Energy metabolism in human obesity.

Obesity results from a chronic imbalance between energy intake and expenditure. Accurate measurements of total energy expenditure of lean and obese individuals with a respiration chamber have clearly shown that obese individuals expand more energy than lean sedentary subjects. Studies on the body composition of obese individuals reveal that not only the fat mass is enlarged, but the fat-free mass is also increased as compared with that of lean subjects. Since basal metabolic rate is proportional to the fat-free mass, obese subjects have a greater basal metabolic rate than lean controls. The energy cost of weight bearing activities such as walking and standing is related to body weight, and is therefore increased in obese individuals. The thermogenic response to food ingestion, the diet-induced thermogenesis, has been found to be reduced in some groups of obese people, but not in all obese individuals. The thermic effect of glucose or to meal ingestion is blunted in obese subjects with insulin resistance. Any alteration in thermogenic responses to a caloric excess can be important to store or to oxidize part of the excessive energy intake. After weight reduction in obese subjects due to a hypocaloric diet, the total 24-hour energy expenditure decreases by 20 to 25 kcal/day for each kilogram of weight loss. Failure to adapt the every day energy intake accordingly will result in body weight gain and relapse of obesity.