Overweight and obesity are not homogeneous phenotypes as individuals differ in terms of the regional distribution of the excess weight or fat. Overweight and obesity are also complex multifactorial phenotypes influenced by both genetic and nongenetic determinants. Heritability of fat mass or percent body fat derived from underwater weighing measurement reaches about 25% of the age and gender adjusted phenotypic variance. Based on twin and parent-child data, it has been reported that the heritability of resting metabolic rate, thermic response to food, and energy cost of submaximal exercise, adjusted for the proper concomitants, is as high as 40%. The level of habitual physical activity also exhibits a significant heritability level on the order of about 25%. Experimental overfeeding with identical twins demonstrates that there are inherited differences in body weight and body composition response. The most important factor identified thus far to account for the individual differences in response to long-term overfeeding is a nutrient partitioning characteristic, namely the proportion of fat vs lean tissue gained. The high gainers are those storing energy primarily in the form of fat, while the low gainers are storing relatively more in the form of lean tissue.
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