Duration-Dependent Changes in Body Composition During Prolonged Fasting: A Systematic Review and Meta-Analysis

Prolonged fasting has gained increasing attention as a non-pharmacological strategy for weight reduction and metabolic health improvement. However, the magnitude and composition of fasting-induced weight loss remain unclear, particularly regarding the relative contributions of fat mass, fat-free mass, and total body water, as well as the role of fasting duration.

A systematic review and meta-analysis was conducted to quantify the effects of prolonged fasting on body composition outcomes.

The PubMed, Web of Science, and Scopus databases were systematically searched for eligible studies reporting pre-post changes in body composition. Standardized mean changes were calculated as Hedges' g and pooled using random-effects models. Duration effects were evaluated using subgroup analyses (≤3 days vs >3 days) and meta-regression with fasting duration as a continuous moderator. Subgroup analyses were also conducted according to baseline weight status (obese vs. healthy/normal weight) and body composition assessment method.

A total of 49 studies met the inclusion criteria, contributing 150 independent effect sizes across eligible outcomes. Prolonged fasting produced significant reductions in body weight (g = -0.38) and BMI (g = -0.42). Decreases were also observed in fat mass (g = -0.25), fat-free mass (g = -0.51), total body water (g = -0.68), and waist circumference (g = -0.55). Subgroup analyses indicated that body weight reductions were greater in obese participants than in healthy individuals and were more pronounced when fasting duration exceeded 3 days. Meta-regression analyses did not reveal significant duration-response relationships for BMI, fat mass, or fat-free mass, although a non-significant trend toward greater body weight reduction with longer fasting duration was observed.

Prolonged fasting reduces body weight, BMI, fat mass, and waist circumference but is also associated with substantial losses in fat-free mass and total body water, indicating that fasting-induced weight loss reflects complex changes in body composition rather than isolated fat loss. No clear duration-dependent effects were observed for most outcomes.