Age-dependent nongenetic influences of birth weight and adult body fat on insulin sensitivity in twins.

OBJECTIVE: We hypothesized a nongenetic influence of birth weight (BW) and twin and zygosity status on dual-energy x-ray absorptiometry determined adult total and regional body composition and a quantitative equal, although independent, importance of adult body composition and BW for insulin sensitivity. DESIGN AND METHODS: This was a clinical study of 110 young (aged 22-31 yr) and 86 elderly twins (aged 57-66 yr) and 32 age-matched controls with measures of BW, adult body composition, and insulin sensitivity. RESULTS: BW was nongenetically, positively associated with height, weight, and total and regional (trunk, arm, and leg) fat mass and percentages in the young twins. In the elderly twins, BW was persistently nongenetically related to height as well as leg fat mass and percentage. Insulin sensitivity was nongenetically influenced by total fat percentage as well as arm and trunk fat percentages, all leading to a decrease in insulin sensitivity by 20% per sd increase in the elderly twins. Conversely, a sd increase in BW led to an increase of 16% in insulin sensitivity. Total body fat and regional fat was similarly associated with insulin sensitivity in the younger twins, whereas no influence of BW was seen. CONCLUSION: BW was nongenetically, positively associated with adult height and total and regional fat mass in an age-dependent manner. BW and total and regional body fat percentages were nongenetically associated with insulin sensitivity in the elderly subjects, supporting an influence of the fetal environment on insulin sensitivity that is quantitatively similar to and independent of the effect of adult adiposity.

Total and regional fat distribution is strongly influenced by genetic factors in young and elderly twins.

OBJECTIVE: Indirect estimates of obesity such as BMI seem to be strongly influenced by genetic factors in twins. Precise measurements of total and regional fat as determined by direct techniques such as DXA scan have only been applied in a few twin studies. The aim of the present study was to estimate the heritability (h(2)) of total and regional fat distribution in young and elderly Danish twins. RESEARCH METHODS AND PROCEDURES: Monozygotic (108) and dizygotic (88) twins in two age groups (25 to 32 and 58 to 66 years) underwent anthropometric measurements and DXA scans. Intraclass correlations and etiologic components of variance were estimated for total and regional fat percentages using biometric modeling. RESULTS: The intraclass correlations demonstrated higher correlations for all fat percentages among monozygotic twins as compared with dizygotic twins. The biometric modeling revealed a major genetic component (h(2)) of total (h(2)(young) = 0.83, h(2)(elderly) = 0.86) and regional fat percentages (trunk, h(2)(young) = 0.82, h(2)(elderly) = 0.85; lower body, h(2)(young) = 0.83, h(2)(elderly) = 0.81; and trunk/lower body, h(2)(young) = 0.83, h(2)(elderly) = 0.71) in both the young and elderly twins. DISCUSSION: The h(2) estimates emphasize that body fat and distribution as determined by DXA scan are under extensive genetic control.