Dieting is often 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. At two years, all calorie-reduced diet types cause equal weight loss irrespective of the macronutrients emphasized.
The energy intake from food is limited by the efficiency of digestion and efficiency of utilization. The efficiency of digestion is largely dependent on the type of food being eaten, while efficiency of utilization is affected by a variety of factors, including age, gender, body weight, hormone levels, and many more.
