THE RESPONSE TO LONG-TERM OVERFEEDING IN IDENTICAL-TWINS

We undertook this study to determine whether there are differences in the responses of different persons to long-term overfeeding and to assess the possibility that genotypes are involved in such differences. After a two-week base-line period, 12 pairs of young adult male monozygotic twins were overfed by 4.2 MJ (1000 kcal) per day, 6 days a week, for a total of 84 days during a 100-day period. The total excess amount each man consumed was 353 MJ (84,000 kcal). During overfeeding, individual changes in body composition and topography of fat deposition varied considerably. The mean weight gain was 8.1 kg, but the range was 4.3 to 13.3 kg. The similarity within each pair in the response to overfeeding was significant (P<0.05) with respect to body weight, percentage of fat, fat mass, and estimated subcutaneous fat, with about three times more variance among pairs than within pairs (r ≈ 0.5). After adjustment for the gains in fat mass, the within-pair similarity was particularly evident with respect to the changes in regional fat distribution and amount of abdominal visceral fat (P<0.01), with about six times as much variance among pairs as within pairs (r ≈ 0.7). We conclude that the most likely explanation for the intrapair similarity in the adaptation to long-term overfeeding and for the variations in weight gain and fat distribution among the pairs of twins is that genetic factors are involved. These may govern the tendency to store energy as either fat or lean tissue and the various determinants of the resting expenditure of energy. (N Engl J Med 1990; 322:1477–82.) THERE are large differences among persons of a given age and sex in body fat content and regional distribution of body fat. The reasons for such differences are not known, but it is increasingly recognized that inheritance is a factor in obesity and regional fat distribution. One line of research studies biologic relatives and relatives by adoption and relies on the tools of genetic epidemiology to estimate the genetic and nongenetic sources of variation in body fat content and regional fat distribution.1,2 Another, more experimental, approach requires the study of the differences between people in the response to a well-defined. © 1990, Massachusetts Medical Society. All rights reserved.