Obesity is a chronic disorder caused by an imbalance of the energy metabolism with high associated burdens. Therefore, huge efforts are being currently devoted in studying new types of hypoenergetic diets and their composition, in order to characterise more specific, long-lasting and safe slimming protocols. A number of investigations are trying to determine the specific influence of the macronutrient distribution in energy-restricted diets on the management of excessive body weight. In this context, very-low-energy diets supplying between 1670 and 3350 kJ (400 and 800 kcal)/d have been beneficial in short-term treatments causing a weight loss of 300-500 g/d. Such strategies place more emphasis on energy restriction than on the macronutrient composition of the diet prescription. Weight loss produced by either low-carbohydrate or low-fat moderately energy-restricted diets ranges from 0.5 to 1.0 kg/week, while diets with high or moderately high protein content have also been applied in weight-reducing programmes by inducing losses of 0.2-0.4 kg/week. Other factors that determine weight loss by dieting are sex, age, initial body weight, race, genetics, regional fat deposition, etc, which must be taken into account to explain the variability in the outcomes of different low-energy diets. Therefore, more research is needed about the impact of diets with different fuel substrates and foods on the characteristics of the weight-loss process.