared to singletons and experience post-natal catch-up growth. However, there is no evidence that twins are at increased risk of cardiovascular disease. Studying whether discordance in size at birth within monozygotic twin pairs is predictive of discordance in later life disease should help resolve whether the association between size at birth and later disease is due to common genetic factors. Results from studies of blood pressure in childhood and adult life looking at these within twin effects are far from conclusive. There are, however, methodological problems in the interpretation of these results, not least of which is the relatively small numbers of twin pairs studied. Studies exploring the effect of zygosity and chorion type on later disease provide may provide a useful extension of the research agenda. In summary, twin studies to date have raised more questions about the foetal origins hypothesis than they have resolved.

Subjective wellbeing and longevity: a co-twin control study

Mental health is increasingly defined not only by the absence of illness but by the presence of subjective well-being (SWB). Previous cohort studies have consistently shown that indicators of SWB predict favorable life outcomes, including better mental and somatic health, and longevity. The favorable effects associated with SWB have prompted new research aimed at raising happiness and wellbeing through individual interventions and public health initiatives. Standard observational studies of individual-level associations, however, are subject to potential confounding of exposure and outcome by shared genes and environment. The present study explored the association between SWB and increased longevity, using twin pair analyses to determine whether the association is consistent with causality or is due to genetic or environmental confounding. The study sample of 3,966 twins aged 70 or older, followed for a median time period of 9 years, was drawn from the population-based Longitudinal Study of Aging Danish Twins (LSADT). The association between SWB, operationalized as affect and life satisfaction, and all-cause mortality risk was examined using between-individual and within-pair survival analyses. As expected, at the individual level, SWB predicted increased longevity. Exposure effects were also present in unadjusted and adjusted within-pair analyses of 400 dizygotic (DZ) pairs and 274 monozygotic (MZ) pairs, indicating that SWB is associated with increased longevity independent of familial factors of genes and shared environment.

Changes in genetic and environmental effects on growth during infancy

BACKGROUND

Accelerated infant growth is a possible explanation for the relation between birth weight and adult diseases.

OBJECTIVE

The aim of this study was to estimate the heritability of infant growth and to examine whether the genetic contribution changes with increasing or decreasing birth weight and gestational age.

DESIGN

Growth (change in weight z score) was analyzed in 522 infants from the East Flanders Prospective Twin Survey for age windows of 0-1, 1-6, 6-12, and 12-24 mo. Structural equation modeling was performed to estimate the relative importance of additive genetic, shared environmental, and unique environmental sources of variance.

RESULTS

We showed no genetic contribution to growth in the 0-1-mo growth period. However, at later ages, the heritability of growth was high at 94% (95% CI: 90%, 96%) from 1 to 6 mo, 85% (95% CI: 80%, 89%) from 6 to 12 mo, and 86% (95% CI: 77%, 91%) in the 12-24-mo growth period. Nevertheless, in the last age window, a model without genetic factors was also statistically plausible. From 0 to 1 mo, the genetic contribution to growth was low in the average birth weight range but higher at both extremes of birth weight. The genetic contribution from 0 to 1 mo increased with increasing gestational age from 36 wk of gestation onward.

CONCLUSIONS

This study shows that genetic factors are not important in early infant growth (0-1 mo), whereas heritability is high after 1 mo. Because many (nutritional) interventions are aimed at influencing early postnatal growth, to target long-term health, these interventions may be most successful if implemented in the first month of postnatal growth.

Register-based research on twins

INTRODUCTION

The Danish Twin Registry (DTR) has for more than 50 years been based on surveys and clinical investigations and over the two last decades also on register linkage. Currently these two approaches are merged within Statistics Denmark.

RESEARCH TOPICS

Here we report on three major groups of register-based research in the DTR that used the uniqueness of twinning. First, we focus on the ''long-term prognosis'' of being a twin compared with being a singleton and show that Danish twins have health trajectories in adulthood similar to singletons, which is a result of interest for twins and their families as well as a test of the fetal origins hypothesis that states that fetal growth restriction has long-term health consequences. Secondly, we summarise some of the most important register-based ''classical twin studies'', e.g. heritability studies on lifespan and exceptional longevity. Finally, we illustrate how the co-twin control method in a register setting can be used to control for the effect of rearing environment and genetic factors in studies of the association between exposures and health.

CONCLUSION

The spectrum of register-based twin studies is very wide and have changed in accordance with methodological and data resource developments.

Birth weight predicts risk of cardiovascular disease within dizygotic but not monozygotic twin pairs: a large population-based co-twin-control study

BACKGROUND

The widely reported inverse association between birth weight and risk of cardiovascular disease (CVD) has sparked theories about early life determinants of adult disease. Within-twin-pair analysis provides a unique opportunity to investigate whether factors shared within twin pairs influence the association.

METHODS AND RESULTS

In a population-based cohort of like-sexed twins with known zygosity born in Sweden from 1926 to 1958, disease-discordant twin pairs were identified through linkage to the National Inpatient and Cause of Death registers between 1973 and 2006. Co-twin-control analyses were performed on twins discordant for cardiovascular disease (n=3884), coronary heart disease (n=2668), and stroke (n=1372). Overall, inverse associations between birth weight and risk of cardiovascular diseases were seen within dizygotic but not monozygotic twin pairs. In dizygotic twins, the odds ratios for a 1-kg within-pair increase in birth weight were 0.74 (95% confidence interval, 0.56 to 0.98) for coronary heart disease and 0.57 (95% confidence interval, 0.37 to 0.88) for stroke. Conversely, no statistically significant associations were found within monozygotic twins (for coronary heart disease: odds ratio, 1.10; 95% confidence interval, 0.73 to 1.68; for stroke: odds ratio, 0.92; 95% confidence interval, 0.48 to 1.80).

CONCLUSIONS

We found an association between birth weight and risk of cardiovascular disease within disease-discordant dizygotic but not monozygotic twin pairs. This indicates that the association between birth weight and cardiovascular disease could be a result of common causes, and that factors that vary within dizygotic but not monozygotic twin pairs may help identify them.

Being Born Under Adverse Economic Conditions Leads to a Higher Cardiovascular Mortality Rate Later in Life: Evidence Based on Individuals Born at Different Stages of the Business Cycle

We connect the recent medical and economic literatures on the long-run effects of early-life conditions by analyzing the effects of economic conditions on the individual cardiovascular (CV) mortality rate later in life, using individual data records from the Danish Twin Registry covering births since the 1870s and including the cause of death. To capture exogenous variation of conditions early in life, we use the state of the business cycle around birth. We find significant negative effects of economic conditions around birth on the individual CV mortality rate at higher ages. There is no effect on the cancer-specific mortality rate. From variation within and between monozygotic and dizygotic twin pairs born under different conditions, we conclude that the fate of an individual is more strongly determined by genetic and household-environmental factors if early-life conditions are poor. Individual-specific qualities come more to fruition if the starting position in life is better.

Determinants of infant growth in four age windows: a twin study

OBJECTIVE

To identify determinants of growth during infancy.

STUDY DESIGN

The sample included 424 twin pairs from the East Flanders Prospective Twin Survey. Multilevel regression analysis was performed and intrapair growth correlations were calculated. The main outcome measure was growth, measured in g/kg/d (0-1 month) or in change in weight z-score (0-6, 6-12 and 12-24 months).

RESULTS

Growth during infancy was associated with birth weight and gestational age. One z-score increase in birth weight resulted in -1.77 g/kg/d less growth from 0-1 month (P < .0001). The effect size decreased with age until -0.02 (P = .70) z-scores less growth from 12 to 24 months. Corresponding numbers for one z-score increase in gestational age decreased from 0.78 (P = .001) to 0.06 (P = .40). From 12 to 24 months, paternal height had a significant positive effect. The difference in growth similarity within the twin pair between monozygotic and dizygotic twins increased from non-significant from 0 to 1 month (P = .49) to a monozygotic:dizygotic ratio approximating 2:1 from 12 to 24 months (P = .002).

CONCLUSION

From 0 to 1 month, environmental factors are most important for growth, whereas genetic factors become more important over time. This is a first step in identifying age windows for future counseling and interventions on the effects of accelerated growth.

Cross-national comparison of sex differences in health and mortality in Denmark, Japan and the US

The present study aims to compare the direction and magnitude of sex differences in mortality and major health dimensions across Denmark, Japan and the US. The Human Mortality Database was used to examine sex differences in age-specific mortality rates. The Danish twin surveys, the Danish 1905-Cohort Study, the Health and Retirement Study, and the Nihon University Japanese Longitudinal Study of Aging were used to examine sex differences in health. Men had consistently higher mortality rates at all ages in all three countries, but they also had a substantial advantage in handgrip strength compared with the same-aged women. Sex differences in activities of daily living (ADL) became pronounced among individuals aged 85+ in all three countries. Depression levels tended to be higher in women, particularly, in Denmark and the HRS, and only small sex differences were observed in the immediate recall test and Mini-Mental State Exam. The present study revealed consistent sex differentials in survival and physical health, self-rated health and cognition at older ages, whereas the pattern of sex differences in depressive symptoms was country-specific.

Shift-work and cardiovascular disease: a population-based 22-year follow-up study

Studies on the association between shift-work and cardiovascular disease (CVD), in particular coronary heart disease (CHD), have given conflicting results. In this prospective population-based study we assessed the association of shift-work with three endpoints: CHD mortality, disability retirement due to CVD, and incident hypertension. A cohort of 20,142 adults (the Finnish Twin Cohort) was followed from 1982 to 2003. Type of working time (daytime/nighttime/shift-work) was assessed by questionnaires in 1975 (response rate 89%) and in 1981 (84%). Causes of death, information on disability retirement and hypertension medication were obtained from nationwide official registers. Cox proportional hazard models were used to obtain hazard ratios (HR) for each endpoint by type of working time. Adjustments were made for 14 socio-demographic and lifestyle covariates. 76.9% were daytime workers and 9.5% shift-workers both in 1975 and in 1981. During the follow-up, 857 deaths due to CHD, 721 disability retirements due to CVD, and 2,642 new cases of medicated hypertension were observed. However, HRs for shift-work were not significant (mortality HR men 1.09 and women 1.22; retirement 1.15 and 0.96; hypertension 1.15 and 0.98, respectively). The results were essentially similar after full adjustments for all covariates. Within twin pairs, no association between shift work and outcome was observed. Our results do not support an association between shift-work and cardiovascular morbidity.

Developmental programming and hypertension

PURPOSE OF REVIEW

There is a growing body of evidence linking adverse events or exposures during early life and adult-onset diseases. After important epidemiological studies from many parts of the world, research now focuses on mechanisms of organ dysfunction and on refining the understanding of the interaction between common elements of adverse perinatal conditions, such as nutrition, oxidants, and toxins exposures. This review will focus on advances in our comprehension of developmental programming of hypertension.

RECENT FINDINGS

Recent studies have unraveled important mechanisms of oligonephronia and impaired renal function, altered vascular function and structure as well as sympathetic regulation of the cardiovascular system. Furthermore, interactions between prenatal insults and postnatal conditions are the subject of intensive research. Prematurity vs. intrauterine growth restriction modulate differently programming of high blood pressure. Along with antenatal exposure to glucocorticoids and imbalanced nutrition, a critical role for perinatal oxidative stress is emerging.

SUMMARY

While the complexity of the interactions between antenatal and postnatal influences on adult blood pressure is increasingly recognized, the importance of postnatal life in (positively) modulating developmental programming offers the hope of a critical window of opportunity to reverse programming and prevent or reduce related adult-onset diseases.

Familial (shared environmental and genetic) factors and the foetal origins of cardiovascular diseases and type 2 diabetes: a review of the literature

Several researchers have argued that observed associations between birth weight and cardiovascular diseases, and type 2 diabetes, may be confounded by familial (shared environmental and genetic) factors. However, most studies have found that shared environmental factors, including socio-economic factors, do not influence the foetal origins of adult diseases. Results from two twin studies suggest that genetic factors may be of importance for the association between birth weight and risks of coronary heart disease, but findings from intergenerational studies are not consistent with genetic confounding. More studies have assessed the importance of genetic factors with respect to risk factors of coronary heart, including raised blood pressure and lipid levels. Recent findings suggest that the association between birth weight and hypertension is independent of genetic factors. In contrast, recent twin and intergenerational studies favour the hypothesis that the association between birth weight and risk of type 2 diabetes is confounded by genetic factors.

Birth weight is forever

Birth weight is associated not just with infant morbidity and mortality, but with outcomes occurring much later in life, including adult mortality, as reported by a paper by Baker and colleagues in this issue of Epidemiology. While these associations are tantalizing per se, the truly interesting question concerns the mechanisms that underlie these links. The prevailing hypothesis suggests a "fetal origin" of diseases resulting from alterations in fetal nutrition that permanently program organ function. The most commonly proposed alternative is that factors, mainly genetic, that affect both fetal growth and disease risk are responsible for the observed associations. Although both mechanisms are intellectually attractive-and may well coexist-we should be cautious to not focus excessively on fetal growth. Doing this may lead us in the wrong direction, as has likely happened in the case of birth weight in relation to infant survival.

Genetic and Shared Environmental Factors Do Not Confound the Association Between Birth Weight and Hypertension

BACKGROUND

Studies have found associations between low birth weight and increased risks of cardiovascular diseases in adulthood. However, these associations could be due to confounding by genetic or socioeconomic factors.

METHODS AND RESULTS

We performed a study on Swedish like-sexed twins with known zygosity who were born from 1926 to 1958. First, to obtain an overall effect of birth weight on risk of hypertension, we performed cohort analyses on all twins (n=16,265). Second, to address genetic and shared environmental confounding, we performed a nested co-twin control analysis within 594 dizygotic and 250 monozygotic twin pairs discordant for hypertension. Birth characteristics, including birth weight, were obtained from original birth records. Information from adulthood was collected from a postal questionnaire in 1973 (body mass index, height, smoking, and alcohol use) and from a telephone interview conducted from 1998 to 2002 (hypertension and socioeconomic status). Hypertension was defined as reporting both high blood pressure and treatment with antihypertensive medication. In the cohort analysis, the adjusted odds ratio for hypertension in relation to a 500-g decrease in birth weight was 1.42 (95% confidence interval, 1.25 to 1.61). In the co-twin control analyses, the corresponding odds ratios were 1.34 (95% confidence interval, 1.07 to 1.69) for dizygotic and 1.74 (95% confidence interval, 1.13 to 2.70) for monozygotic twins.

CONCLUSIONS

In the largest twin study on the fetal origins of hypertension, we found that decreased birth weight is associated with increased risk of hypertension independently of genetic factors, shared familial environment, and risk factors for hypertension in adulthood, including body mass index.

Self-rated health and age: a cross-sectional and longitudinal study of 11,000 Danes aged 45-102

AIMS

The aim of this study was to explore and describe self-rated health in middle-aged and elderly Danes using both a cross-sectional and a longitudinal design. Global and (age) comparative self-rated health are examined and compared.

METHODS

This study is interview based and comprises data on 11,294 Danes aged 45-102 with more than 1,900 participants aged 90 years and older.

RESULTS

As expected, global self-rated health declines with age in both cross-sectional and longitudinal analyses. In contrast, comparative self-rated health either increases or remains stable with age in cross-sectional analyses while in longitudinal analyses there is a slight decline in comparative self-rated health.

CONCLUSIONS

The age-trajectory of global self-rated health is similar in individuals and populations. For comparative self-rated health, however, the individual on average experiences a slight decline, whereas on the population level comparative self-rated health either increases or remains stable. The explanation for this is likely to be higher mortality and higher non-response among the participants with the poorest self-rated health.

Maternal undernutrition during early to mid-gestation in the ewe results in altered growth, adiposity, and glucose tolerance in male offspring

This study utilized maternal undernutrition from early to midgestation in the ewe to determine the impact(s) of intrauterine growth restriction on postpartum growth of male offspring and the potential mechanisms involved. Multiparous ewes were fed 50% (nutrient-restricted) or 100% (control-fed) of their nutrient requirements (NRC, 1985) between d 28 and 78 of gestation, and then all ewes were fed 100% of the NRC requirements from d 79 through lambing. Male lambs born to nutrient-restricted (n = 9) and control-fed (n = 9) ewes exhibited similar BW (5.8 vs. 6.0 +/- 0.3 kg) and crown-rump lengths (53.8 vs. 55.4 +/- 1.0 cm) at birth. At 63 and 250 d of postnatal age, wether lambs were subjected to a glucose tolerance test, in which a bolus of glucose was administered i.v. to evaluate changes in glucose and insulin concentrations. After i.v. glucose administration at 63 d of age, lambs from nutrient-restricted ewes exhibited a greater area under the curve for glucose (AUCg; 6,281 vs. 5,242 +/- 429; P < 0.05) and insulin (AUCi; 21.0 vs. 8.6 +/- 1.9; P < 0.001) than lambs from control-fed ewes. After glucose administration at 250 d of age, lambs from nutrient-restricted ewes had greater AUCg (7,147 vs. 5,823 +/- 361; P < 0.01) but a lower AUCi (6.4 vs. 10.2 +/- 1.9; P = 0.05) than lambs from control-fed ewes. Lambs from nutrient-restricted ewes were heavier (26.6 vs. 21.8 +/- 2.3 kg; P < 0.05) and had more backfat (0.30 vs. 0.21 +/- 0.03 cm, P < 0.05) by 4 mo of age than the lambs from control-fed ewes. At slaughter at 280 d of age, lambs from nutrient-restricted ewes remained heavier than lambs from control-fed ewes, had greater (P < 0.05) amounts of kidney and pelvic-area adipose tissue, and tended (P < 0.10) to have reduced LM and semitendinosus muscle weights as a percentage of HCW. These data demonstrate that a bout of maternal undernutrition during early to midgestation in sheep increased BW and fat deposition during adolescence and dysregulated glucose uptake in the absence of any change in birth weight.

Effects of twinning, birth size, and postnatal growth on glucose tolerance and hypothalamic-pituitary-adrenal function in postpubertal sheep

Low birth weight is associated with postnatal physiological changes, including impaired glucose tolerance and increased cortisol secretion, that may predispose to disease in adulthood. Twins are born lighter than singletons, but there are conflicting data regarding the association between birth weight and postnatal physiology in twins. We studied glucose tolerance and ACTH and cortisol responses to a combined corticotropin-releasing hormone and arginine vasopressin (CRH + AVP) challenge in postpubertal female twin (n = 7 twin pairs) and singleton (n = 13) sheep from the same flock. There were no differences in glucose tolerance between twins and singletons and no association with birth weight. Twins had a greater ACTH (P < 0.05), but not cortisol, response to CRH + AVP than singletons. ACTH area under the curve was inversely related to birth weight in both singletons [R(2) = 0.31, P = 0.05; -8,311 (SD 3,736) pg.min.ml(-1).kg(-1)] and twins (R(2) = 0.49); in twins, this was due to the within-twin pair rather than the between-twin pair coefficient in the regression analysis [P = 0.02, -26,856 (9,806) vs. P = 0.1, 8,619 (4,950) pg.min.ml(-1).kg(-1)]. We conclude that the reduced fetal growth in twins has postnatal consequences for hypothalamic-pituitary-adrenal function and that this is determined by factors specific to the fetus (within-twin pair) rather than by shared maternal factors (between-twin pair). Studies investigating the associations between fetal growth and postnatal outcomes in twins benefit from an appropriate singleton control group and from analyses evaluating the contribution from both between- and within-pair coefficients in twins.

Association between height and coronary heart disease mortality: a prospective study of 35,000 twin pairs

An inverse association between height and risk of coronary heart disease (CHD) is well demonstrated, but it is not known whether this association is because of genetic factors, socioeconomic background, or other environmental factors. Four population-based twin cohorts with register-based follow-up data on CHD mortality from Denmark (1966-1996), Finland (1975-2001), and Sweden (1963-2001 and 1972-2001) were used to investigate this question; response rates varied between 65% and 86%. Together, the cohorts included 74,704 twin individuals (35,042 complete twin pairs) with 5,943 CHD deaths during 1.99 million person-years of follow-up. Cox and conditional logistic regression models were used. Per 1-standard deviation decrease in height, height was inversely associated with CHD mortality in men (hazard ratio = 1.08, 95% confidence interval (CI): 1.04, 1.12) and in women (hazard ratio = 1.06, 95% CI: 1.01, 1.10). A twin who had died from CHD was on average shorter than the co-twin within monozygotic pairs (odds ratio = 1.27, 95% CI: 1.12, 1.44, with no sex difference), whereas a weaker association was found within dizygotic pairs in men (odds ratio = 1.01, 95% CI: 0.91, 1.13) and in women (odds ratio = 1.14, 95% CI: 1.01, 1.28). The inverse association between height and CHD mortality found within monozygotic discordant twin pairs suggests that this association is because of environmental factors that directly affect height and CHD risk.

Genetic influence on human lifespan and longevity

There is an intense search for longevity genes in both animal models and humans. Human family studies have indicated that a modest amount of the overall variation in adult lifespan (approximately 20-30%) is accounted for by genetic factors. But it is not known if genetic factors become increasingly important for survival at the oldest ages. We study the genetic influence on human lifespan and how it varies with age using the almost extinct cohorts of Danish, Finnish and Swedish twins born between 1870 and 1910 comprising 20,502 individuals followed until 2003-2004. We first estimate mean lifespan of twins by lifespan of co-twin and then turn to the relative recurrence risk of surviving to a given age. Mean lifespan for male monozygotic (MZ) twins increases 0.39 [95% CI (0.28, 0.50)] years for every year his co-twin survives past age 60 years. This rate is significantly greater than the rate of 0.21 (0.11, 0.30) for dizygotic (DZ) males. Females and males have similar rates and these are negligible before age 60 for both MZ and DZ pairs. We moreover find that having a co-twin surviving to old ages substantially and significantly increases the chance of reaching the same old age and this chance is higher for MZ than for DZ twins. The relative recurrence risk of reaching age 92 is 4.8 (2.2, 7.5) for MZ males, which is significantly greater than the 1.8 (0.10, 3.4) for DZ males. The patterns for females and males are very similar, but with a shift of the female pattern with age that corresponds to the better female survival. Similar results arise when considering only those Nordic twins that survived past 75 years of age. The present large population based study shows genetic influence on human lifespan. While the estimated overall strength of genetic influence is compatible with previous studies, we find that genetic influences on lifespan are minimal prior to age 60 but increase thereafter. These findings provide a support for the search for genes affecting longevity in humans, especially at advanced ages.

Birth Characteristics and Risk of High Systolic Blood Pressure in Early Adulthood

BACKGROUND

Although an inverse association between size at birth and blood pressure has been found in several studies, few studies have adjusted for the influence of socioeconomic and familial effects.

METHODS

We investigated whether the association between birth weight and systolic blood pressure in young men is confounded by socioeconomic factors in adolescence or familial factors (ie, common genes and shared environment). Our population-based cohort study comprised 330,768 Swedish men born between 1973 and 1981, and conscripted for military service between 1991 and 2000. The analyses of family effects were restricted to 89,856 siblings from the initial cohort. A high systolic blood pressure at conscription was defined as a systolic blood pressure >/=140 mm Hg. Birth weight for gestational age <-2 standard deviation scores was considered "light for gestational age."

RESULTS

Compared with men who had normal birth weight for gestational age, men who had been born light for gestational age were at increased risk of high systolic blood pressure (odds ratio = 1.14; 95% confidence interval = 1.07-1.22), even after adjustment for important confounders such as socioeconomic status. The increase in risk of high systolic blood pressure related to 1 standard deviation score decrease in birth weight for gestational age was similar within families (1.08; 1.04-1.12) and between families (1.05; 1.03-1.08).

CONCLUSIONS

This study suggests that low birth weight for gestational age slightly increases the risk of high systolic blood pressure, and that the association appears not to be confounded by socioeconomic or familial effects.

Programming of glucose-insulin metabolism in adult sheep after maternal undernutrition

The present study examines the effects of late vs. early gestation undernutrition on adult glucose-insulin homeostasis in sheep and investigates whether the lower birth weight of twins alters glucose-insulin handling in adult life. Pregnant sheep were fed to requirement (100% intake) from day 0 of gestation to term [ approximately 147 days of gestation (dGA), control singles (CS) n = 5; control twins (CT) n = 5] or to 50% requirement from days 0-30 dGA [nutrient restricted during early gestation (NRE); n = 5] or day 110-term [NR during late nutrition (NRL); n = 4]. At all other times, NR sheep received 100% intake. All sheep lambed naturally; offspring were weaned at 10 wk and were reared on pasture until 1 yr of age. At this time, indwelling catheters were inserted, and 2-4 days later, basal metabolic and endocrine status and responses to an intravenous glucose tolerance test (IVGTT) and feeding were assessed. Adipose and skeletal muscle were then sampled after humane euthanasia and were analyzed for expression of insulin-signaling proteins and GLUT4. Between groups, birth weight of singletons was similar and increased relative to twins. At 1 yr of age, weights were similar between groups. The areas under the curve for glucose and insulin during the IVGTT were greater in NRL vs. other groups, indicating glucose intolerance. This was associated with reduced adipose, but not muscle, GLUT4, and increased adipose tissue mass. Adult glucose-insulin homeostasis in sheep was unaffected by fetal number. In conclusion, prenatal undernutrition, specifically during late gestation, affects adult offspring intermediary metabolism, and, in particular, glucose-insulin homeostasis.

Embryonic and fetal programming of physiological disorders in adulthood

In the past decade, data from numerous epidemiological studies have indicated strong inverse associations between birth weight and risk of coronary heart disease, hypertension, type 2-diabetes, and other diseases in adulthood. The "Barker hypothesis" thus postulates that a number of organ structures and functions undergo programming during embryonic and fetal life. This developmental programming determines the set points of physiological and metabolic responses in adult life. Alterations of nutrient availability during gestation may lead to developmental adaptations, via hormonal maneuvers by the embryo and fetus that readjust these set points. These adaptive measures have short-term benefits to the embryo and fetus, so that the newborn will be better prepared for the adverse environment (e.g., undernutrition). However, adequate nutritional support during postnatal life that enables catch-up growth may create metabolic conflicts that predispose the adult to aberrant physiological functions and, ultimately, increased risk of disease. It is plausible that other adverse in utero conditions, including exposure to developmental toxicants, may similarly alter adult disease susceptibility. This article provides an overview of the Barker hypothesis, its supporting evidence, the current advances in understanding the biological mechanisms underlying this phenomenon, and its implications for developmental toxicology.

Fetal programming: the perspective of single and twin pregnancies

Multiple pregnancy is associated with increased risk of adverse consequences for both mother and fetus(es), including increased rates of maternal hypertension and pre-eclampsia, spontaneous abortion, Caesarean delivery, low birthweight, birth prematurity, perinatal mortality, admission to neonatal intensive care and extended length of care, respiratory distress, cerebral palsy, developmental delay, contact with disability services and mortality to age 5 years. Premature birth, which affects 97% of triplets and 53.3% of twins in Australia, is not the sole factor involved. The rate of multiple pregnancy in Australia is 1.7%. This compares to 22.1% for pregnancies resulting from assisted reproduction technology (ART). As a result, 21.8% of babies born from ART are from a multiple pregnancy, in comparison to the USA where the majority of babies born from ART are from a multiple pregnancy. Additionally, the population rate of multiple births is rising due to the more frequent use of ART and continued multi-embryo transfers, which is operating against a background of rising implantation rates within ART clinics. Twins have been of interest from a programming perspective. However, analysis of associations between crude birthweight and subsequent metabolic risk factors or mortality in adulthood from chronic disease indicate that adaptations in pregnancy to support multi-fetal growth are not identical to fetal growth restriction in singleton pregnancies. Indeed, the process of ‘maternal constraint’ is incompletely understood and confounds such comparisons. From a programming perspective, it is a challenge to identify in twin pregnancies the transition from physiological adaptation to pathological growth restriction. Growth disparity between twins has been more illuminating of subtle adverse effects for the smaller of twin pairs in both blood pressure and insulin resistance in adulthood. Interestingly, these effects can be observed in both dizygotic and to a lesser degree in monozygotic twins, which indicates a role for both genetic and environmental factors in these measures. This suggests that, consistent with experimental studies in other species, the relationship between impaired growth in utero and chronic disease in later life is not simply mediated by a common genetic pathway.