We are in the midst of an epidemic of obesity. This would seem to contradict the well established fact that height and weight and shape are determined by genetics.
From April 1971 through June 1974, dietary intake for 1 day was obtained by interviewing 28,043 sample persons selected to represent the 194 million US population, aged 1 to 74 years. This was the first National Health and Nutrition Examination Survey, NHANES I (pronounced n-hanes one). This project was conducted by the folks at the National Center for Health Statistics. To date we have conducted three, ever more detailed NHANES. The last, NHANES III, finished collecting data in 1994 and it is still being analyzed.
All three NHANES taken together indicate a dramatic increase in weight and calorie consumption over time despite massive increases in Weight Watchers revenues during the same period. This must be what happened to the dinosaurs. Other studies in the United States and studies from England and other developed countries show the same trends.
But the NHANES data and other studies have shown that the most obese segments of the population are the ones who eat the least. There are many other internal inconsistencies that point up the fact that epidemiology is only the first tentative baby step in scientific understanding. One thing left out of the NHANES survey data so far, is some measure of exercise.
Maybe that's the key to this confusion. NHANES III interviewers did ask some questions about exercise that are still being analyzed. But a different project, The Minnesota Heart Health Program (MHHP), has reported on exercise. They studied three communities in the upper midwestern part of the USA, and found no evidence connecting overweight with physical activity, either occupational or during leisure time. They did find the same secular trends as the NHANES surveys â€“ everybody is getting fatter and eating more, but there was no connection between overweight and dietary fat, total calorie intake, or smoking cessation.
The increase in weight of our population is too fast to blame on genetics. That leads to the conclusion by many observers that the environment - too much good food - is making us fatter. Nurture rather than Nature.
This tendency for humans to get fatter quicker than their genes can change, may be because humans, for most of their brief history on earth, have been prone to repeated famine and hard labor and dire stress. We've always carried a heavy burden of worms and other parasites. We've had frequent fevers and diarrhea, loss of teeth at an early age, dangerous hungry enemies like other humans etc. etc. Yikes! A tendency to grab weight and fat is a real asset to survival under these more long standing circumstances. The genes for obesity have been accumulating over a long time.
Whether or not we can really understand these secular trends for weight, they are not unique. Asthma is another disease with a strong familial component that is getting more common and more severe. Over the past 100 years intelligence has increased steadily. Someone scoring in the 90th percentile for IQ 100 years ago would be in the 5th percentile today. We are also getting taller. Smarts and height have strong genetic components too.
Finally, the evidence from epidemiology seems to show little connection between weight and diet and exercise.
Epidemiology, as with all science, raises more questions than it answers.