The search for the causes of obesity has involved genetic abnormalities and endocrine and neural lesions. Although evidence suggests that genetics plays an important role in body weight regulation, rapid increases in obesity rates do not seem to be caused by significant genetic changes within populations. Total energy expenditure and total energy intake are not the only factors that regulate body fat. Nitrogen and carbohydrate balances are eased by the capacity of the organism for adjusting amino acids and glucose oxidation rates, respectively. Regarding fat, this mechanism is considerably less precise; a fat intake increase does not stimulate its oxidation on the same basis. In addition, dietary fat is stored very efficiently as body fat. Elevated carbohydrate ingestion enhances glycogen reserves, which usually are much smaller than the maximum capacity of storage and enlargement of these stores, thus stimulating this nutrient's oxidation. These data point to a very well controlled carbohydrate balance in the body. Various studies show lack of efficiency of the hyperlipidic diet in stimulating satiety. Signals arising from the gastrointestinal tract play a fundamental role in regulation of appetite and energy intake, and evidence indicates that the gastrointestinal and hormonal mechanisms involved in the suppression of appetite and in energy intake are compromised in obesity. A high-fat diet is important in its origin. Additional studies are necessary to explain the mechanisms that lead to adipose tissue retention resulting in a fat-rich diet.