Morning or evening? Time of day does not affect body composition, cardiopulmonary performance, and muscle strength responses to combined training in women with obesity

Introduction Obesity is characterized by adipocyte dysfunction, accompanied by impairments in oxidative metabolism and physical function. Although physical training is known to improve oxidative capacity, body composition, and functional performance, the magnitude of these adaptations may be influenced by several modulatory factors. Among these, circadian rhythm has emerged as a potential determinant of exercise-induced adaptations due to daily fluctuations in metabolic and physiological processes; however, its influence in women with obesity remains unclear. Therefore, this study aimed to investigate the effects of combined training performed at different times of the day (morning vs. evening) on body composition, cardiopulmonary performance, and muscle strength in previously untrained women with obesity. Methods This randomized clinical trial (NCT06601660) included women with obesity (waist circumference ≥88 cm) aged 18–50 years. Participants were randomly assigned to a control group (CG; n = 13), a morning training group (MT; training performed between 07:00 and 10:00 a.m.; n = 17), or an evening training group (ET; training performed between 06:00 and 09:00 p.m.; n = 19). Assessments conducted before and after the intervention included body composition (InBody® 770), cardiopulmonary performance evaluated by cardiopulmonary exercise testing (CPET), muscle strength assessed by the one-repetition maximum (1RM) test, and cardiovascular parameters evaluated by blood pressure (BP) and heart rate (HR); the last three were performed at both a.m. and p.m. times. Only participants in the MT and ET groups underwent the intervention, which consisted of an 8-week combined training program performed three times per week. Each session included both resistance and aerobic exercise, with progressive increases in exercise intensity and duration throughout the intervention period. Participants assigned to the CG underwent assessments only. Results Significant group-by-period interactions were observed for VO 2peak and bench-press and leg-press 1RM, whereas maximal treadmill velocity showed a significant period effect without a significant group-by-period interaction. Bench-press 1RM increased in MT during morning and evening testing by 5.79 and 5.31 kg, respectively, and in ET by 4.97 and 5.50 kg (all p < 0.001; d = 2.37–2.76), with gains in both training groups exceeding those observed in CG. Leg-press 1RM increased in MT by 22.20 and 29.01 kg and in ET by 18.40 and 17.11 kg during morning and evening testing, respectively ( p ≤ 0.001; d = 1.13–1.92); however, only MT showed a greater overall change than CG. VO 2peak increased in MT during morning testing [2.42 mL·kg −1 ·min −1 ( p = 0.004; d = 1.04)] and in ET during evening testing [1.57 mL·kg −1 ·min −1 ( p = 0.048; d = 0.67)]. Maximal treadmill velocity and CPET duration improved over time, but their changes did not differ significantly among groups. Body-composition and cardiovascular adaptations were limited, with waist circumference increasing only in CG. Conclusion Eight weeks of combined training performed either in the morning or evening improved maximal muscle strength and cardiopulmonary performance in women with obesity, while producing limited changes in body composition and cardiovascular measures. No consistent evidence of superior adaptations according to training time was observed, suggesting that both schedules can effectively promote health-related adaptations.