The influence of birth weight and genetic factors on lipid levels: a study in adult twins

Developmental origins of cardiometabolic health outcomes in twins: A systematic review and meta-analysis

BACKGROUND AND AIMS

Studies of twins can reduce confounding and provide additional evidence about the causes of disease, due to within-pair matching for measured and unmeasured factors. Although findings from twin studies are typically applicable to the general population, few studies have taken full advantage of the twin design to explore the developmental origins of cardiometabolic health outcomes. We aimed to systematically review the evidence from twin studies and generate pooled estimates for the effects of early-life risk factors on later-life cardiometabolic health.

METHODS AND RESULTS

An initial search was conducted in March 2018, with 55 studies of twins included in the review. Risk of bias was assessed using the Newcastle-Ottawa Scale, and eligible studies were included in a meta-analysis, where pooled estimates were calculated. Twenty-six studies analysed twins as individuals, and found that higher birthweight was associated with lower SBP (β = -2.02 mmHg, 95%CI: -3.07, -0.97), higher BMI (β = 0.52 kg/m², 95%CI: 0.20, 0.84) and lower total cholesterol (β = -0.07 mmol/L, 95%CI: -0.11, -0.04). However, no associations were reported in studies which adjusted for gestational age. Few of the included studies separated their analyses into within-pair and between-pair associations.

CONCLUSIONS

Early-life risk factors were associated with cardiometabolic health outcomes in twin studies. However, many estimates from studies in this review were likely to have been confounded by gestational age, and few fully exploited the twin design to assess the developmental origins of cardiometabolic health outcomes.

No evidence of a higher 10 year period prevalence of diabetes among 77,885 twins compared with 215,264 singletons from the Danish birth cohorts 1910-1989

AIMS/HYPOTHESIS

Previous Danish twin studies have found a highly increased risk of precursors of type 2 diabetes as well as a higher prevalence of type 2 diabetes among twins compared with singletons. Likewise, small-scale studies of Danish twins have shown that monozygotic twins have a higher risk of developing precursors of type 2 diabetes compared with dizygotic twins. In the present register-based study, the 10 year period diabetes prevalence in Danish twins is compared with that in a random sample of Danish citizens. Furthermore, the 10 year period prevalence of diabetes in monozygotic twins is compared with that in dizygotic twins.

METHODS

The study population consisted of twins (n = 77,885) identified in the Danish Twin Registry, and a 5% random sample (n = 215,264) from the birth cohorts 1910-1989. We identified diabetes patients by means of three nationwide Danish health registers.

RESULTS

The number of identified diabetes cases among males was 6,677 (6.24%) for singletons vs 2,271 (5.68%) for twins (difference = 0.56% [0.29-0.83%]). The number among females was 6,143 (5.67%) for singletons and 1,722 (4.54%) for twins (difference = 1.13% [0.88-0.38%]). Restriction to various birth cohorts, known zygosity and known type 2 diabetes did not alter the overall conclusions. The difference between monozygotic twins (males, 5.29%; females, 4.40%) and dizygotic twins (males, 5.77%; females, 4.63%) was non-significant.

CONCLUSIONS/INTERPRETATION

Danish twins do not have an increased risk of developing diabetes compared with singletons, and the risk of diabetes among monozygotic twins does not differ from that of dizygotic twins.

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.

Developmental plasticity and human disease: research directions

The conceptual basis of the 'developmental origins' paradigm has converged on the role of developmental plasticity responding to signals from the early environment, with heightened risk of disease if the induced phenotype does not match the later environment. Nevertheless, numerous questions remain, including the current burden of disease that can be attributed to early environmental factors; the pathways, mechanisms and windows of plasticity; the identification of early markers of environmentally induced change; and the feasibility, costs and benefits of intervention. A focused agenda of research is needed to convince policy makers of the importance of developmental factors in human disease.