Dieting is the practice of eating food in a regulated fashion to decrease, maintain, or increase body weight. Dieting is often used in combination with physical exercise to lose weight in those who are overweight or obese. Some people, however, follow a diet to gain weight (usually in the form of muscle). Diets can also be used to maintain a stable body weight. Diets to promote weight loss are generally divided into four categories: low-fat, low-carbohydrate, low-calorie, and very low calorie. A meta-analysis of six randomized controlled trials found no difference between the main diet types (low calorie, low carbohydrate, and low fat), with a 2–4 kilogram weight loss in all studies.