Introduction Resting energy expenditure (REE) may fluctuate during the menstrual cycle (MC), due to the physiological effects of estradiol (E2) and progesterone. This study examined changes in REE and metabolic hormones (leptin, ghrelin, thyroid hormones), and dietary intake in two hormonally distinct groups, naturally menstruating women (NoOC) and women using monophasic combined oral contraceptives (COC). Methods Measurements included REE by indirect calorimetry, body composition by bioimpedance, and blood samples for hormone analysis in the early follicular and midluteal phases of the MC in the NoOC group (n = 38) or the active and inactive phases of the COC cycle (COC; n = 19). Participants recorded their food intake for 3 d after measurements. A secondary analysis was completed for the NoOC group without REE outliers (difference between measurements >1.5 × interquartile range, n = 4). Results In the NoOC group, luteal phase REE was 40 kcal higher than follicular phase REE (95% confidence interval (CI), -2 to 82 kcal·d-1, d = 0.20, P = 0.061). Leptin (d = 0.35, P < 0.001), triiodothyronine (T3; d = 0.26, P = 0.05), and fat intake (d = 0.48, P = 0.027) were higher, and thyroxine (d = 0.21, P = 0.041) was lower in the luteal phase. After excluding outliers, REE was 44 kcal higher in the luteal phase than in the follicular phase (95% CI, 12-76 kcal·d-1, d = 0.22, P = 0.007). In the COC group, the mean difference in REE was -2 kcal (95% CI, -82 to 79 kcal·d-1) between active and inactive phases, whereas T3 was higher in the inactive phase (d = 0.01, P = 0.037). Conclusions REE increases only slightly from the follicular to the luteal phase but remains unchanged between COC phases. Increases in T3, leptin, and fat intake during the luteal phase might echo metabolic fluctuations that parallel female sex hormones during the MC.
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