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 4.3–13.3 kg. The similarity within each pair in the response to overfeeding was statistically-significant (p < 0.05) with respect to body weight, percentage of fat, fat mass, and estimated subcutaneous fat, with about 3× 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 6× 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.
[Note the extreme individual-differences in weight gain:]
…There were considerable differences between persons with respect to changes in body weight and body composition with overfeeding. Although the mean increase in body weight was 8.1 kg, the standard deviation was 2.4 kg and the range 4.3–13.3 kg. The individual differences in body weight and body composition as a result of overfeeding, however, were not distributed randomly among the 24 men. This is demonstrated by the similarity of the absolute changes within pairs shown in Table 1: The F ratios of the variances between pairs to the variances within pairs were about 3, and the intraclass correlation coefficients computed on the basis of the changes resulting from overfeeding were clustered around 0.50. The within-pair similarity was slightly less for fat-free mass (about 0.4). The within-pair similarity for the changes in body weight is illustrated in Figure 1.
…The average gain in fat mass was 5.4 kg or about 210 MJ (52,220 kcal), whereas the gain in fat-free mass was 2.7 kg or ~11.5 MJ (2754 kcal). On average, about 121 MJ (29,000 kcal) did not appear as weight gain when constants were used to convert tissue gains into energy equivalents,17 and presumably this energy was dissipated in some way. One third of the weight gained by the group as a whole was in the form of fat-free mass, a proportion comparable with that reported previously.4, 9 The man who gained the most weight (13.3 kg) had no evidence of energy dissipation by any mechanism, whereas in the man who gained the least weight (4.3 kg) only about 40% of the extra calories were deposited as body tissues. The men who gained more fat than lean tissue tended to gain more weight and to gain more fat in the truncal-abdominal area. On the basis of the correlations we found, 37–44% of the gains in weight or trunk fat were accounted for by the increments in the proportion of fat and lean tissues…the findings that some persons were more prone than others to store fat on the trunk, in the abdominal cavity, or both are of considerable clinical interest.