That's the popular theory, but there is some evidence that the issue is more complicated than that. E.g. diets are incredibly ineffective at losing weight. Peop

That's the popular theory, but there is some evidence that the issue is more complicated than that. E.g. diets are incredibly ineffective at losing weight. People artificially forced to gain weight by eating a lot quickly return to their previous weight after they stop. And identical twins maintain very similar weights even when they are raised in separate environments. And lab animals have actually gotten fatter over time for unknown reasons.