Moderate cardiorespiratory fitness is positively associated with resting metabolic rate in young adults.

OBJECTIVE To determine whether moderate cardiorespiratory fitness (CRF) or moderate to vigorous physical activity (MVPA) is associated with elevations in resting metabolic rate (RMR) similar to findings previously observed in endurance athletes. PARTICIPANTS AND METHODS Using a cross-sectional design, we measured CRF, RMR, body composition, energy expenditure, and time in MVPA via an arm-based activity monitor in 423 young adults (mean age, 27.6 years). Based on the results of a fitness test, participants were classified into CRF tertiles (low, moderate, or high) by sex. RESULTS There were significant differences among the low-, moderate-, and high-CRF groups for mean ± SD body mass index (calculated as the weight in kilograms divided by the height in meters squared) (28.1 ± 4.1, 25.1 ± 3.4, and 23.6 ± 2.5, respectively; P<.001) and fat mass (28.8 ± 9.7, 20.5 ± 8.2, and 14.8 ± 6.5 kg, respectively; P<.001) but not fat-free mass (53.1 ± 11.5, 53.5 ± 12.4, and 54.7 ± 12.1 kg, respectively; P=.49). There were no differences in mean ± SD unadjusted RMR among the groups (1533.2 ± 266.2, 1519.7 ± 267.6, and 1521.9 ± 253.9 kcal/d, respectively). However, after statistical adjustment for differences in body composition, the moderate- and high-CRF groups had a higher RMR compared with low-CRF individuals by 39.7 and 59.9 kcal/d, respectively (P<.05). After further adjustment for MVPA, RMR was higher in the high-CRF group compared with the low-CRF group by 51.2 kcal/d (P<.05). CONCLUSION In this large sample of young adults representing a range of CRF, there was a positive stepwise gradient in RMR across tertiles of CRF independent of body composition. Also, MVPA was independently associated with RMR, although this relationship was modest. These findings underscore the multidimensional role of CRF and MVPA on health. TRIAL REGISTRATION clinicaltrials.gov Identifier: NCT01746186.