Gary Taubes, the nationally acclaimed science writer, explains why exercise and weight loss are not physiologically linked:

The one thing that might be said about exercise with certainty is that it tends to makes us hungry. Maybe not immediately, but eventually. Burn more calories and the odds are very good that we’ll consume more as well. And this simple fact alone might explain both the scientific evidence and a nation’s worth of sorely disappointing anecdotal experience.

The New York magazine article is a good read, and includes Taubes’ signature style of including historical perspective to frame the issue. And he gets to the underlying philosophy behind the “calories in / calories out” theory of weight loss: the idea that the body is a thermodynamic black box that has to respond to the balance of calories taken in and calories expended.

But we are not gasoline engines. Taubes explains that the thermodynamic black box theory (TBBT) fails to take into account the role of fat tissue in our metabolism. Studies showed fat people maintaining or gaining weight while eating less than thin counterparts. But the thin people were more active. The correlation seemed to support the TBBT theory. But studies of overweight people who increased activity substantially, including a controlled study where Finnish researchers trained overweight men and women to run a marathon, showed that they maintained their weight.

Overweight people who have tried the “diet and exercise” remedy can commiserate with their Finnish brothers and sisters.

Taubes relates that our bodies have been shown to try and maintain certain levels of blood sugar, hormones, etc. We have evolved to try and counteract the entropy of our environment and gradual breakdown of our bodies. Our survival depends on it:

The key is that among the many things regulated in this homeostatic system—along with blood pressure and blood sugar, body temperature, respiration, etc.—is the amount of fat we carry. From this biological or homeostatic perspective, lean people are not those who have the willpower to exercise more and eat less. They are people whose bodies are programmed to send the calories they consume to the muscles to be burned rather than to the fat tissue to be stored—the Lance Armstrongs of the world. The rest of us tend to go the other way, shunting off calories to fat tissue, where they accumulate to excess. This shunting of calories toward fat cells to be stored or toward the muscles to be burned is a phenomenon known as fuel partitioning.

What is the mechanism for storing calories as fat? Insulin, working in concert with an enzyme, lipoprotein lipase (LPL), determines if energy should be burned or stored as fat.

Low carb dieters know that one effect of eating a low carb diet is to even out the insulin response. Those of us with insulin resistance also know that as we even out the insulin response, our cells become less resistant to the insulin in the bloodstream, and more of the glucose in our blood is used for muscular energy rather than stored as fat.

A low carb diet is more than a weight loss diet; it is a lifestyle choice that leads to more stable weight for a lifetime. This is possible because, unlike exercise, eating lower carbohydrates in your daily diet aids in the feeling of being satisfied with the amount you have eaten. When you aren’t hungry, its easier to not reach for that snack.