Sex Differences in Fat Oxidation Depend on Age, Body Mass Index, and Exercise Modality: A Systematic Review and Meta-Analysis

OBJECTIVE: To synthesize current evidence on sexual dimorphism in maximal fat oxidation (MFO) and its corresponding exercise intensity (FATmax), exploring key moderators influencing these parameters across populations. METHODS: We conducted a systematic review and meta-analysis, registered in PROSPERO (CRD42024563620), following Preferred Reporting Items for Systematic Reviews and Meta-Analyses 2020 guidelines. PubMed, Scopus, and EBSCOhost databases were searched from January 2001 to April 2024 for English- and Spanish-language studies examining MFO and FATmax during graded exercise tests. Studies were included if they reported sex-specific MFO and/or FATmax values derived from indirect calorimetry. Both cross-sectional and longitudinal designs were considered. Exclusion criteria included pooled-sex data, inaccessible full texts, and nonexercise-based MFO assessments. RESULTS: Thirty-six studies (n = 8,990; 5,133 females, 3,857 males) were included. Absolute MFO was significantly lower in females (MD: -0.06 g/min), but no significant differences were observed in relative MFO (standardized mean difference: 0.202, p = .096) or FATmax (mean difference: 1.29% VO2max, p = .254). Cardiorespiratory fitness, body mass index, and exercise modality were key moderators in both sexes. Treadmill exercise was associated with higher MFO, irrespective of sex. Notably, body mass index and age influenced sexual dimorphism in fat oxidation, with age showing a curvilinear (quadratic) relationship with FATmax, suggesting complex age-related adaptations (p = .008). CONCLUSIONS: This review questions the assumption that women consistently oxidize more fat than men during exercise. The influence of sex on fat metabolism appears context-dependent, emphasizing the need for individualized interpretation. Longitudinal studies are required to evaluate whether these differences confer cardiometabolic protection.