Sparing Muscle While Losing Fat: A Narrative Review of Ketogenic Diets and Lean Mass Preservation During Weight Loss

Background/Objectives: Clinically meaningful weight loss reduces fat mass but is commonly accompanied by loss of fat-free mass (FFM). Ketogenic diets are of interest because ketone bodies have plausible anabolic and anti-catabolic actions. This narrative review examined whether these mechanisms translate into preservation of lean compartments or skeletal muscle during weight loss in clinical studies. Methods: MEDLINE was searched via PubMed from database inception to 14 August 2026 using terms related to ketogenic exposure, body composition, muscle metabolism, and weight loss. Evidence was synthesised according to dietary context, energy and protein intake, population, exercise exposure, measurement modality, and timing relative to carbohydrate restriction or reintroduction. Results: Acute human studies demonstrate biological effects of ketone bodies on muscle protein turnover and signalling, but these findings do not establish long-term tissue preservation during ketogenic weight-loss diets. Supervised very-low-energy ketogenic diets generally produce weight loss composed predominantly of fat mass; however, comparative meta-analyses report small yet larger reductions in FFM or lean body mass than non-ketogenic regimens. Because most studies used hydration-sensitive compartment methods rather than direct anatomical imaging of skeletal muscle, the contributions of glycogen, water, and non-muscle lean tissues remain uncertain. Energy deficit, protein intake, resistance exercise, baseline adiposity, age, metabolic health, and training status may modify the observed response. Conclusions: Current evidence is insufficient to determine whether ketosis itself has either a protective or harmful effect on contractile skeletal muscle during weight loss. Ketogenic diets may be effective short-term options for selected adults with obesity, particularly within supervised protocols, but adequate protein, resistance exercise, and monitoring of strength and physical function remain the best-supported muscle-preserving strategies.