A fad diet is one that gains rapid popularity for a time and then falls out of fashion. Most fad diets are food-specific or revolve around a gimmick. For example, the cabbage soup diet requires you to eat only cabbage soup for each meal for a week. The paleo diet, which limits what you can eat to what was available during the Paleolithic period, is also considered a fad diet. More recently, diet advocates have also classified the alkaline diet, which is based on the hypothesis that the acidity of foods affects your body’s pH balance and that a higher pH leads to weight loss, as a fad diet.