Age-related decline in RMR in physically active men: relation to exercise volume and energy intake. (on-line article)

Rachael E. Van Pelt, Frank A. Dinneno, Douglas R. Seals, and Pamela Parker Jones

Human Cardiovascular Research Laboratory, Center for Physical Activity, Disease Prevention and Aging, Department of Kinesiology and Applied Physiology, University of Colorado, Boulder 80309; and Department of Medicine, Divisions of Cardiology and Geriatric Medicine, University of Colorado Health Sciences Center, Denver, Colorado 80262

Am J Physiol Endocrinol Metab 281: 633-639, 2001

The primary new findings from the present study are as follows. First, RMRadj {RMR was measured by indirect calorimetry (ventilated hood system) after an overnight fast and ~24 h after exercise. Because RMR is related to fat-free mass (FFM; r = 0.76, P < 0.001, current study), FFM was covaried to adjust RMR (RMRadj).} declines with age in men who are highly physically active. Second, the lower RMRadj in the older compared with young physically active men is associated with a lower exercise volume and lower estimated daily energy intake. Finally, it appears that both exercise volume and estimated energy intake are independently related to RMRadj.

It should be noted that FFM remains the primary determinant of the age-related decline in RMR. The present results extend our understanding of the age-related decline by adding insight into other factors that influence RMR after FFM has been accounted for.

In the present study, RMR was strongly and directly related to FFM, as observed previously in sedentary male populations (7, 18). The lower RMR with age in the physically active men, however, was not due to a lower FFM, because our analysis was adjusted for this important influence. Rather, our results indicate that RMR per unit FFM is reduced with age in physically active adult males. Because all subjects were euthyroid, and thyroid hormone levels were similar among groups, differences in thyroid function do not explain our observed differences in RMR. Thus some intrinsic decline in the rate of cellular respiration appears to occur with age in both sedentary and active men.

Recently, we reported that, in contrast to the present findings in men, RMR adjusted for body composition does not decline significantly across age in healthy women who regularly perform endurance exercise (24). These physically active women, however, did not differ with respect to exercise volume or energy intake. In the present study, physically active men were similarly recruited but were much more varied in their exercise volumes and energy intakes. Such variability allowed us to examine the potential influence of these two factors on RMR.

We first hypothesized that an age-associated decline in exercise volume might play a role in the lower RMRadj with age in the active men. This hypothesis was supported by the observation that there was no age-related difference in RMRadj in subgroups of young and older physically active men matched for total weekly hours of endurance exercise. It should be noted that subjects were not matched for exercise intensity, and it is likely that the young subjects exercised at a higher intensity than the older subjects did, because their energy intake was significantly higher. Thus they were likely expending more energy per hour of exercise. Nevertheless, our results are consistent with the findings in young males of Tremblay et al. (23), who used a similar quantification of exercise volume and reported that RMR was higher in a subgroup who exercised 12-16 h/wk compared with a subgroup exercising 6-10 h/wk.

We additionally hypothesized that age-associated declines in energy intake might play a role in declines in RMRadj with age in active men. In the present study, reductions in daily energy intake across age were the single strongest univariate correlate of age-associated differences in RMRadj. Moreover, when subgroups of the men matched for estimated energy intake were compared, there were no age-related differences in RMRadj. Furthermore, both estimated energy intake and exercise volume remained significantly correlated with RMRadj when the other was held constant by use of partial correlation analysis, supporting the concept that exercise volume and energy intake may exert independent influences on age-associated differences in RMRadj.

The common link between RMRadj, exercise volume, and energy intake may involve the concept of "energy flux." Bullough et al. (2) reported that young endurance exercise-trained males studied in a high state of energy flux (i.e., high exercise-induced energy expenditure matched by high daily energy intake) had an elevated RMR compared with a low energy flux state. Thus, in the present study, it is possible that the older physically active men were in a relatively lower state of energy flux than their younger counterparts, who exercised more and consumed more energy. Our observation that subgroups of men matched for exercise volume or energy intake do not differ with respect to RMRadj supports this possibility. A comprehensive study of all components of energy balance could add further insight into potential age and exercise-related changes in energy flux.

In the present study, RMRadj and O2 max were positively related in the overall population (r = 0.42, P < 0.001). However, it seems unlikely that there is a direct causal relationship between these two variables, as there were no differences in RMRadj between the young and older physically active men matched for either exercise volume or energy intake despite lower O2 max in the older men. It may be that there is a genetic component of energy-consuming processes that is common to both O2 max and RMRadj and thus, at least partially, determines their relation.

There are at least three limitations to the present study that should be noted. First, as with all cross-sectional studies, we cannot discount the possibility that genetic or constitutional factors influenced our findings. Second, there is error inherent in the self-reported measures of energy intake. However, we do not have reason to believe that under- or overreporting of dietary intake was more prevalent in one group compared with another. Thus relative differences among the four groups should be valid. Furthermore, a recent study has related energy intake to basal metabolic rate (EI/BMR) to categorize subjects as either under (EI/BMR <1.35)- or over (EI/BMR >2.4)-reporters (10). Our young and older physically active men appear to be reporting energy intakes accurately on the basis of EI/RMR ratios of 1.69 0.05 and 1.68 0.04, respectively (not different, P = 0.83). Third, our quantification of exercise volume is based on duration of endurance exercise/week and does not quantify potential differences in exercise intensity between young and older subjects. It is likely that young subjects expended more energy for any given exercise volume due to higher intensity of exercise.

In conclusion, the results of the present study support the idea that RMRadj declines with age in men who regularly perform endurance exercise. Moreover, it appears that age-associated reductions in exercise volume and daily energy intake contribute to this decline in RMRadj. Our results have important physiological and clinical implications regarding the role of regular endurance exercise in the prevention of age-related obesity. In men, body weight and fatness increase with age and are associated with increased morbidity and premature mortality from cardiovascular and metabolic diseases. Declines in energy expenditure that occur disproportionate to declines in energy intake likely contribute to these age-related increases in body weight and fatness. Although RMRadj declined with age in our physically active men in conjunction with declines in exercise volume and energy intake, RMRadj did not differ between young and older physically active men who performed the same volume of endurance exercise (in terms of hours/week, regardless of intensity) and/or consumed the same number of calories per day. This would suggest that men who are able to maintain high levels of exercise and energy intake with age might better maintain RMR (per unit of fat-free tissue), which in turn might play a role in their lesser increase in body weight and fatness with age. This is supported by our observation of smaller age- related differences in percent body fat in the subgroups matched for energy intake and exercise volume, as well as similar RMRadj and adiposity in older physically active and young sedentary subjects. Furthermore, the higher energy diet appropriate for physically active older adults increases the likelihood that older physically active individuals can meet their daily micronutrient needs.

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