Current understanding of the etiology of obesity: genetic and nongenetic factors.

Four types of human obesities are defined by topography of fat deposition. The focus of the paper is on the genetic determinants and the nongenetic correlates of the first type, which is commonly defined as excess weight-for-height or excess body fat without a particular concentration of fat in a given area of the body. The heritability of fat mass or percent body fat derived from underwater-weighing assessment of body density reaches approximately 25% of the age- and gender-adjusted phenotypic variance. The within-identical-twin-pair resemblance in the response to a standardized overfeeding protocol emphasizes the importance of the genotype in determining individual differences in body weight and body composition. In this regard, the proneness to store energy primarily as fat or as lean tissue is a major determinant of the response to a caloric surplus.