Читать книгу Clinical Obesity in Adults and Children - Группа авторов - Страница 72

Traditionally the most favored model for separation of the genetic component of variance is based on studies of twins, as monozygotic co‐twins share 100% of their genes and dizygotes 50% on average. Heritability estimates the proportion of the total variance attributable to genetic variation under a polygenic model by comparing the similarity of a trait within monozygotic twins with the similarity within dizygotic twins. Heritability is a function of both the number of genes influencing a phenotype and the proportion of phenotypic variation accounted for by each of these genes. The advantage of studies of the heritability of BMI is that age‐dependent influences of genes or environmental factors are the same for both twins. Genetic contribution to the BMI has been estimated to be 64–84% [5].

The study of monozygotic twins reared apart has all the advantages of a twin study but does not rely on the equal environmental exposure assumption. Correlation of monozygotic twins reared apart is virtually a direct estimate of the heritability, although monozygotic twins do share the intrauterine environment, which may contribute to lasting differences in body mass in later life. Estimates vary from 40 to 70%, depending on the age at the separation of twins and the length of follow‐up. Longitudinal data from Virginia looking at adult twins and their offspring have reported a heritability of 69% [6]. Studies of Swedish twins have suggested a heritability of 0.70 for men and 0.66 for women [7], whilst a heritability of 0.61 was observed in a cohort of UK twins [8]. In a meta‐analysis of results derived from Finnish, Japanese and American archival twins, Allison observed similar correlations [9]. In addition, Price and Gottesman have shown that these correlations did not differ significantly between twins reared apart and twins reared together, and between twins reared apart in relatively more similar (i.e. with relatives) versus less similar environments [8].

Familial resemblance in food intake has been reported in parents and their children [10], although the extent to which this is genetically determined is unclear. Twin data suggest that there are notable genetic influences on overall food intake, size and frequency of meals. Bouchard and Tremblay have shown that about 40% of the variance in resting metabolic rate, thermic effect of food, and energy cost of low to moderate‐intensity exercise may be explained by inherited characteristics [11]. In addition, significant familial resemblance for level of habitual physical activity has been reported in a large cohort of healthy female twins [12].