Short-Term Effects of a Low-Energy Ketogenic Diet versus a Mediterranean Diet on Body Composition Parameters in Adults with Obesity

Background Low-energy ketogenic diets (LEKD) are increasingly used for rapid weight loss in clinical practice, but short-term body-composition effects versus hypocaloric Mediterranean diets remain undefined. This study assessed whether LEKD better preserves fat-free mass (FFM) than a low-energy Mediterranean diet (LEMD) in adults with obesity requiring rapid weight loss. Methods In this prospective quasi-experimental study, 50 adults (BMI ≥30 kg/m 2 ) were allocated to LEKD (800–900 kcal/day; <50 g/day carbohydrates) or LEMD (1200–1400 kcal/day) based on a structured questionnaire and preference; 31(18 LEKD, 13 LEMD) completed 6 weeks intervention. Anthropometry and bioelectrical impedance were measured at baseline (T0), 7–10 days (T1), and 45 days (T2). Linear mixed models tested diet, time, and interaction effects. Results At T2, ≥5% weight loss was achieved by 94.4% of LEKD vs 46.2% of LEMD participants; ≥10% weight loss occurred in 16.7% versus 0% (p=0.005). LEKD produced greater decreases in body weight (β=-1.85 kg; p=0.038), BMI (β=-0.62 kg/m 2 ; p=0.0026), and abdominal circumference (p=0.0086). Fat mass declined over time in both groups (p<0.0001) with no significant between-diet difference. FFM decreased slightly, with a greater reduction in LEKD (β=-1.29 kg; p=0.0139). Total body water percentage increased and phase angle decreased (p=0.0084), regardless of diet. Mild gastrointestinal and neurological symptoms were similar between groups. Conclusion Both diets improved short-term weight and fat mass. LEKD achieved faster abdominal and total weight reduction but at the cost of a small additional FFM loss, supporting short, supervised use before longer-term balanced diet and physical activity to preserve function.