The only low calorie event in people's lives was starvation. Those who could cope with a temporary lack of food were the ones who survived. Our bodies, therefore, have developed this built in mechanism to help us survive in the face of low food intake. When researchers compare overweight and thin people, they find that they eat roughly the same numbers of calories. What makes overweight people different is the amount fat they eat.
Thin people tend to eat less fat and more complex carbohydrates.
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