Effects of One-Month Very-Low-Calorie Ketogenic Diet on 24 h Energy Metabolism and Body Composition in Women with Obesity

Context: Very-low-calorie ketogenic diet (VLCKD) is used for weight loss and management of obesity-related comorbidities. Objective: We aimed at evaluating the effects of VLCKD on body composition and energy metabolism. Methods: This prospective outpatient study included 17 women with obesity (mean age 41.6 yrs; BMI 37.5 kg/m2) who followed a 1-month VLCKD (700-800 kcal/day, CHO 11%, fat 46%, protein 43%) at the University Hospital of Pisa. Measurements of 24-hour energy expenditure (24hEE) and substrate oxidation were conducted in a metabolic chamber at day 1 (V1), day 8 (V2), and day 29 (V3). Body composition was assessed by DXA. Twenty-two women with obesity fed a balanced isocaloric diet served as controls. Results: Compared to controls, carbohydrate oxidation (CarbOx) was lower, whereas fat oxidation (FatOx) and protein oxidation (ProtOx) were higher in the VLCKD group at V1. CarbOx decreased by 65%, while FatOx increased by 11% at V3. The rate of protein oxidation was already higher than in controls at V1 and remained stable throughout the study. After 1 month, body weight decreased by 7%, reflecting an 8.8% reduction in fat mass (FM) and a 5.6% reduction in lean soft tissue (LST). A 10% decrease in 24hEE and 24 h sleeping metabolic rate was observed at V3 compared to V1. Conclusion: VLCKD promotes weight loss in women with obesity. Our findings highlight the shift in energy metabolism towards increased fat oxidation accompanied by a modest increase in protein oxidation, a decrease in LST and a reduction in EE.