Does Fasting Change Your Gut and Your Brain?

“Intermittent fasting triggers surprising changes in the brain.” That was the ScienceDaily headline this spring, and the story underneath it comes with a gut-bacteria angle too. If you fast, it’s a tempting story: skip breakfast, rewire your cravings.

The paper behind it is real, and it’s interesting. Published in 2023, it reports a small study of a diet most people reading the headline have never tried.

The short version

A Chinese research team recruited 41 adults with obesity for a two-month program of heavy calorie cuts every other day. The analysis focused on 25 who lost weight: they dropped an average of about 7.6 kg, showed lower resting activity in some eating-related brain regions, and had shifts in several gut bacteria. At several checkpoints, bacterial abundance correlated with activity in particular brain regions. There was no control group, so the study can’t separate the effects of fasting, eating less and losing weight.

What the study actually did

The paper is Zhou and colleagues, Frontiers in Cellular and Infection Microbiology, December 2023, by a team including researchers at the PLA General Hospital in Beijing and Henan Provincial People’s Hospital. The authors call the method intermittent energy restriction, and it looked nothing like a daily eating window.

Four days of normal eating to record each person’s usual calorie intake.
A 32-day tightly controlled phase. On restricted days, a dietitian supplied meals at two-thirds, then a half, then a third, then a quarter of that person’s usual intake, stepping down every eight days. Restricted days alternated with days of eating freely at home.
A 30-day looser phase. Every other day, a 500-calorie target for women and 600 for men.

All 25 analyzed participants had obesity, with a BMI between 28 and 45, and a well-controlled condition such as high blood pressure, diabetes or high blood lipids. Along the way the team ran resting brain scans (fMRI), blood work and stool sequencing at four points.

That’s closer to alternate-day fasting under a hospital’s eye than to anything you’d run off a phone timer.

What they found

Weight. Average body weight fell from 97.5 kg to 89.9 kg, with drops in waist size, body fat and systolic blood pressure. Some blood markers, including fasting glucose and cholesterol, improved at one or more check-ins. Skeletal muscle went down too, which is easy to miss in the coverage and is what you’d expect from a cut this steep.

Brain. Resting activity fell in a handful of regions the authors link to eating behavior: part of the orbitofrontal cortex early on, the putamen by the end of the strict phase, and the anterior cingulate cortex and dorsolateral prefrontal cortex by the end of the program. The authors’ read is that this “may promote weight loss.” The reward circuit, the part most people picture when they hear “cravings,” showed no significant change.

Gut. E. coli dropped across several time points. Three species often described as beneficial (Faecalibacterium prausnitzii, Parabacteroides distasonis and Bacteroides uniformis) rose during the strict phase. The detail that rarely makes the headline: those increases peaked midway and drifted back toward baseline levels by the end. Overall gut diversity followed the same arc, rising and then falling, so it finished about where it started.

Gut and brain together. The team tested correlations between bacteria and brain regions at each time point. For example, at baseline, higher E. coli abundance went with lower activity in part of the orbitofrontal cortex. That association was present before the intervention, so it isn’t evidence of a change caused by fasting.

How much to trust this

Some of this is solid and some of it is a long way from settled.

Only the successes were analyzed. 41 people were recruited, 35 finished, and the analysis covers the 25 who “successfully lost weight.” Anyone who didn’t respond was left out, which tilts every result toward a good story.

No comparison group. Everyone did the same diet, and every change is measured against that person’s own baseline. Losing 8% of your body weight in two months changes a lot of things at once, so there’s no way to separate “fasting did this” from “eating far less did this” or “losing weight did this.”

Many correlations in a small sample. The methods specify a Spearman correlation coefficient of 0.2 or more and a p-value below 0.05. Many bacteria and several brain regions were tested at four time points, without a reported multiple-testing adjustment for that correlation analysis, so some links could turn up by chance. The conclusion acknowledges that the results “did not establish causation,” although parts of the discussion argue that the microbial changes promote weight loss.

It doesn’t measure cravings. The scans were taken at rest, not while people looked at food or reported hunger. “Lower activity in an appetite-related region” and “fewer cravings” aren’t the same finding.

The diet isn’t 16:8. Days at a quarter of normal intake are a different intervention from skipping breakfast. Nothing here tells you what a daily eating window does to your gut or your brain.

None of that makes the paper worthless. It’s an early, exploratory look at how the gut and brain shift during rapid weight loss, and it says so. The problem is the jump from that to “fasting rewires your brain.”

What to do with this

Read gut-and-brain headlines as hypotheses. This is the same pattern as the autophagy story: a real mechanism, mostly early or indirect evidence, and marketing that runs well ahead of it. We went through that one in what fasting does and doesn’t do for your cells.
Don’t copy this protocol on your own. Days at a quarter of your normal intake were run with a hospital team, dietitian-built meals and blood work. That’s not a weekend experiment, especially if you take medication for blood sugar or blood pressure.
Work the levers you can actually pull. This study doesn’t hand you a gut-or-brain trick. If the goal is eating less in a way you can keep, the practical levers are still a window you’ll stick to, enough protein, and getting through the hunger that comes and goes in waves.
Keep the tools honest too. Atomic Fast’s timer displays phase labels at fixed elapsed-hour thresholds. It does not measure a biological phase, gut bacteria or brain activity. What it can do is keep the fast and the food in one place: log a meal by photographing it, typing a quick description, or scanning a barcode, then see today’s fast and meals together on the daily timeline.

Frequently asked questions

Does intermittent fasting change your gut bacteria? In this study, several species shifted during a strict alternate-day calorie-cut program, and many of the shifts faded back toward baseline by the end. With 25 people and no control group, it can’t show the fasting itself caused them, and it says nothing about daily 16:8.

Does fasting reduce cravings by changing the brain? The study found lower resting activity in some regions linked to eating behavior, not in the reward circuit, and it never measured cravings directly. The authors argue that the brain changes contribute to weight loss, but the design can’t establish that causal link.

Is this the same as 16:8 fasting? No. Participants alternated normal days with days at a fraction of their usual intake, down to a quarter, and later with 500 to 600 calorie days. That’s much closer to alternate-day fasting than to a daily eating window.

Did people lose muscle on this diet? Skeletal muscle fell along with body fat, which is common with steep, fast weight loss. Enough protein and some resistance training are the usual advice for limiting it, though this study didn’t test either.

Should I take probiotics while fasting? This study didn’t test probiotics, so it gives no basis for them. If you’re thinking about supplements for a gut condition, ask a doctor or dietitian.

Sources

Sources checked: October 11, 2026.

Zhou et al. (2023) — Dynamical alterations of brain function and gut microbiome in weight loss — The original study: 25 selected responders, no control group and repeated gut/brain measurements.
ScienceDaily (2026) — Intermittent fasting triggers surprising changes in the brain — The later headline discussed in the opening; this is coverage of the same study, not independent evidence.

Track your fast by the clock and log the meals around it with Atomic Fast on the App Store or Google Play.

This article is for general education and isn’t medical advice. If you’re pregnant, managing a health condition, taking medication, have a history of disordered eating, or are considering fasts longer than 24 hours, talk to a healthcare professional first.