Here’s a good study of PSMF (Van Gaal LF, Snyders D, De Leeuw IH, Bekaert JL. Anthropometric and calorimetric evidence for the protein sparing effects of a new protein supplemented low calorie preparation. Am J Clin Nutr. 1985;41:410-540).
A commercial protein sparing modified fast (PSMF) preparation has been evaluated for the protein sparing effects in 15 morbidly obese patients. During a 500 kcal preparation, given during a 6-week period, mean body weight and BMI decreased significantly. Total body fat decreased from 55.8 to 41.4 kg and lean body mass and arm muscle circumference (AMC) remained unchanged. Using indirect calorimetry and under the same degree of energy expenditure, carbohydrate metabolic consumption was significantly diminished (166 to 61 g/24 hr; p less than 0.001) but fat consumption was increased (116 to 155 g/24 hr; p less than 0.05) while the metabolic turn-over of protein was unchanged. This new presented PSMF preparation seems to present the typical properties of a protein sparing modified fast.
Here’s another study (Bistrian DR, Winterer J, Blackburn GL, Young V, Sherman M.. Effect of a protein-sparing diet and brief fast on nitrogen metabolism in mildly obese subjects, J Lab Clin Med. 1977 May;89(5):1030-5).
Five young, mildly obese females consumed a formula diet providing total calories at 1.2 X basal energy expenditure and egg white protein, 1.5 gm./kg. ideal body weight (IBW), for 1 week (period 1). During period 2, lasting 3 weeks, a protein-sparing modified fast (PSMF) consisted of similar amounts of egg white without nonprotein calories followed by a 1-week total fast (period 3). In the final period (4), a PSMF with the use of meat protein, 1.5 gm./kg. IBW, alone was given for 1 week.
Nitrogen balance (Nbal) measurements were made in all periods, and periods 2, 3, and 4 were compared with period 1. No significant difference existed between period 1 and period 2, although Nbal improved weekly during period 2. Nitrogen excretion in period 3 was similar to that found after 3 weeks of total fasting and was significantly negative, whereas positive balances occurred in period 4 with both periods being significantly different from period 1.
Thus nitrogen equilibrium in the PSMF can be achieved with the use of protein free from fat, confirming earlier experience with meat protein. The metabolic adaption which occurs during total fasting and results in reduced nitrogen loss develops in the course of PSMF. After a deficit in lean body mass is produced, net protein anabolism can be achieved by a PSMF despite insufficient dietary energy.
Here’s another study (Chang J, Kashyap SR. The protein-sparing modified fast for obese patients with type 2 diabetes: what to expect. Cleve Clin J Med. 2014;81:557-565).
Here’s the second figure from the paper: