Hormonal Cycling And Substrate Oxidation At Rest And During Exercise: A Systematic Review And Meta-analysis

Abstract

Fluctuations of endogenous and exogenous sex hormones across the menstrual cycle (MC) and oral contraceptive (OC) cycle have been suggested to influence rest and exercise substrate metabolism in females. However, there is no clear consensus on the relationship between substrate oxidation and MC or OC phase in females. PURPOSE: This systematic review and meta-analysis aimed to examine the impact of MC and OC cycle phase on substrate oxidation in females at rest and during moderate-intensity continuous exercise. METHODS: Five databases were searched to identify studies including female participants assessed during ≥2 hormonal cycle phases and substrate oxidation determined at rest and/or during moderate-intensity continuous exercise via indirect calorimetry. Data were excluded if substrate metabolism was experimentally manipulated, collected earlier than 5 min into exercise, or if a physiological “steady-state" could not be achieved during exercise. Pairwise meta-analyses examined the effect of MC phase, follicular versus luteal, and OC phase, inactive versus active, on the standardized mean difference (SMD) of each substrate oxidation measurement. Study quality was assessed using a modified Downs and Black checklist, and certainty of the evidence was determined using GRADE criteria. RESULTS: 51 eligible trials were identified for analysis. Analysis of MC phases from 24 studies for resting trials (n = 487) and 23 studies for exercise trials (n = 441) observed that RER was unaffected in the luteal versus follicular phase at rest (SMD = 0.03, 95% CI = -0.16, 0.21, P = 0.78; very low certainty) or during exercise (SMD = -0.01, 95% CI = -0.20, 0.19, P = 0.95; very low certainty). Analysis of OC phase from 7 studies (n = 122) found that RER was lower in the active versus inactive OC phase during exercise (SMD = -0.40, 95% CI = -0.77, -0.03, P = 0.03; low certainty). CONCLUSIONS: We found no effect of MC phase on substrate oxidation at rest or during moderate-intensity continuous exercise. The active OC phase may be associated with greater lipid oxidation during moderate-intensity continuous exercise than the inactive OC phase, indicating a possible effect of exogenous rather than endogenous hormones on substrate oxidation during exercise.