What most of these diets had in common was an idea that is still popular today: eat fewer calories and you will lose weight. Even the low-fat craze that kicked off in the late 1970s–which was based on the intuitively appealing but incorrect notion that eating fat will make you fat–depended on the calorie-counting model of weight loss. (Since fatty foods are more calorie-dense than, say, plants, logic suggests that if you eat less of them, you will consume fewer calories overall, and then you’ll lose weight.)