The Effect Of Morning Or Evening Exercise On Cardiovascular Fitness And Body Weight

Abstract

PURPOSE: To compare the effect of 8 weeks of morning exercise or evening exercise on cardiovascular fitness and body composition in healthy women. METHODS: Fifty-five participants were randomized to either a morning exercise group (AM) (25.3±4.1 yrs; 25.2±4.7 kg/m2; 37.9±7.6 %BF) or an evening exercise group (PM) (25.4±6.7 yrs; 23.5±3.9 kg/m2; 34.1±6.1 %BF). The AM group completed exercise between 6:30-9:30am and the PM group between 6:30-9:30pm, both for 4 days per week (3 days supervised), 45 minutes per session, and for 8 weeks. All participants followed an exercise program of moderate-intensity treadmill walking (40-59% heart rate reserve [HRR]) and progressed to include vigorous-intensity exercise (60-89% HRR). Cardiovascular fitness was determined using a maximal treadmill protocol and indirect calorimetry. Body weight/composition was determined using a digital scale and dual-energy x-ray absorptiometry (DXA). RESULTS: Eighty-two percent of participants completed the study; 21 (78%) from the AM group and 25 (86%) from the PM group, with 94.75% of the prescribed exercise sessions completed. Using intent-to-treat analysis, there was not a group*period interaction for V02peak or time-to-completion during the treadmill test (ps>0.05); however, with groups combined, time-to-completion during the fitness test significantly improved (F=6.66; p=0.013). For completers in the AM group, 76% gained weight while 36% gained weight in the PM group. For completers, body weight increased by 0.79±1.16 kg in the AM group (F=5.05; p=0.0361) and decreased by 0.21±1.46 kg in the PM group. Intent-to-treat analysis showed a significant group*period interaction (F=5.12; p=0.029). This trend persisted with control of baseline body weight (F=5.02; p=0.0301). However, there was not a significant group*period interaction for total body fat (g) or fat-free mass (g) (ps>0.05). CONCLUSIONS: Eight weeks of exercise may improve exercise performance but does not appear to be a function of time of day of exercise. However, these results suggest the possibility that evening exercise may be slightly superior for body weight maintenance. As these differences are small and body composition did not differ between groups, caution should be used when interpreting these data; however, additional research may be warranted.