Prolonged caloric restriction reduces basal metabolic rate and lean mass, which necessitates proportional reductions in refeeding calories to prevent fat regain.

Study: Independent and Combined Effects of Resistance Training and Whey Protein on Skeletal Muscle Mass and Function in Individuals with MASLD Under Caloric Restriction

When people ate much less food for 4 weeks, those who lifted weights didn’t lose muscle, even if they didn’t drink extra protein. This suggests the body might not always lose muscle during dieting, which partly supports the idea that you need to be careful when eating normally again.

Study: Effects of Diet Macronutrient Composition on Weight Loss during Caloric Restriction and Subsequent Weight Regain during Refeeding in Aging Mice

When mice ate much less food for a while and then got to eat as much as they wanted, they gained back more weight — especially if they ate a lot of fat. This shows that eating normally again after dieting can make you gain fat unless you eat less than before.

Study: Low 24-hour core body temperature as a thrifty metabolic trait driving catch-up fat during weight regain after caloric restriction.

When rats ate less for a while and then ate normally again, their bodies stayed colder and burned fewer calories—even though they ate the same as before. This made them gain more fat than muscle, showing why people might gain fat back after dieting if they don’t eat fewer calories when restarting.

Study: Caloric restriction induces energy-sparing alterations in skeletal muscle contraction, fiber composition and local thyroid hormone metabolism that persist during catch-up fat upon refeeding

When people eat very little for a long time, their muscles slow down and burn less energy—even after they start eating normally again. This makes them more likely to gain fat if they eat back to normal amounts.