What does fasting one day a week do to your gut microbiome?

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.