Objective. - To investigate the effects of aging on mechanisms of body energy regulation and thereby determine the causes of unexplained weight loss in older persons, a factor predisposing to premature death and disability. Design. - Dietary intervention study. Setting. - Metabolic ward and outpatient. Participants. - The subjects were 35 healthy younger and older men of normal body weight consuming a diet of typical composition and performing usual activities. Main Outcome Measures. - Subjects were either overfed by a mean (±SD) of 4.09 (±0.26) MJ/d (n=17) or underfed by 3.17 (±0.68) MJ/d (n=18) for 21 days. Measurements were made of changes in body weight, body composition, and energy expenditure during overfeeding or underfeeding, and of subsequent voluntary nutrient intakes and changes in body weight. Results. - There was no significant effect of aging on changes in body composition, body weight, or energy expenditure with overfeeding or underfeeding. However, following overfeeding, younger men exhibited spontaneous hypophagia, whereas the older men did not (mean [±SD] changes in energy intake relative to control values were -2.11 [±2.18] and 1.55 [±2.11] MJ/d, respectively; P=.006). As a result, the younger men lost the excess body weight gained during overfeeding but the older men did not. Similarly, following underfeeding, the younger men exhibited hyperphagia while the older men did not (mean [±SD] changes in energy intake relative to control values were 1.88 [±2.31] and -0.52 [±1.54] MJ/d, respectively; P=.02), and as a result the older men failed to regain the weight lost during underfeeding. Conclusions. - These results in 35 men suggest that aging may be associated with a significant impairment in the ability to control food intake following overeating or undereating. Since overeating and undereating occur routinely as part of the normal pattern of energy regulation, the findings reported herein may help to explain the vulnerability of older persons to unexplained weight gain and weight loss.
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