Why We Get Fat

I just posted a video about why we get fat. It results from cravings for high-calorie foods. These cravings were given to us by natural selection. They were an essential part of survival back in cave man days. Cravings made us eat every bit of food we could find. Our bodies then stored excess calories in the form of body fat. We would feast on food when it was briefly available, then live off of the stored calories in our body fat during times when food was not available—times of deprivation or famine.

Body fat is the safest way for us to store calories. The calories are safe from pests and mildew. And body fat has advantages other than just storing calories, which is its main function in natural selection. Visible accumulations of fat can make sexual characteristics more noticeable, and thus be favored by sexual selection as well. Also, since we walk upright on two legs, we also sit upright, and it is very nice to have a fat butt to sit on.

But today, our bodies are storing calories for a famine that never comes. There are more obese people than hungry people in the world today. Today, for our health, we have to deliberately do things for which our caveman ancestors had no choice. We must limit our calorie intake, and make sure many of those calories are in healthy food—that is, just about the only kinds of foods available to our evolutionary ancestors. We also need to make sure we get plenty of exercise, something our evolutionary ancestors had no choice about.

This is just one example of many in which modern medical problems are best understood in the light of our evolutionary history.

Eat Food, Not Too Much, Mostly Plants

This is the subtitle to Michael Pollan’s book In Defense of Food. Those seven words delineate a perfectly adequate diet: for anyone; for anyone interested in saving the planet; and for borderline diabetics.

Last winter I was diagnosed with the beginning of diabetes. I am not yet at the stage that requires any medication, and the fact that since that time I have lost 20 pounds might mean that I am safe. My response is to find creative solutions that I can enjoy. I am posting occasional entries about diabetes on this site if they relate to evolution and/or to the environment in which we have evolved and in which we live. I am not turning this blog into a diabetes forum.

Diabetes is considered to be a disease of civilization. That is, many scientists believe it results from physiological adaptations that were perfectly good in the Stone Age but which are maladaptive for civilized people. I can see that this argument makes sense for obesity. The ability to store food in the form of fat, during a time of feasting, and then use it up later during famine, was utterly essential in the Stone Age, when your tribe might get to feast on a dead mammoth then have nothing for a long time. But today, when food is continuously available, obesity results from fat storage. As Pollan said in The Omnivore’s Dilemma, our bodies are preparing for a famine that never comes. Not only that, but fat storage was a sign of affluence in the Stone Age; being fat was therefore attractive to potential mates. There is a fair amount of Paleolithic pornography to this effect. Therefore both natural and sexual selection favored the potential for obesity in Paleolithic times.

But diabetes? How could the inability to control blood sugar levels be adaptive under any set of conditions? Some scientists say that having high blood sugar levels (that is, a reduced ability of cells to absorb and use the sugar) was adaptive because it kept our Stone Age ancestors from using up their blood sugar too fast. Sorry, but this sounds like one of those just-so stories that modern evolutionary science is trying to get away from. The nascent science of evolutionary medicine would be well served to leave such stories behind. Of course, I could be wrong, in which case I will post a revision of the opinion here stated. Diabetics need to eat lots of small meals; this is precisely what our Stone Age forebears did not always have the luxury of doing, especially when they spent all day chasing down big game and when wild grains, roots, nuts, and berries were unavailable.

Some people propose that we adopt the Paleolithic Diet. Without going into detail, I’ll just say that this is not too useful unless you adopt the Paleolithic Lifestyle—running through the forest or across the savanna all day.

Michael Pollan’s advice, to eat food, not too much, most plants, is good advice for all of us. People with diabetic tendencies could include a bias against starch and sugar.

I will be participating in Botany 2013, the big meeting of botanists, starting next weekend. If I have a chance, I will blog from there.