A disclosure before we go further: I develop soup cleanse products, which is a reason to read the evidence sections closely rather than take my word for it. Where the research is thin, I’ve said so.
What a water fast sets in motion
Liver glycogen runs out within about a day, and the body turns to fat. Fatty acids can’t fuel the brain, so the liver converts them to ketones, which accumulate over the days that follow. Blood ketones reach roughly 4 to 7 millimoles per liter after two weeks of fasting, and in prolonged starvation the brain can take most of its energy, up to about three-quarters, from them.
Ketones leave in the urine alongside sodium, a loss called the natriuresis of fasting. The resulting dip in sodium and fluid can bring dizziness on standing, fatigue, and headache, the same cluster the day-two headache post describes for a cleanse.
Muscle takes part too. In a 2014 study in PLOS One, eight healthy men fasted for 72 hours, and net release of the amino acid phenylalanine from their thigh muscle rose, a sign of protein breakdown. The muscle post covers why three days stays well short of the range where that becomes a concern, and the metabolism post covers resting energy use, which rose in a separate fasting study.
What seven days of complete fasting measured
The most useful recent measurement is a seven-day complete fast published in Nature Communications in 2025. Participants lost 5.8 kilograms: 4.6 of lean mass and 1.4 of fat. Maximal leg strength did not change. Peak oxygen uptake fell 13 percent, and high-intensity endurance capacity fell 10 to 15 percent, because the body burned less carbohydrate. The muscle post and the post on why the weight comes back draw on the same study.
What the best-studied fast allows
Water-only fasting is the version most people picture. The best-documented fasting program is a different thing. In a 2019 study in PLOS One, 1,422 people fasted for 4 to 21 days at a clinic in Germany under the Buchinger guidelines. They drank 2 to 3 liters of water a day and ate about 250 calories: a small glass of juice at noon and a cup of vegetable soup in the evening, which came to 25 to 35 grams of carbohydrate a day. Clinic staff monitored them daily.
Adverse effects were reported in under 1% of participants, there were no fatalities, and about 93% reported no feeling of hunger. That result belongs to a supervised setting with screened participants, and an observational study can’t show that fasting caused the improvements people reported.
Where a soup cleanse sits
Picture a line. At one end, a water fast supplies nothing. A little further along, the Buchinger program supplies about 250 calories. At the far end is ordinary eating. A soup cleanse falls between, and the pot decides where. The 3-day guide itself calls this kind of plan a low-calorie liquid diet and leaves portions to your hunger, so it sits closer to the fasting end than the eating end. A pot with legumes or grains in it moves toward the middle, because it carries protein, more energy, and carbohydrate.
Two things differ from a water fast whatever the recipe. Salted broth puts sodium back, which is why the guide asks you to season every batch. And the food supplies some energy, and some protein when it includes legumes. The shared effects show up in smaller form: lower sodium, the day-two headache, hunger, and a lighter scale weight that is mostly water, as the post on why the weight comes back lays out.
Day by day
Line the sequences up. By 18 to 24 hours the liver’s glycogen is largely gone. On day two, ketones climb and sodium leaves in the urine, and the caffeine clock behind the day-two headache peaks 20 to 51 hours after the last coffee. In the Vienna study, blood glucose fell from 4.9 to 3.5 millimoles per liter over 84 hours, as the metabolism post reports. By day three of a complete fast, ghrelin has fallen and hunger has blunted, as the hunger post describes. A soup cleanse shares the early part of that sequence and softens the rest by whatever the pot supplies.
How long fasts run
Duration changes the risk, and the research spans a wide range. The physiology studies run 24 to 84 hours. The seven-day study sits above that, and the German clinic program runs 4 to 21 days under supervision. The 2013 consensus guidelines for supervised fasting set a first-time week of one preparation day, five fasting days, and two refeeding days. On the refeeding side, NICE counts little or no food for more than 5 days as a risk factor. A three-day cleanse sits at the short end of all of these, and a water fast of five days or more belongs with a clinician.
Coming off
The return from a fast follows its own rules. For people at high risk of refeeding problems, hospitals start at 10 calories per kilogram a day and build to full needs over 4 to 7 days. A healthy adult after a short fast does not need that ramp, and the return should still be slow. The come-off post covers the criteria and the order.
Who should skip a fast
Everyone on the 3-day guide’s skip list should skip a water fast, and a fast removes more than a soup cleanse does, so the list applies at least as strongly. The clinic study’s safety record came with daily monitoring and participants screened for contraindications. Nothing on this site can offer that. Anyone who wants to fast should do it with a clinician.
Common questions
- Is a soup cleanse a fast?
- It includes food, so it is not a fast in the strict sense. The 3-day guide calls this kind of plan a low-calorie liquid diet, so it shares some of a fast’s effects, such as lower sodium, and how many depends on the pot.
- Does a soup cleanse put you in ketosis?
- Not necessarily. Carbohydrate holds ketone production down, and soups built on starchy vegetables and legumes supply it. Nobody has measured ketones during a soup cleanse.
- Is water fasting riskier than a soup cleanse?
- A water fast removes more than a soup cleanse does. The best-documented fasting program allows about 250 calories a day of juice and vegetable soup and runs under daily medical monitoring.
- What does a seven-day fast do to strength?
- In a 2025 study of seven days with no food, maximal leg strength did not change, and high-intensity endurance capacity fell 10 to 15 percent.