Starting in the mid 1890’s, American insurance companies began recording the heights and weights of men and women seeking life insurance. From that time to the mid-1970’s, the weights of typical American men and women remained remarkably constant. Over these 80 years, the average middle-aged woman weighed 145-150 pounds, and the average man, 165-170 pounds. But starting in the mid-1970’s our weights suddenly began to shoot up so that the average man or woman today weighs 25 pounds more. As we all participate in the American obesity epidemic, almost everyone is heavier today than their counterparts of 40 years earlier. Why?

In one sense, the answer is easy: We are heavier because we are eating more. The average number of calories per person in the American food supply was actually lower in 1965 (3100 calories) than it was in 1909 (3500), but then began to go up and is now more than 3900, an increase of 25%. The increase is the same when allowances are made for waste and spoiling, and there is a similar jump in the number of calories based on peoples’ reports of what they have eaten. So why are we eating many more calories than we used to?

Some experts say we are eating more because we are so addicted to fat and sugar that we cannot stop ourselves from eating too much when our foods our laced with these tempting, tasty ingredients. But if we look at the share of calories in our food supply from sugars, it fell during the time we were gaining weight, and the share from all fat also fell until 1997 when it started to rise again. Others blame all carbohydrates, but their percentage has not changed either.

And there was no reason we could not have been eating more forty or fifty years ago if we had wanted to. Sugar was just as sweet, and there was no lack of appealing foods and sugary creamy desserts. In fact, for those who remember, food tasted even better when it was prepared with plenty of animal fat. People could have easily afforded to buy more food, and to eat more cookies, cakes, ice cream, and drink more sugar-sweetened sodas, but they didn’t. Those with higher incomes—and even better access—weighed less, not more. Men and women would usually add some pounds between their late teens and middle age, but much less than they do now, and most people seemed to be content with their weight. There were hardly any books about dieting, and no weight loss programs, joggers, or fitness clubs.

To better understand why we are eating more today, we need to consider how our brains control our desire for food. Our appetite and weight are carefully regulated by an ancient part of the brain, the hypothalamus. This almond-sized bundle of nerve tissue makes sure that animals eat enough to maintain their body functions and activities, but not enough to get fat. (Most animals have no more than five percent of their weight in fat.) In regulating our hunger, the hypothalamus monitors the levels of the sugars, amino acids, and fatty acids in our blood which are the end products of our digestion. And it pays particular attention to fat. Not only is fat the best source of calories, but it makes up about a fifth of the cells in our bodies not counting water, and half of our brain cells. And there are two “essential” types of polyunsaturated fat in our cells, omega-3 and omega-6, that can only come from our diets.

If we compare the current American diet with our diets forty years ago, we find similar levels of sugars, amino acids, and total fats, but the amounts of the two types of omega fats have changed very dramatically. In a natural diet of grains, meat, dairy foods, fruits, and vegetables, there is a bit more omega-6 than omega-3, but today there is more than twenty-times more omega-6 than omega-3. This shift in the proportion of these different fats is by far the biggest change in our diets over the past forty years.

One reason for this shift is a three-fold increase in our consumption of processed vegetable oils made from corn and soybeans; these oils are now the major source of fat in the American diet, and they are ubiquitous. They are used to make processed foods, including fried foods, fast foods, snack foods, and baked goods. Since most vegetables and grains have only a small amount of fat, there would be no way we could consume this much vegetable oil without the industrialized chemical processing of corn and soybeans.

The other reason for the remarkable change in the omega balance of our diets is the decision to increase meat production by feeding corn and soybeans to immobilized cattle, pigs, and poultry instead of letting them graze on the grass that has been their natural diet for fifty million years. While grass has much more omega-3 than omega-6, corn is almost all omega-6. An analysis of a hair from CNN’s medical correspondent in 2007 showed that 69% of the carbon atoms in his body came from corn! 

To make matters worse, vegetable oils high in omega-6 interfere with our getting the omega-3 which is naturally present in food. While fish and seafood are especially rich in the active forms of omega-3, when they are fried or cooked with vegetable oils, the omega-3 they contain is no longer available to us.

But is it possible that this dramatic change in the balance between omega-3 and omega-6 fats could be making us eat more? There is good reason to think so. They are much more in balance in the western European diet, and people there weigh much less. American men and women who have high levels in their blood of the most active forms of omega-3 and low levels of active omega-6 weigh forty pounds less than those with high omega-6 and low omega-3. The average ratio of omega-6 to omega-3 in Americans is more than five to one, while in the Japanese, who have very little obesity, there is more omega-3 than omega-6. There is also another clue from an unusual form of vitamin E that is especially abundant in corn and soybean oils. The level of this odd vitamin in our blood tells how much corn and soybean oil we are eating; and the higher the level, the more we weigh.  

We are also learning more about how omega fats influence the appetite-regulating cells in the hypothalamus. These cells are rich in receptors for endocannabinoids, our body’s form of the ingredient in marijuana that increases appetite. Because these are made from the active form of omega-6, arachidonic acid, more omega-6 in the diet means more munchy-promoting endocannibinoids. Arachidonic acid is also the source of inflammatory types of signaling molecules called eicosanoids which are also linked to increased weights. Omega-3-based eicosanoids have the opposite effect.

The most active form of omega-3, DHA, is critically important for the growth, development, and functioning of our brain, and the hypothalamus may also be able to sense how much is in the blood. Those with higher levels of DHA in their blood tend to have lower levels of arachidonic. People are hungrier after meals high in omega-6 than meals high in omega-3, even if the total amount of fat is the same.

Ours is the only country in the world that has transformed its diet so radically. This change was actually thought to be beneficial because of the mistaken belief that increasing omega-6 would reduce the risk of heart disease. The western European diet today is much closer to the diet we had forty years ago, with 41% more animal than vegetable fat, and, not surprisingly, their weights are very similar to our weights back then. And their death rates from heart disease are also much lower than ours.

While our highly industrialized methods of food production have temporarily lowered the cost of calories in our food supply, we know that they cannot be sustained in the future. And, unfortunately, the unprecedented shift in our diets from a natural pattern that has been maintained for tens of thousands of years to a sudden and unnatural dependence on corn, soybeans, and their oils has now made us the fattest people on the planet. There is more on diet and weight in our book, Why Women Need Fat.

 Written by Will Lassek and Steve Gaulin

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