Protein-sparing modified fast in the treatment of severe obesity: weight loss and nitrogen balance data

Summary

This 1980 Italian study addressed a specific operational question in PSMF design: how much protein is enough to spare nitrogen during severe caloric restriction? Twenty-five severely obese patients (16 women, 9 men) were assigned to one of four 4-week conditions: total fasting; an 80 kcal-PSMF (about 17 g protein per day); a 180 kcal-PSMF (about 40 g protein per day); or an alternating 80/180 kcal regimen. The researchers measured weight loss and nitrogen balance carefully across all four protocols. Both PSMF arms produced rapid weight loss comparable to total fasting, but the higher-protein conditions (40 g/day, with or without the lower-protein alternating phases) produced substantially less negative nitrogen balance. Nitrogen loss was significantly reduced from the third week of treatment onward, demonstrating that the metabolic adaptation that protects body protein takes time to engage and that adequate protein intake during that window matters disproportionately. The paper helped establish dose-response thinking in PSMF protocols — protein intake is not a binary "supplemented vs not" variable but a graded one with thresholds.