"Darwin Made Me Fat!"

Do our Stone-Age genes make us overeat?

Posted Aug 27, 2012

Even though many Americans say they don’t accept Darwin’s theory of evolution, it has become quite fashionable to blame our Stone-Age genes for the current American obesity epidemic. But we seem to have forgotten that we had the same Stone-Age genes 40 years ago when very few Americans were obese. And obesity rates are still quite low today in the same European countries which provided the genetic heritage for many of the Americans who are now struggling with overweight and obesity.

The idea that evolution is responsible for our expanding waistlines is appealingly simple and plausible. Calorie-rich sugar and fat were hard to come by during the Stone-Age, the period that encompasses 99 percent of the history of our species. And because we needed lots of calories, our brains naturally evolved to make us feel good when we made the effort to get some.

The richest source of sugar could be found in the nests of honey bees who were naturally reluctant to give it up. Even today, for Hadza hunter-gatherers in Tanzania, honey is by far the favorite food, and men suffer to bring home more of it when their wives are nursing. Juicy ripe fruits were the next most sugary foods, when we could manage to get to them before other animals, insects, molds, and yeasts. And the best source of fat was the meat, organs, and bone marrow of animals that had to be chased down, killed, butchered, and kept (or stolen away) from dangerous predators.

We needed lots of energy because of our big brains, the hallmark of human evolution. Brains are the most demanding organ in the body when it comes to both calories and fat. Because sugar and fat were both scarce and necessary, evolution programmed us to get as much as we could.

Today, we no longer struggle to meet these needs. Sugar and fat literally fall in our laps as food companies make sure to put plenty into the processed foods that make up a big part of our diet. But our Stone-Age brains still give us the same big reward when sugar or fat hits our tongues. So, according to this view, it is very difficult for us to limit how much of them we eat, and we helplessly overeat and put on excess pounds. The mismatch between food preferences that evolved under scarcity and the current oversupply is driving the obesity epidemic.

The key problem with this seemingly attractive explanation is that we don’t have to go all the way back to the Stone-Age to find lower weights. While more than one in three adult Americans are obese today, in the 1960s, it was only one in seven or eight. The average adult then weighed twenty pounds less than now. But sugar was just as sweet, and fat was even tastier than it is now because most of it came from animals rather than corn and soybeans. Our share of calories from sugar and fat are actually lower today than they were then.

And what about people in western European countries? Their weights and obesity rates are as low as ours were four decades ago. Do they have much less fat and sugar in their diets? No. Their calorie share from fat is the same as ours, while their share from sugar is almost as big. Denmark, for example, has sugar levels as high as ours and one fifth as much obesity.

So what is different about the American diet today? It turns out that it is unlike any diet that human beings have ever eaten during the past two million years. Instead of getting most of our fat from animal foods, the way our ancestors did and as Europeans still do, we now get most of it from vegetable oils. These highly concentrated and unnatural “foods” are produced by the industrialized chemical processing of millions of tons of soybeans and corn. The American agribusiness machine pumps out eight million tons of these oils a year, enough slick yellow fluid to provide 600 calories a day for every man, woman, and child.

To make matters worse, what animal fat we do get now is also much different than it used to be. Instead of feeding our farm animals the grass they evolved to eat over millions of years, we feed them corn, so that their fat is now more like corn oil. Our farm animals have been swept up in the same tsunami of vegetable fat that has engulfed us. And unlike grass, which is high in healthy omega-3 fats and low in unhealthy omega-6, corn and soybean oils are very high in omega-6 and very low in omega-3. Where Stone-Age people had equal amounts of these two omega fats, we have 15-20 times more omega-6 than omega-3!

Unfortunately, these two omega fat groups have opposite effects on our weights. Where omega-3 reduces weight and appetite, omega-6 stimulates overeating and weight gain. Those of us with high omega-6 and low omega-3 in our diets, like most Americans, weigh 40 pounds more than those with high omega-3 and low omega-6 (like most Japanese, who have very low obesity rates).

So Darwin is trying to tell us something about why we are so much fatter than we used to be. But it’s not the amount of sugar and fat in our diets that is the cause of our problem. Instead, it’s the kinds of fats we are eating, with a fat balance that is completely unnatural and would be impossible without squeezing a few drops of fat out of billions of soybeans and corn kernels. To escape the obesity epidemic, we don’t need to go all the way back to the Stone Age. A return to the diet we had in the middle of the 20th century, and which most Europeans still have today, should suffice. For more, see our book Why Women Need Fat.

Written by Will Lassek and Steve Gaulin