Is Weight Gain Caused by Genetics | Heredity Factors for Obesity

Weight Gain and Heredity

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ls your body weight dependent on your environment or your genes? That question is often stated as nature versus nurture. Is it true that if you inherit genes to be lean, heavy or frankly obese that dieting and exercise won't help you? Or is it true that people are overweight because they consume too many calories? These puzzling questions have been at least partially answered in recent years. Yes, you can inherit genes that increase your tendency to be either lean or plump.

Two studies reported in The New England Journal of Medicine support this observation. But don't throw caution to the wind and decide that diet is not important, it is. However, one must accept that both diet and exercise often have their limitations in affecting what you weigh and where the fat is. Dr. Albert Stunkard and colleagues at the University of Pennsylvania have been studying twins to assess the role of genes in body weight for some years. In an earlier study they showed that the body type, from lean to overweight, was highly correlated with adopted individuals' natural parents and not correlated with adoptive parents clearly a strong indication that the genes these adopted individuals inherited were more important than their environment or eating habits within the family that reared them.

Stated another way, if your parents are lean, you have a god chance to be lean, and if they are overweight, you are more likely to be overweight. This time Dr. Stunkard's team did a statistical analysis on twins. They studied 93 pairs of identical twins reared apart and 154 pairs of identical twins reared together in the same environment. They also studied 218 pairs of non-identical twins reared apart and 208 pairs of non-identical twins reared together.


They found that identical twins reared apart had a strong correlation in their weight/height characteristics. In fact they were just as similar as the identical twins reared together, ruling out any effect of a shared rearing environment on the weight/height characteristic. They studied the differences in weight/height between each of the two twins to each other (intra pair) compared to the differences between one pair of twins to another pair of twins (between pairs). What they really demonstrated with this well-designed study was that genes were responsible for about 70% of the differences in what people weigh. That was true for all different weight/height groups as adults, and had little or no correlation with childhood environment.

The second important study, also published in the same issue of The New England Journal of Medicine, was done by a team of scientists from Haval University in Quebec, Canada. Besides being intensive, it was a small and very well-controlled study of 12 pairs of identical twins. Such detailed metabolic studies within the hospital environment involve a lot of effort, but because they are so well controlled, it is possible to draw very meaningful conclusions from such studies. The twins were fed 1,000 calories a day more than they usually needed, six days a week for a 100-day period (just over 14 weeks). That means 84 days of over feeding for a total of 84,000 extra calories.


They found that each twin tended to have the same weight change and body composition change as his other twin, but there was a wide difference between one pair of twins to other pairs to twins. The average weight gain over the 100 days was 8.1 kilograms (18 pounds), and the range from 4.3 kilograms (9.5 pounds) to 13.3 kilograms (30 pounds). Physical activity and all aspects of lifestyle were controlled within the hospital environment so that all the twins were treated the same. There was variation between one identical twin and the other identical twin, so genetics alone was not the total answer, but it was by far the most important factor.

These were not fat men, as the average percent of bodyweight as fat was only 11.3% before over feeding and increased to 17.8% after over feeding. In addition, the intra-twin pairs tended to increase bodyfat deposits in the same places, within the abdomen, under the abdominal skin or elsewhere. That means bodyfat distribution in the presence of weight gain was strongly influenced by inherited factors. That is especially important as individuals who tend to gain weight in the abdomen versus the hips are much more likely to develop high blood pressure, diabetes and heart attacks.

There were differences in the changes in body composition - how much of the weight gain was fat and how much lean tissue. The average gain in fat was 5.4 kilograms (12 pounds) and the average gain in lean tissue was 2.7 kilograms (6 pounds). The investigators calculated that meant 52,220 of the excess calories were converted to fat and 2,754 calories were used for an increase in lean tissue. That leaves an average of 29,000 calories that were not accounted for. To be continued (with an answer to what happened to those 29,000 extra calories, plus advice for controlling body weight even if your genes are stacked against you.)

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