In a previous blog I discussed the historical perspective that Gary Taubes gives in his new book Good Calories, Bad Calories: Challenging the Conventional Wisdom on Diet, Weight Control, and Disease. Now I'd like to discuss the fascinating way of looking at weight gain, hunger, and exercise that Taubes, a science journalist, came to believe makes the most sense.

One can view the problem of why people gain weight in two different ways. The traditional theory is that people lack self-control and simply eat too much and don't exercise enough. This makes them fat. You can think of this as the overeating theory.

The other theory is that some underlying hormonal imbalance makes people gain weight even when they don't eat very much. The process of storing fat -- and also storing sugar in the liver in the form of glycogen -- depletes the bloodstream of energy-producing substances, namely, fatty acids and glucose. This, in turn, triggers hunger and makes people (or animals) eat more and move less in order to conserve energy. I'll call this the metabolic theory.

In other words: you're not fat because you eat too much and don't exercise enough. You eat too much and don't exercise enough because you're fat.

Here's how the metabolic theory is supposed to work. Normally, after meals, insulin levels rise, and insulin tells the body to store the extra food as glycogen in the liver and as fat in the fat cells. It also tells the body not to break down any of that fat or glycogen to produce energy. You already have enough energy-producing substances from the food you just ate.

Between meals, insulin levels fall, so the process is reversed. Some glycogen is converted to glucose, and fats are released into the bloodstream (as fatty acids). The fatty acids as well as the glucose can be burned for energy.

In a healthy person, this process works very well. Your blood glucose levels stay pretty level, and you don't get hungry between meals.

But something in our modern lifestyle makes this system go crazy. According to the metabolic theory, this "something" is insulin. Insulin levels become too high because you eat too many processed carbohydrate foods. The high insulin levels then stimulate insulin resistance.

If all the cells in your body were equally sensitive to the insulin resistance, you might be OK. But the fat cells seem to become less insulin resistant than other cells. Imagine that your insulin levels doubled. If the insulin resistance in the muscle cells also doubled, the muscle cells would take up the usual amount of glucose from the blood. But if the insulin resistance in the fat cells remained the same, your fat cells would store twice as much fat. [The mathematics of all these effects is obviously more complex than this; I've simplified for the sake of argument.]

And because in order to store fat in the fat cells you'd remove both fatty acids and glucose from the blood, your blood would become deficient in energy-producing substances. As a result your body would tell you to eat -- and you'd get hungry. As an extra safety valve, your body would also tell you not to exercise, because it wouldn't want you to waste your "scarce" energy. You'd actually have a ton of energy stored in your fat cells, but because insulin levels were too high, that fat couldn't get out to be used.