Five-day fasting cures are in fashion. Whole clinics have made them their speciality, and everyone knows someone who came back a few kilos lighter. One question keeps coming back, though. What is left three months later?
A team in Berlin has just published a serious trial on the subject. I am telling you about it because it is interesting. Not because I recommend fasting for five days. My protocol stops at around 28 hours, and I explain why further down.
A trial run in Berlin
The study is called LEANER. It was run at Charité university hospital and the Max Delbrück research institute in Berlin and published in Genome Medicine on 16 September 2026 (Kaufhold et al., 2026). Thirty-eight healthy adults aged 20 to 50, with a BMI between 20 and 30, half of them women. A random draw decided who fasted straight away and who waited, the second group serving as the comparison.
It was not a water fast. For five days, participants were allowed 200 to 250 kcal a day as vegetable juices and broths, with water and herbal teas as they liked. They fasted at home, supported in small groups by fasting counsellors. They were followed for twelve weeks.
What is left after twelve weeks
Right after the fast, BMI had dropped, mainly through fat loss. At twelve weeks, part of it had come back. What remained was mostly fat. The muscle lost during the five days had fully returned.
Resting energy expenditure had fallen by 62 kcal a day at the end of the fast, then went back to normal. So did most blood and blood pressure markers. For days 2 to 4 after eating again, blood sugar stayed a little higher than usual during the day. And 83% of participants reported side effects during the fast, all mild and short-lived.
You also have to look at who really kept anything. The researchers compared each weight loss with the normal fluctuations of the control group. Only 38% of the fasters had a loss beyond those fluctuations at twelve weeks. For the other 62%, nothing clear.
The microbiome shifts, then returns
During the fast, the make-up of gut bacteria changed markedly. Butyrate in the stool even dropped, while other work had reported the opposite. Then almost everything went back to where it started. At twelve weeks, only seven species remained lastingly changed.
That matches what I write in my guide. In a German study on a ten-day fast, the microbiome had returned to its starting state three months later. One big isolated fast does not change the gut for good.
The starting microbiome hints at what comes next
This is the most original part of the study. The researchers looked for what set apart, before the fast, the people who would keep their weight loss. A computer model kept four markers. Two gut bacteria, an Oscillibacter and a Faecalibacterium, LDL cholesterol and blood pressure. The Oscillibacter weighed the most, and those who kept their loss had more of it. LDL counted for little.
The model was then tested on three other groups. It worked well in people with metabolic syndrome who had fasted for five days. It did less well in healthy volunteers who had fasted for 6 to 12 days. The authors themselves call the results exploratory, and point out that a bacterium that predicts does not prove that it acts.
The limits
The sample is small, and the participants were healthy. Nobody recorded what they ate after the fast, so it is impossible to say what comes from the fast and what comes from habits in the following weeks. The study does not compare fasting with another diet. And one of the authors is employed by the company behind the Buchinger Wilhelmi clinic, one of the best known for fasting cures. As for the model, it is a research tool, not a test you can take.
Why my protocol stops well before
Five days is far beyond what I suggest. The Totem Fast stays occasional. At most once a week and up to 24 hours. Beyond 24 hours, only once or twice a month, and the protocol stops at around 28 hours. Seventy-two hours is a safety limit, not a goal, and beyond that a fast is done only under medical supervision.
Fasts of 24 to 48 hours are enough to trigger the switch to fat, the hormonal response and the anti-inflammatory effect of ketones. The deeper effects take several days, and that is exactly where medical supervision becomes essential. Some people should not fast without medical supervision. In particular if you have diabetes or gout, a past or present eating disorder, if you are pregnant, breastfeeding or trying to conceive, over 65 or under 18, talk to your doctor first.
And coming out of the fast matters as much as the fast itself, as I explain in What to eat after a 24 hour fast.
That is why I prefer short fasts repeated through the year to one big fast. Your body does not need five days to switch to its fat stores. One well-placed day is enough.
This article is for information only and does not replace medical advice.