Your weight and shape are strongly determined by your genes.
There are not many studies of the genetics of obesity because they all show the same thing. Why spend any more effort to study that particular question? One way to study genes is to study twins. In the May 24, 1990 issue of the New England Journal of Medicine, there appeared a study report by Dr. Stunkard and co-workers entitled, "The body-mass index of twins who have been reared apart." Albert Stunkard is a psychiatrist at the University of Pennsylvania in Philadelphia. He is famous for his obesity research and his name will appear often in our story. Why is a psychiatrist an important authority on obesity?
For this study Stunkard and al. used the Swedish Twin Registry, a treasure trove that includes information about nearly 25,000 pairs of twins born between 1886 and 1958. They were able to look at subgroups of 93 pairs of identical twins reared apart, 154 pairs of identical twins reared together, 218 pairs of fraternal twins reared apart, and 208 pairs of fraternal twins reared together. The average age was 58.6 years at the time they checked up on the destiny of the twin pairs. They compiled their weights and indexes of weight. Regardless of where or how they lived, the twin pairs resembled each other in size and shape and weight. To quote the final conclusion from the abstract, "... genetic influences on body-mass index are substantial, whereas the childhood environment has little or no influence."
Two other studies by Stunkard and others, using different twin registries, show the same results. Finally, in "An adoption study of human obesity," Dr. Stunkard and company used the Danish Adoption Register to look at this question. They found that more than 80% of the variability of body size and shape (body mass index) was explained by the genetic parents. The size of the adoptive parents didn't matter. Their conclusion was, "that genetic influences have an important role in determining human fatness in adults, whereas the family environment alone has no apparent effect."
Another interesting report appeared in the same issue of the New England Journal of Medicine as the first Stunkard study I mentioned above. This study by Claude Bouchard, Ph.D. and 9 co-authors from Laval University Medical Center near Montreal, Quebec, was titled "The response to long-term overfeeding in identical twins." They watched what happens when a diet containing 1000 extra Calories per day was fed to 12 pairs of identical twin young men aged 19 to 27 years. The twins ate the 1000 extra Calories 6 days a week for 100 days. On the seventh day of each week they took a break and ate only their usual number of calories. Thus they were overfed for 84 of the 100 days; the total excess intake was 84,000 Calories. Physical activity was limited during the study to such activities as reading, playing video games, playing cards, and watching television, as well as an outdoor walk for 30 minutes per day. None of the men were obese nor did they come from obese families. At the beginning of the study, the average weight was 60.3 kilos or about 132 pounds They gained an average 17 1/2 lbs The most gained was 29 1/4 lbs and the least was 9 1/2 lbs. (By the rule that 1 pound equals 3500 calories they should have gained 24 pounds.) Within each twin pair, the similarity of how much weight they gained and where they gained it was striking even though there was substantial variation of weight gain among the pairs.
Swedish twins, Danish adoptees and sedentary identical twin men from the Canadian province of Quebec are all just like rats. The amount of their weight gain and where they put it on is determined by their genes and not by how much they eat.