Effects of age on energy expenditure and substrate oxidation during experimental overfeeding in healthy men

Relatively little is known about the influence of age on energy regulation during energy imbalance. We compared the effects of overfeeding on changes in energy expenditure, substrate oxidation, and energy deposition between young men (age 23.7 +/- 1.1 [SEM] years) and older men (age 70.0 +/- 7.0) of normal body weight who were leading unrestricted lives. Changes in total energy expenditure, resting energy expenditure (REE), the thermic effect of feeding (TEF), respiratory quotient (RQ), and body energy content were determined in response to overeating by 4.09 +/- 0.07 Megajoule (MJ)/day for 21 days in 16 healthy subjects consuming a typical diet. After excluding data from one young subject with unusual results and adjusting for individual differences in excess energy intake, there was a tendency towards a smaller increase in REE in older men compared to the young men (p = .07) which was accounted for by their lower fat-free mass (p = .016). There was also a significantly smaller increase in resting energy expenditure averaged over fasting and fed states (i.e, REE + TEF) with overfeeding in older men than in young men (p < .01). Combined, these smaller increases in energy expenditure with overfeeding in the older subjects averaged an estimated 365 kilojoule (kJ)/day (8.9% of the excess energy intake) (p < .02). There were also significant effects of age on fasting RQ (p < .001) and the change in RQ with overfeeding (p < .001), but no significant increase in energy expenditure for physical activity and thermoregulation with overfeeding in either age-group. These results are consistent with the suggestion that older individuals experience both a reduction in the ability to increase energy expenditure, and an alteration in the pattern of substrate utilization, in response to overfeeding. These changes may promote cumulative increases in body energy during normal cycles of positive energy balance unless compensated for by adaptive variations in energy intake.