To believe otherwise is to believe we can find a really good perpetual motion machine to solve our energy problems. It won’t work, and neither will changing the source of calories permit us to disobey the laws of science.

Did you ever ask whether people respond differently to diets of different compositions?

Dr. Rudolph Leibel, now an obesity researcher at Columbia University, and I took people who were of normal weight and had them live in the hospital, where we diddled with the number of calories we fed them so we could keep their weights absolutely constant, which is no easy thing. This was done with liquid diets of exactly known calorie content.

We kept the number of calories constant, always giving them the amount that should keep them at precisely the same weight. But we wildly changed the proportions of fats and carbohydrates. Some had practically no carbohydrates, and some had practically no fat.

What happened? Did people unexpectedly gain or lose weight when they had the same amount of calories but in a diet of a different composition?

No. There was zero difference between high-fat and low-fat diets.

Why is it so hard for people to lose weight?

What your body does is to sense the amount of energy it has available for emergencies and for daily use. The stored energy is the total amount of adipose tissue in your body. We now know that there are jillions of hormones that are always measuring the amount of fat you have. Your body guides you to eat more or less because of this sensing mechanism.