Environmental (non-genetic) causes of obesity: overview
We have spent quite a bit of time discussing genetic factors, theoretical and proven, that might contribute to the high prevalence of obesity in the U.S. In summary, it is very clear that genetic factors are very important but they cannot fully explain why we have so much obesity, and particularly why we have so much more of it recently; I don’t think we can postulate a large upsurge in obesity genes over the past 20-30 years. So, what are the non-genetic factors that might explain our rising BMIs?
Remember the Pimas?
We briefly discussed the Pimas in an earlier posting. They are an indian tribe that started out in the Sierra Madre of Southern Mexico. Over many years, many of the Pimas migrated northward to what is now the southwestern U.S. Many of them live on the Gila River Indian Reservation, just outside of Phoenix, AZ. The Pimas have the highest known prevalence of diabetes (type 2) of any genetic group- about 50%. The Pimas also have a very high prevalence of obesity, including people many people with BMIs over 40. What is particularly interesting about the Pimas is that those who are still living in the Sierra Madre show virtually no obesity and very little diabetes. Yet, the Sierra Madre Pimas consume more calories, on average, than the New Mexico Pimas. What gives? The answer is activity; the Sierra Madre Pimas have to work hard to survive and their levels of physical activity are far higher than in their U.S. cousins.
Another factor that may be important is the diet composition itself. In the Sierra Madre, the Pimas eat mostly what they grow and raise- lots of corn and occasional chicken. The New Mexico Pimas eat what most other people in the U.S. eat- lots of highly processed foods which are high in fat. I know the U.S. diet is not as healthy as the Sierra Madre diet but I am not certain if diet composition itself contributes to the high prevalence of obesity in the U.S. Pimas and non-Pimas. Certainly it is easier to consume lots of calories when one eats lots of calorie dense foods, but that is a different issue.
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