Duration-Dependent Body Composition Responses to Water-Only Fasting: A Systematic Review, Meta-Analysis, and Meta-Regression

OpenAlex · English

This project is a systematic review, meta-analysis, and meta-regression designed to comprehensively evaluate the effects of strict water-only fasting on body composition and anthropometric outcomes in adults. Water-only fasting, defined as complete abstinence from caloric intake with unrestricted water consumption, has gained increasing attention as a non-pharmacological intervention for weight loss and metabolic health. Despite growing interest, existing evidence remains fragmented, heterogeneous, and inconsistent regarding the magnitude, consistency, and duration-dependence of body composition changes induced by water-only fasting. The primary purpose of this project is to quantitatively synthesize available human evidence on pre–post changes in body weight, body mass index (BMI), fat mass, fat-free mass (lean mass), total body water, and waist circumference following water-only fasting interventions lasting at least 24 hours. A secondary objective is to examine whether fasting duration moderates these outcomes, both categorically and continuously. To address this aim, fasting duration is evaluated using predefined subgroup comparisons (≤3 days vs. >3 days) and meta-regression models with fasting duration treated as a continuous moderator. The study follows a predefined and transparent methodological framework consistent with PRISMA guidelines. Eligible studies are identified through systematic searches of PubMed, Web of Science, and Scopus, supplemented by manual reference screening. Only peer-reviewed human studies implementing strict water-only fasting protocols and reporting quantitative pre–post body composition or anthropometric data are included. Effect sizes are calculated using Hedges’ g and pooled using random-effects models to account for between-study heterogeneity. To ensure statistical independence, within-study pooling and biomarker-by-study aggregation strategies are applied when multiple outcomes or groups are reported within the same study. Expected outcomes of this project include (1) pooled estimates of the overall effects of water-only fasting on key body composition and anthropometric parameters, (2) clarification of which outcomes demonstrate consistent and meaningful changes across studies, and (3) evaluation of whether longer fasting durations are associated with progressively greater effects. Particular attention is given to distinguishing absolute changes (e.g., fat-free mass and total body water in kilograms) from relative composition measures (percent values), thereby improving interpretability of fasting-induced adaptations. By providing a rigorous quantitative synthesis and explicitly examining duration-dependent effects, this project aims to improve the evidence base surrounding water-only fasting, inform interpretation of body composition changes observed during fasting interventions, and identify gaps requiring further controlled research. The findings are intended to support researchers and clinicians in evaluating the physiological implications of water-only fasting while emphasizing the importance of fasting duration when interpreting reported outcomes.