A protein-sparing modified fast depletes liver glycogen, suppresses insulin levels, and shifts metabolism toward fat oxidation.

Study: The Protein-Sparing Modified Fast Diet

This diet helps people lose weight quickly by making the body burn fat instead of carbs, which means less sugar and insulin in the blood. The study showed teens lost a lot of weight on this diet, which only happens when the body switches to burning fat.

Study: Feasibility of protein-sparing modified fast by tube (ProMoFasT) in obesity treatment: a phase II pilot trial on clinical safety and efficacy (appetite control, body composition, muscular strength, metabolic pattern, pulmonary function test)

This diet made people lose weight mostly from fat, not muscle, and their bodies started burning fat for energy instead of sugar — which means insulin went down and liver sugar stores were used up.

Study: Effect of diet on the metabolic response to infection: protein-sparing modified fast plus 100 grams glucose and yellow fever immunization.

When people eat very little sugar and mostly protein, their body switches to burning fat for energy and keeps insulin low — this study shows that’s what happens with this diet, even when the body is fighting an infection.

Study: Nitrogen Metabolism and Insulin Requirements in Obese Diabetic Adults on a Protein-Sparing Modified Fast

This diet gives you just enough protein to keep your muscles but almost no carbs or calories, so your body runs out of stored sugar and starts burning fat instead — which also makes your insulin levels drop. You can tell because your urine showed ketones (fat-burning byproducts) within a few days.