Does intermittent fasting help your gut health?
Intermittent fasting appears to improve the gut microbiome in people with overweight or a metabolic condition, but in healthy individuals no effect on the gut has been demonstrated after short-term fasting. If you want to promote gut health, focus first on getting enough fibre in your diet.
In people with overweight or a metabolic condition, intermittent fasting measurably improves the composition of the gut microbiome. In a randomised study of 39 adults with metabolic syndrome, after eight weeks the gut produced more short-chain fatty acids (substances that nourish the gut lining and suppress inflammation) and fewer pro-inflammatory bacterial waste products entered the bloodstream. Those changes went hand in hand with improved cardiovascular risk factors.
For people with a healthy weight the picture is different. In a randomised crossover study of sixteen healthy adults, early time-restricted eating (all calories before 14:00) had no measurable effect whatsoever on the gut microbiome, transit speed, or gas production in the large intestine after nine days. Blood sugar levels did improve. This is an important nuance: if you are already healthy, you should not immediately expect short-term fasting to change anything in terms of gut health.
In animal research the effects are more positive and better supported mechanistically. Mouse studies show that a so-called fasting-mimicking diet reduces gut inflammation, promotes recovery of the gut lining, and shifts the microbiota in a favourable direction. A small clinical study in people with a gut disease subsequently also found lower inflammatory markers. Note: the lead author of that research has a financial interest in the company that sells the associated product.
Two review articles argue that intermittent fasting can positively influence the gut microbiome via the gut's circadian rhythm, but both draw on existing literature and generate no new data. For the theory that fasting-related microbiota changes could suppress cancer, no clinical data are yet available. That idea remains entirely in the research phase.
Which form of fasting works best and for whom is not yet clear. The positive results come mainly from studies in people with overweight or a metabolic disorder; in shorter fasting protocols in healthy individuals the effect is absent. If you are a healthy person looking to improve your gut microbiome, a varied, fibre-rich diet is currently better supported by evidence than any fasting protocol.
Evidence is based on two randomised studies (n=39 and n=16), one small pilot study, two mouse studies with limited human data, and three review articles without their own study participants. No meta-analyses are available.