5-Day Modified Fast at Home: What a Trial Found

A randomized controlled trial published on 1 June 2026 in Genome Medicine tested a five-day modified fasting program that 64 healthy adults followed at home, eating about 600 kcal a day. During and just after the program, participants lost weight, and several blood and gut measures shifted. One month later, the changes in the gut microbiome and the blood metabolome could no longer be told apart from the control group.

Safety first. This was a study in healthy volunteers who were enrolled in a trial and assessed repeatedly by a research team. It is not a plan to copy alone. Eating about 600 kcal a day for five days is a serious restriction. If you are pregnant or breastfeeding, have diabetes or take blood-sugar medication, have a history of eating disorders, take other regular medication, or are under 18, talk to a doctor before fasting, and do not attempt a multi-day fast without medical guidance. This post is general information, not medical advice.

What the study did

The trial had two arms. Of the 64 participants, 32 were assigned to the modified fasting program, and the rest were told to continue with their usual eating and lifestyle.

The program lasted 5 days. It was hypocaloric, at about 600 kcal a day, and ketogenic. This is why it is called a modified fast: participants ate a small amount, in contrast to a fast with no food at all. The abstract does not describe the menu, so this post does not either.

Who	64 healthy adults
What was compared	A 5-day program of about 600 kcal a day against usual eating
Primary endpoints	Changes in blood pressure and body weight
Secondary outcomes	Ketosis, glucose and lipid metabolism, inflammatory markers, antioxidant capacity, well-being
Also explored	Blood metabolomics and gut metagenomics
When measured	Baseline, end of the program, after food reintroduction, one month later

The trial was registered at ClinicalTrials.gov (NCT05821660) on 6 April 2023, before recruitment began.

What changed during and just after the five days

Weight. Participants on the program lost weight compared with controls, a difference that was statistically significant (p < 0.001).

Blood pressure. At the end of the fasting period, the change in blood pressure was not significant. After food was reintroduced, both systolic and diastolic blood pressure were significantly lower than in the control group (p < 0.05 and p < 0.01).

Blood chemistry. The program reduced glucose levels and coagulation factors. Blood metabolomics showed a significant decrease in markers of chronic inflammation.

Well-being. Physical well-being increased significantly.

Gut microbiome. Sequencing of the gut microbiome showed significant changes in the relative abundance of 11 bacterial species, and in 52 carbohydrate-active enzymes, the tools bacteria use to break down carbohydrates. The pattern reflected an increase in enzyme families that work on host-derived glycans. In plain terms, with little food arriving, the bacterial community shifted toward species able to live on substances the body itself supplies.

The authors also compared these effects with data from an earlier group who had done a five-day prolonged fast, and report similar metabolic changes.

A note on figures. The abstract gives numbers for weight and blood pressure, but it is not clear from the abstract alone whether they are totals or rates of change. Because their meaning is ambiguous, this post describes the direction of each change and leaves those numbers out.

What had faded a month later

This is the part that deserves the most attention. One month after the program, none of the differences in the gut microbiome or the blood metabolome were statistically different from the control group.

The authors' own summary uses the word for it: the program "transiently" improves cardiometabolic health and physical well-being in healthy individuals. Transient means temporary.

The abstract does not state separately whether the weight or blood pressure differences were still present at one month, so that remains open.

What the trial does not show

That the benefits last. By the authors' account they are transient. Five days produced a short-lived shift, not a lasting reset.

That it works the same in people who are unwell. Everyone in the trial was healthy. The authors' conclusion that the program is safe applies to healthy individuals in a study setting. It says nothing about people with medical conditions or on medication.

A clean result on the main blood pressure question. Blood pressure was one of two primary endpoints, and at the end of the fasting period the change was not significant. The significant reduction appeared at the later measurement, after food reintroduction.

An effect free of expectation. The control group simply carried on as usual, so participants knew which group they were in. That matters most for self-reported outcomes such as well-being.

Independence from a fasting provider. The first and last authors are based at a fasting clinic in Germany, and the paper's competing-interests statement says four of the authors are employees of the clinic's company. Three more are employees of a second company. That does not make the findings wrong, and declaring it is exactly what authors should do. It is a reason to want replication by groups with no stake in fasting programs.

Much about scale. Sixty-four people is a small trial, and the gut and metabolomic analyses were exploratory.

What this means if you fast

If you are drawn to longer fasts, this trial is useful mainly for setting expectations. A single five-day program moved many markers in a favorable direction, and within a month the body's bacterial and metabolic profile had returned to where the control group was. Anyone presenting one extended fast as a permanent change is going beyond this evidence.

It also shows how much structure sits around a study like this: healthy volunteers, a defined program, a planned return to food and follow-up measurements. If you and your doctor decide a longer fast is appropriate for you, preparation and refeeding matter as much as the fasting days. These guides cover how to structure longer fasts and hydration and electrolytes during extended fasting, and this one explains ketosis and fasting.

QoFast is a fasting tracker app for iPhone and Android that supports extended fasts of 3, 5 or 7 days or a custom length, each with an optional preparation phase and a refeeding phase, alongside water and mood logging. It gives general information, not medical advice, and it does not replace medical supervision. It is free to download, with an optional Pro subscription: App Store and Google Play.

FAQ

Was this a water fast? No. Participants ate about 600 kcal a day for five days on a ketogenic program. The authors call it a modified fast.

Did the benefits last? Not the ones tracked at one month. The gut microbiome and blood metabolome differences were no longer statistically different from the control group, and the authors describe the improvements as transient.

Is a five-day fast at home safe? The authors concluded that this program was safe in healthy individuals taking part in a trial. That is not a general statement about fasting alone at home, and it does not cover people with health conditions or on medication.

Who ran the study? A team led from a fasting clinic in Germany, with collaborators at several universities and research institutes. Several authors are employed by the clinic's company, which the paper declares.

Sources

• Grundler F, Ducarmon QR, Holley A, et al. "Health benefits of a five-day at-home modified fasting program: a randomised controlled trial." Genome Medicine, 1 June 2026, volume 18, article 80. DOI: 10.1186/s13073-026-01681-3. PubMed: 42226305.