12-Day Fast: What MRI Showed in Liver and Belly Fat

A study published on 7 September 2026 in Frontiers in Nutrition used MRI to look inside 32 people before a 12-day fast, at the end of it, and again one and four months after they started eating. Liver volume fell sharply during the fast, visceral (deep belly) fat kept trending down for four months, and liver fat moved in opposite directions depending on where a person started. There was no control group, and two of the authors were employed by a fasting clinic group.

Safety first. The fast in this study took place inside a clinical trial, and the full paper states that it was supervised by medical doctors and nurses. Twelve days is far outside what anyone should attempt alone. Nothing below is a plan to copy. 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.

What the study did

The scans were part of the GENESIS clinical trial, which the paper describes as a prospective, monocentric, single-arm interventional study. In plain terms: one center, one group, everyone fasted, and each person was compared with their own earlier scans.

Who	32 participants (16 men and 16 women, according to the full paper)
Intervention	A 12-day fast
Scans	Before, at the end of the 12 fasting days, and 1 and 4 months after refeeding
Method	Quantitative chemical-shift encoded MRI, with automated measurement of the liver and of visceral and subcutaneous fat
Comparison group	None

Two details from the full text matter for how you picture this. First, it was not a water-only fast: energy intake was limited to 200 to 250 kcal a day, taken as fruit juice, vegetable soup and a little honey. Second, the fast came with a clinical setting around it, including medical supervision and a program of light physical activity.

MRI has an advantage over a bathroom scale here. Weight tells you that something left the body. This kind of MRI estimates how much fat sits in the liver, how large the liver is, and how much fat is stored under the skin (subcutaneous) versus around the organs (visceral), without an injection.

What it found

Liver volume dropped. The abstract reports that fasting "strongly reduced" liver volume, beyond the expected reductions in weight and blood triglycerides. The full text puts the average change at the end of the fast at about 21% and reports that liver volume had returned to baseline values at the one-month scan. So this looks like a temporary change in the organ during the fast, not a lasting shrinkage.

Visceral fat kept going down. Both subcutaneous and visceral fat volumes changed over time. The authors single out a "continuous downward" reduction in visceral fat up to 4 months after fasting. The abstract gives no volumes or percentages for this, so neither do we.

Liver fat went two ways. This is the most interesting result:

• In healthy participants, liver fat content increased temporarily during the fast. That increase had reversed after 1 month.
• In participants with liver steatosis (fatty liver), liver fat content decreased. How well that decrease persisted varied during follow-up.

A temporary rise in liver fat in healthy people during a fast is not as strange as it sounds. When you stop eating, fat is released from stores and travels through the liver to be processed. The study does not tell us whether that brief rise matters.

Fat composition shifted slightly. In adipose tissue, the proportion of monounsaturated fatty acids decreased during fasting, with no change in saturated or polyunsaturated fatty acids.

What it does not show

The authors conclude that their results "suggest that long-term fasting can help prevent metabolic dysfunction-associated fatty liver disease". Read that sentence against the design, because the design cannot carry all of it.

• No control group. The full paper says so directly: the absence of a control group limited the ability to infer causality and makes it hard to separate the effect of fasting from the rest of the intervention. People in a clinic program also walk, rest and often change how they eat afterwards. A continued fall in visceral fat over four months could reflect what people did after the fast as much as the fast itself.
• Small sample. 32 people, most of them without fatty liver. The finding in people with steatosis rests on a subgroup, and the paper acknowledges that limited numbers restrict how far it generalizes.
• Nobody was followed for disease. The study measured fat on scans. It did not test whether anyone avoided liver disease.
• Who ran it. Two authors were employed by Buchinger Wilhelmi Development & Holding GmbH, a fasting clinic group, which also co-funded the study. The paper states the funder had no involvement in design, analysis or the decision to publish. That is worth knowing, not a reason to dismiss the data.
• It is not a treatment. If you have fatty liver disease, this is research on a supervised program, not something to try by yourself. Decisions about liver disease belong with your doctor.

What this means if you fast

For most people who fast, the practical message is modest. A fast of this length changes organs in ways you cannot feel and cannot monitor at home. That is an argument for supervision, not for ambition.

If you do shorter extended fasts, the same principles apply at a smaller scale: prepare, keep up fluids and electrolytes, and take refeeding seriously. Two of our guides cover that ground: how to structure longer fasts without discomfort and the extended fasting hydration and electrolyte checklist. For a general picture of what the body is thought to do as the hours pass, see fasting stages.

It is also a useful reminder that the scale misses things. Two people can lose the same weight while their livers respond differently.

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

FAQ

Does a 12-day fast reduce liver fat? In this study it depended on the starting point. Liver fat decreased in participants with liver steatosis and temporarily increased in healthy participants, with that increase gone after 1 month. The study had no control group.

Did belly fat stay down after the fast? The authors report a continuous downward trend in visceral fat up to 4 months after fasting. Without a comparison group, the study cannot say how much of that was due to the fast and how much to what participants did afterwards.

Can I do a 12-day fast at home? No. The fast in this study was part of a clinical trial and was supervised by medical doctors and nurses. A fast of this length should not be attempted alone.

Was this a water fast? No. According to the full paper, participants took in 200 to 250 kcal a day.

Sources

• Viallon M, Leporq B, Ratiney H, de Toledo FW, Mesnage R, Croisille P. "Impact of long-term fasting on liver and abdominal adipose tissues monitored by quantitative chemical-shift encoded MRI." Frontiers in Nutrition, 7 September 2026. DOI: 10.3389/fnut.2026.1868854, PubMed 42769954, full text on PubMed Central