To verify safety respect to weight loss, cardiometabolic diseases of short-term Very low-calorie ketogenic diets (VLCKDs, <800 kcal day-1).Randomized cross-over trial with placebo. The study had no. 2 dietary treatment (DT), conducted in two arms: (1) VLCKD1 in which 50% of protein intake is replaced with synthetic amino acids; (2) VLCKD2 with placebo. The VLCKDs (<800 kcal day-1) were different in term of protein content and quality each arm lasted three weeks (wks). Between the two arms a 3-wks washout period was performed to avoid additive effects on DT to follow. At the baseline, at start and end of each arm, all the subjects were evaluated for their health and nutritional status, by anthropometric analysis, body composition (Dual X-ray Absorptiometry (DXA), Bioimpedentiometry, biochemical evaluation, and Peroxisome Proliferator-Activated Receptor γ (PPAR) γ expression by transcriptomic analysis.After VLCKD1 were reduced: Body Mass Index (BMI) (Δ%=-11.1%, p=0.00), Total Body Water (TBW) (p<0.05); Android Fat Percentage (AFP) (Δ%=-1.8%, p=0.02); Android Fat Mass (AFM) (Δ%=-12.7%, p=0.00); Gynoid Fat Mass (GFM) (Δ%=-6.3%, p=0.01); Intermuscular Adipose Tissue (IMAT) (Δ%= -11.1%, p=0.00); Homeostasis Model Assessment of Insulin Re-sistance (HOMA-IR) (Δ%=-62.1%, p=0.01). After VLCKD1 a significant increase of uricemia, cre-atinine and aspartate aminotransferase (AST) (respectively Δ%=35%, p=0.01; Δ%=5.9%, p=0.02; Δ%=25.5%, p=0.03). After VLCKD2 were reduced: BMI (Δ%=-11.2%, p=0.00); AFM (Δ%=-14.3%, p=0.00); GFM (Δ%=-6.3%, p=0.00); Appendicular Skeletal Muscle Mass Index (ASMMI) (Δ%=-17.5%, p=0.00); HOMA-IR (Δ%=-59,4%, p=0.02). After VLCKD2, uricemia (Δ%=63.1%, p=0.03), and Vitamin D levels (Δ%=25.7%, p=0.02) were increased. No significant changes of car-diovascular disease (CVD) indexes were observed after DTs. No significant changes of PPARγ lev-el in any DTs.21-days VLCKDs not impair nutritional state; not cause negative changes in global measurements of nutritional state including sarcopenia, bone mineral content, hepatic, renal and lipid profile.