Resting metabolic rate and diet-induced thermogenesis in young and elderly subjects: relationship with body composition, fat distribution, and physical activity level.

To investigate the relationship between age and energy expenditure, resting metabolic rate (RMR) and diet-induced thermogenesis (DIT; for 180 min after a 1.3-MJ meal) were measured by indirect calorimetry in 56 young and 103 elderly subjects. In addition, the influence of body composition, body-fat distribution, and physical activity level on this relationship was studied. RMR was significantly lower in elderly (3.98 +/- 0.46 and 3.33 +/- 0.39 kJ/min for men and women, respectively) than in young (5.29 +/- 0.53 and 4.08 +/- 0.33 kJ/min for men and women, respectively) subjects, which persisted after adjustment for body composition. DIT was significantly lower in older than in younger men (126 +/- 27 vs 154 +/- 34 kJ/180 min) but not in women (111 +/- 26 vs 115 +/- 25 kJ/180 min). The difference in men disappeared after adjustment for body composition. No clear relation between physical activity level and RMR or DIT was observed. These results demonstrate a relationship of age per se with RMR but not with DIT.