What does fasting one day a week do to your gut microbiome?
Intermittent fasting demonstrably changes your gut microbiome, but the effects are likely temporary if you stop; for healthy people, the benefit is not yet well substantiated.
Fasting one day a week falls under the broader umbrella of 'intermittent fasting', and several studies show that these kinds of fasting patterns clearly alter the composition of your gut microbiome. Shifts in abundance have been measured in at least 16 bacterial genera, and overall diversity also increases. That sounds positive, but most studies lasted only a few weeks, and in some participants the changes disappeared as soon as they stopped fasting.
An eight-week study in people with metabolic syndrome shows that intermittent fasting increases the production of short-chain fatty acids. These are substances that protect the gut lining and suppress mild inflammation. At the same time, levels of bacterial pro-inflammatory agents in the blood declined. Moreover, the shifts in the gut microbiome were associated with measurable improvements in fat mass, oxidative stress, and other cardiovascular risk factors. This is not a coincidence but was found in a controlled study.
Whether the gut microbiome is the direct link between fasting and those health effects has not yet been fully proven in humans. Animal studies and mechanistic studies suggest that the gut microbiome acts as an intermediary between fasting and changes in your metabolism and immune system, but hard causality in humans is still lacking.
A small pilot study in MS patients and mouse studies show comparable patterns: greater bacterial richness and growth of specific protective bacterial families. How well this translates to healthy people fasting one day a week is uncertain. The studies are small and focus on sick populations.
If you want to fast exactly one day a week, bear in mind that the effects on your gut microbiome are likely to be temporary if you stop later on. Long-term data in humans are almost entirely absent. The strongest indications of a beneficial effect apply to people with overweight or metabolic syndrome, not so much to healthy individuals.
Based on one review (PMID 36950759), two RCTs/clinical studies (PMID 33017844, 38806467), one animal study with a small pilot study in MS patients (PMID 29874567), and several mechanistic reviews (PMID 38719726, 28715993, 36678130). Study PMID 38806467 combined fasting with a protein protocol and was funded by a commercial party. None of the studies focuses specifically on exactly one fasting day per week.
Does intermittent fasting help your gut health?
For healthy people and people with metabolic syndrome there is reasonably consistent evidence that intermittent fasting has a beneficial effect on gut bacteria and inflammation markers. If you have a chronic bowel disease, discuss fasting with your doctor first, as evidence in that group is still lacking.
Can a disrupted gut microbiome weaken your immune system?
A disrupted gut microbiome demonstrably dysregulates your immune system and increases the risk of chronic inflammation. Eating fermented foods is the best-supported way to do something about it.
What does a high-fat diet do to your gut microbiome?
A high-fat diet noticeably disrupts the gut microbiome: less bacterial diversity, more harmful species and a leaky gut wall that fuels low-grade inflammation. That relationship is consistent in animal research and plausible in humans, so if you want to protect your gut health, it pays to keep an eye on the proportion of fat in your diet.
What does smoking do to your gut and gut microbiome?
Smoking disrupts your gut microbiome and increases the risk of intestinal inflammation, gastrointestinal complaints and certain forms of cancer. Quitting smoking is the most direct way to reduce these risks.
Does fasting reset your immune system?
Fasting lowers inflammatory markers and improves metabolic risk factors, but a true 'reset' of the immune system has not yet been firmly demonstrated. The strongest gains are in weight and blood pressure; for specific immune diseases it is still too early for conclusions.
Can you permanently change your gut microbiome through diet?
Diet does indeed change your gut microbiome, but lasting change requires lasting adjustment of your dietary pattern; a temporary diet has only a temporary effect.