Influence of 8-week endurance training on resting energy expenditure and body composition in women

Endurance training potentially induces changes in resting energy expenditure (REE), body composition, and metabolic hormones, but well-controlled studies accounting for hormonal status of women are sparse on this topic. This study investigated how 8-week moderate intensity endurance training (MIET) affects these outcomes in recreationally active women. Naturally menstruating women (NoOC, n = 17) and combined oral contraceptive users (COC, n = 8) were assessed in the luteal/follicular phases and active/inactive phases before and after MIET. REE and respiratory exchange ratio (RER; by indirect calorimetry), body composition, estradiol, progesterone, free triiodothyronine (fT3), acylated and unacylated ghrelin were measured at rest. Linear mixed models with time, group, and their interaction were used to analyze the results in two stratified phase comparisons. No interaction effects were observed, but a main effect of time for RER was observed in the pooled sample, indicating a 5.6% increase (p = 0.004) at follicular/inactive phase. The COC group also had higher fT3 concentrations at all time points (p < 0.05). Two participants from different groups showed consistent ≥10% REE changes. An 8-week MIET did not induce detectable changes in REE, body composition, or metabolic hormones on a group level, but high individual variation was observed.