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How do genetic factors influence the simplistic calories in - calories out = surplus = fat?
Rephrasing that - clearly, we all live on a surplus diet. Yet most of us aren't getting fatter and fatter - at least not as fast as a simple caloric calculation would have it. Thus the body must (I think) be able to adjust the metabolic rate upwards as well as downwards (which I have read about, the starvation mode.) The extent of this capability must then be determined by one's genetics.
I do believe that there is a subset of people who have a genetic predisposition to obesity, however, I feel that this is a very small percentage.
If you look at the epidemiology of weight gain in the last 50 years it is pretty clearly tied to the changes in what we eat and the ease of obtaining foods that are calorie dense. The surplus diet has different effects because some people eat more of the surplus. If a 5 foot 7 inch woman should be eating about 1500 - 1600 calories per day (depending on activity) and she eats about 2000, the weight gain will be about a half pound per month. She will eat less when she sees herself gaining weight and her weight will rise and fall some, but over her lifetime she will end up with a body mass index near the obese level.
People are gaining weight -- a LOT of it and at an alarming rate. I see this every day in my practice. I see it every day with many of my friends and aquaintances. The stats don't lie. The U.K. is catching up to the obesity epidemic fast. About 25% of the adult population there is obese. There's evidence of this in those who move from Asian cultures to Western society. The Japanese who move to the U.S. gain weight. The rates of weight gain in their country are now mimicking those of America in the 60's and 70's as they adopt a calorie dense Western diet of KFC and McDonalds.
Certainly there are changes in the metabolic rate as people gain weight that makes it harder for them to lose. This is pretty well documented. There are some hormonal issues that we are not even close to understanding. Still, at the most basic level we know that for the vast majority of humans it is a fairly simple equation of calories in vs. calories out.
Part of the problem is that perceptions have changed in the last 50 years along with the weight gain. There's great research that shows a much lower percentage of us shown pictures of overweight and obese people consider them to be overweight or obese. This has dropped in the last 20 years from about 40% able to identify someone as overweight to 20%. With that perception change it's easy to see why we find the problem easy to ignore. Most people don't think that there is a problem.
Great question! Thanks for writing.
Timothy S. Harlan, MD, FACP