It’s simple: Eat less. Sometimes combined with the directive move more, this mantra has a clear point. If you can’t lose weight, you are either stupid or lazy—or, probably, both. See also: Calories in, calories out.
But if things were that simple, diets would work. Middle-aged people would not suddenly start gaining weight despite eating and moving similarly year after year. No one would have to endure the presence of that one friend with the “fast metabolism” who can eat anything he wants. And who, even though he knows you’re on a diet, says through his overstuffed mouth, “I couldn’t even gain weight if I tried.”
Instead, it is becoming clear that some people’s guts are simply more efficient than others’ at extracting calories from food. When two people eat the same 3,000-calorie pizza, for example, their bodies absorb different amounts of energy.
And those calorie-converting abilities can change over a person’s lifetime with age and other variables. The question is, why? And is it possible to make changes, if a person wanted to?