A disclosure before we go further: I develop soup cleanse products, so read the evidence sections closely and check my claims against the sources. Where the research is thin, I’ve said so.
What the scale change is made of
Three days of reduced eating takes off a few pounds, and the breakdown of that weight tells you more than the number. In a seven-day complete fast published in Nature Communications in 2025, participants lost 5.8 kilograms: 4.6 kilograms of lean mass and 1.4 of fat. Lean mass counts everything that isn’t fat, so it includes water and glycogen as well as muscle. The water comes from three places: liver glycogen and the water stored with it, sodium and the water that follows it, and less food in the gut. Muscle glycogen barely moves in a short fast. Most of the water returns within days of normal eating, which is why the scale looks different a week later, as the post on why the weight comes back explains.
Where muscle enters the picture
Liver glycogen runs out first, within 18 to 24 hours of reduced intake. Once that happens, the body leans on two things to keep blood sugar steady: fat, and gluconeogenesis, which manufactures new glucose partly from amino acids. That second part is why muscle loss can’t be dismissed outright. Some protein turnover does increase early in a fast.
But the studies that show meaningful muscle loss run much longer than three days. A tightly controlled ten-day water fast found a marker of muscle protein breakdown rising until day five before the body’s protein-sparing response caught up and it started falling again. That is the shape of the risk: it builds over days and does not flip on like a switch. At three days the body is still drawing mostly on glycogen and fat, with tissue a small part of the supply.
What the 72-hour studies measured
In a 2014 study in PLOS One, eight healthy men were studied twice, once after an overnight fast and once after 72 hours without food. They drank water and went about ordinary activity, with no exercise. After 72 hours, net release of the amino acid phenylalanine from forearm muscle had risen, a sign of protein breakdown, and signaling through mTOR in the muscle had fallen. The authors note that insulin resistance in muscle and the production of ketones spare glucose and limit protein breakdown during a fast, and that fasting still causes a progressive loss of protein over time. The autophagy post covers the same mTOR signal.
Reviews of the field add that fasts of 12 to 72 hours lower the rate of protein synthesis in the body and the legs while raising breakdown. These are changes in rates, measured in a laboratory. By 72 hours the shift toward breakdown is measurable, and it is small against the size of the muscle.
What seven days of fasting did to strength
The 2025 seven-day study measured the thing readers care about. Maximal isometric and isokinetic leg strength was unchanged after seven days with no food. Peak oxygen uptake fell 13 percent, and high-intensity endurance capacity fell 10 to 15 percent, because the body burned less carbohydrate. Strength held while hard, sustained effort got harder. The paper’s background adds that grip strength stays well preserved through the first two weeks of fasting and declines after that.
That fits the guide’s instruction to skip hard training. What drops first is the capacity for demanding work, and the muscle that produces strength stays.
Protein is the lever
A 2024 trial in Clinical Nutrition isolated what protein does. Protein intake was held at 1.0 gram per kilogram a day across fasting and fed days, and daily muscle protein synthesis came out the same on alternate-day fasting, on continuous energy restriction, and on a normal diet, at 1.18, 1.13, and 1.18 percent a day.
The adult RDA is 0.8 grams per kilogram, which is 56 grams for a 70-kilogram adult. A cup of cooked lentils carries about 18 grams of protein and 230 calories, roughly a third of that day’s RDA. Strained vegetable broth carries close to none, and one brand’s bone broth lists 9 grams a cup. How much protein a soup cleanse supplies depends entirely on what goes in the pot.
Why a soup cleanse differs from the water-fast numbers
Everything above describes water fasting: zero calories, nothing but water. A soup cleanse leaves some food in the day. Broth carries some protein, and the calories from blended vegetable soups, however modest, reduce how hard the body has to lean on gluconeogenesis to begin with. The water-fast research is the more conservative comparison, and even there, three days doesn’t reach the range where researchers see the concerning numbers.
What would change this
Duration is what determines this. It is the same reasoning behind not recommending the 5-day version to first-timers: more days means more time on that same curve, and the body’s protein-sparing response needs time to catch up to early proteolysis. Someone doing this for the first time gets the gentlest version of the mechanism described here. Someone stacking cleanses back to back, or extending well past five days, is running a different experiment.
What it adds up to
The weight that comes off in three days is mostly water and a little fat, with too little muscle to justify restructuring your protein intake. If you want to protect against even the small, early proteolysis, drink your broth and eat the protein in your soups. Protein intake is the lever the 2024 trial isolated, and three days isn’t long enough for muscle to be the thing to worry about.
Common questions
- Will a 3-day soup cleanse make you lose muscle?
- Not meaningfully. The studies that show meaningful loss run much longer than three days, and in a seven-day complete fast maximal leg strength did not change.
- Can you work out during a soup cleanse?
- In a seven-day complete fast, strength held and high-intensity endurance fell 10 to 15 percent. The guide says to skip hard training.
- How much protein do you need during a soup cleanse?
- No study has set a figure for a three-day cleanse. The adult RDA is 0.8 grams per kilogram, 56 grams for a 70-kilogram adult, and a cup of cooked lentils carries about 18 grams.