The Effect Of Age On Substrate Utilization During Graded Exercise Testing

Aging is associated with declines in functional capacity and physical activity that contribute to diminished ability to perform tasks of daily life. Previous research indicates a greater reliance on carbohydrate metabolism at ∼50% VO2 max in elderly compared to young subjects. However, substrate utilization profiles are poorly denned over a full range of exercise intensities across the adult age span. PURPOSE: To test the hypothesis that as exercise intensity increases during a graded exercise test (GXT), increasing age is associated with a disproportionately greater reliance on carbohydrate metabolism, as indicated by respiratory exchange ratio (RER). METHODS: We analyzed GXT data for 190 healthy men (20-92 years of age) enrolled in the Baltimore Longitudinal Study of Aging. Mean VO2 for each stage was expressed both as an absolute value and as a percentage of VO2 max. To test our hypothesis, mixed effects regression models were used to assess possible age-by-exercise intensity interactions. RESULTS: Whether exercise intensity was expressed in absolute or relative terms, significant age-by-exercise intensity interaction effects were found, indicating a more rapid elevation in RER with increasing age occurred during GXT (P<0.05). CONCLUSIONS: Older persons exhibited a disproportionately greater reliance on carbohydrate rather than fat metabolism across a full range of exercise intensities. This specific substrate utilization profile may contribute to the greater fatigability, lower levels of physical activity, and loss of functional capacity that characterize many older persons.