Destiny rides again as twins overeat

Article Abstract:

A question that has long concerned scientists and lay persons alike is whether obesity is inherited or caused by eating habits learned in childhood. The clearest answer to date is revealed in an article by Stunkard et al. which appears in the May 24, 1990 New England Journal of Medicine. Twins who had been reared either together or apart were evaluated for body-mass index, a measure of obesity. While both identical and fraternal twins were studied, the group that provided the most direct information about the inheritance of obesity was the identical twins reared apart. The similarity between two twins with identical genetic makeup who were raised in separate households gives an indication of the strength of genetic influences. These twins were very similar in body-mass index; it was found that genetics explained 70 percent of the differences in body-mass index and that childhood environment had either minimal influence or no impact at all. Other previous studies had also found that inheritance was very powerful in programming for obesity. But it would be unfortunate if these findings were interpreted as meaning that food habits in the home are unimportant. On the contrary, there is much evidence that childhood eating behavior influences whether the inherited tendency towards obesity is realized. Another study in the same issue of the New England Journal of Medicine, by Bouchard et al., involved intentional overfeeding of 12 pairs of identical twins to see if pairs of twins would gain the same amount of weight distributed in the same places on the body. There was much more variability in response to overfeeding between twin pairs than within pairs in this carefully controlled study. Again the strong influence of genetics was demonstrated, particularly in the accumulation of abdominal fat. This distribution of fat stores has been linked to risk of diabetes, high blood pressure and elevated blood cholesterol. (Consumer Summary produced by Reliance Medical Information, Inc.)

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The response to long-term overfeeding in identical twins

Article Abstract:

It is well known that individuals of the same age and sex vary greatly in both the amount of total body fat and its distribution on the body. One reason for this variation in the population may be genetics; inheritance almost certainly influences the development of obesity. Scientists study the effect of genetics on body fat by comparing close relatives, and perhaps the most valuable information can be gleaned from examining identical twins. In theory, the influence of genetics should be the same for twins, and any differences between them should be due to environmental factors. Twelve pairs of identical twins, who were all sedentary young men, were studied to measure their responses to prolonged overfeeding. Six days a week for 100 days, each subject consumed 1,000 extra calories per day over and above their usual caloric intake, which had been measured for two weeks before the overfeeding began. Weight gain and the distribution of body fat were studied at the end of the overfeeding period; within the group as a whole, these results varied greatly. While the average weight gain was 18 pounds, the range was from 9 to 29 pounds. The results of overfeeding were significantly similar within the twin pairs, in terms of weight, percent body fat, fat mass, and subcutaneous (under the skin) fat. For these measures there was approximately three times as much variance among pairs in the group as there was within the twin pairs. The similarity within pairs was especially clear for abdominal fat and fat distribution. The authors conclude that the most probable reason for the similarities within twin pairs was the influence of genetics. Specifically, the tendency to store excess calories as either fat or muscle tissue may be inherited, as may be the metabolic rate while the body is at rest. (Consumer Summary produced by Reliance Medical Information, Inc.)

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The body-mass index of twins who have been reared apart

Article Abstract:

While research studies have established that genetics influences the development of obesity, it is not known how strong this influence is and how it compares with the effect of environment and the way the individual was raised. Body-mass index, a measure of weight divided by height, is considered an indicator of leanness or obesity. Body-mass index was found to be similar between adopted children and their biologic parents and siblings, but not between the children and their adoptive parents. A study of twins was conducted to quantify the relative influences of genetics and environment on body-mass index. The subjects were 93 pairs of identical twins that were reared apart, 154 pairs of identical twins raised together, 218 pairs of fraternal twins reared apart, and 208 pairs of fraternal twins raised together. Body-mass index was highly correlated within the twin pair for both identical twins reared apart and those raised together, and the similarity between twins raised separately was only slightly less than the similarity between twins reared in the same household. The results indicate that inheritance has a substantial impact on body-mass index, while the environment during childhood has minimal or no impact. These findings are in agreement with those from previous studies of twins and adopted children. (Consumer Summary produced by Reliance Medical Information, Inc.)

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