Temporal changes in physiological parameters in Chinese adults during a month-long 5:2 fasting regimen: an exploratory observational study.

BACKGROUND: The 5:2 intermittent fasting regimen (5 days of unrestricted eating + 2 days of strict calorie restriction) has been widely adopted as a weight management strategy. However, the short-term temporal changes in body composition and its sex-specific effects on body composition remain unclear. Particularly, the relationship between weight loss and lean body mass during the weight reduction process has not been sufficiently studied. METHODS: This retrospective cohort analyzed serial body composition data from 109 overweight/obese adults (28 Men/81 Female; age 31.4 ± 7.5 vs. 34.4 ± 9.9, p = 0.193) undergoing 4-week 5:2 fasting with weekly monitoring. Exclusions: age <18y or ≥60y, treatment <4 weeks, or irregular follow-up. RESULTS: Both sexes exhibited linear reductions in weight, body fat percentage, and visceral adipose tissue (p < 0.05). A modest decline in lean mass occurred, indicating that muscle preservation deserves attention in future studies. Weight loss correlated with baseline muscle mass (r 2 = 0.060, p = 0.027) and basal metabolic rate (BMR; r 2 = 0.059, p = 0.029) in female, whereas no such associations were found in males (p > 0.05). Age showed no significant association with outcomes (p > 0.05). CONCLUSION: The 5:2 regimen induces controlled multi-parameter improvements in adiposity metrics, with a concomitant trend toward lean mass reduction that merits attention. Baseline muscle mass and BMR were significant correlates of weight loss in females, suggesting that muscle-centric strategies may be particularly relevant for this subgroup. In males, weight loss appeared to be primarily associated with energy intake regulation rather than baseline body composition. These findings are observational and hypothesis-generating; causality cannot be inferred. Future prospective studies are warranted to validate these sex-specific associations.