If losing weight and keeping it off weren’t such a struggle, there would be no diet books, no Weight Watchers, and maybe even no daytime TV talk shows. Clearly, it’s very hard to slim down and stay that way. But why? Don’t our bodies want us to be fit and trim?

Well, maybe not. Some scientists and physicians have a theory to explain the body’s seeming obstinacy: Each of us has a personal “set point,” the weight our bodies prefer. Any deviation from that weight, the theory goes, and the body will employ tricky tactics to get right back to the set point.

While the idea of a set point is accepted by many (but not all) scientists, how that point may be set by the body is not well understood. Most believe that genes play a significant role; “obesity genes” may well be the blueprints that shape, or at least try to shape, the set point.

And if that turns out to be true? Researchers say it’s no reason to throw up your hands and ditch a sensible diet and regular exercise. Healthy living is still within your reach. But the set point would help explain why a good-faith effort to lose weight can be so challenging.

*Weighty Basics
Set point or not, the factors that determine weight gain or loss are simple:

Calories In
— Calories Out
= Change in Weight

Calories are a measure of energy. If your calorie intake (in food) and output (in activity) are equal, your weight change will be zero. If you consume more calories than you burn, you’ll gain weight, and conversely, if you burn more than you consume, you’ll lose weight (about 1 pound for every 3,500 calories).

Calculating “calories in” is straightforward: Find out how many calories are in the foods you eat, and add them up (don’t forget beverages like soda and juice). Figuring your “calories out”—your total energy output—is a bit more complicated. It is a combination of the calories you burn moving around (through exercise and other physical activity) and your “resting energy output.” That’s the amount of energy your body uses just to keep you alive, and it actually makes up the bulk of your total energy output. The body may not be able to dictate how much you exercise (although even that is debated), but research shows that the body can tweak your weight by changing how much energy you expend at rest.

When people talk about “metabolism,” they often are referring to resting energy output. An efficient metabolism, known colloquially as a “slow” metabolism, spends the calories in the food you eat wisely, and so there are more leftover calories to put in the bank—that is, in fat tissue. In other words, if you’re trying to lose weight, an efficient metabolism is a hurdle to overcome.