[Obesity. Impairment of energy intake or of energy expenditure].

Obesity may result from two mechanisms: a chronic excess of energy intake and/or a reduced energy expenditure. Most studies failed to show a correlation between food intake and body weight; however, no efficient mechanism to spare energy has been demonstrated in obese subjects. Although an impaired thermic effect of food has been reported in obese subjects, this effect does not result in a substantial economy of energy. Total energy expenditure in humans is positively correlated with body size. This demonstrates that energy requirements of obese subjects is larger than those of lean sedentary individuals. The reason for these discrepant observations is the under-reporting of food intake by obese subjects. Obesity is therefore mainly due to a chronic excess of energy intake. Investigations on nutrient balances have shown interesting mechanisms which are relevant for the development of obesity. Carbohydrate balance is accurately regulated: ingested carbohydrates over 24 hours are mainly oxidized during this period of time. The other metabolic pathways of carbohydrates, such as de novo lipogenesis or synthesis of non essential amino acids, are of minor importance. Another property of carbohydrates is to elicit a greater satiating effect than lipids. Protein balance is also accurately regulated; the daily protein ingestion is matched by a similar amount of protein oxidation. In addition, protein ingestion has the greatest satiating effect among the three macronutrients. By contrast, lipid balance is not regulated because lipid oxidation. Overfeeding with a mixed diet shows that fat oxidation is negatively correlated with energy and fat intakes.(ABSTRACT TRUNCATED AT 250 WORDS)