There is evidence that intermittent fasting alters the gut microbiome, and while this is often believed to improve gut health, some studies are now raising doubts about whether that is always the case.
The idea behind intermittent fasting is relatively straightforward: instead of focusing on what you eat, you focus on when, cycling between periods of fasting and eating. The most popular approach limits all meals and snacks to the same 8-hour window each day.
Proponents of intermittent fasting claim it can slow ageing, prevent overeating, regulate circadian rhythms and boost immunity. Many of these benefits are believed to stem from its impact on the gut, particularly the trillions of microbes residing within it. But how intermittent fasting affects gut health is far from settled.
Advertisement
For instance, in a new study, Guodo Cao at Anhui Medical University in China and their colleagues found that intermittent fasting . Compared with five mice that ate freely, an equal number that fasted had significantly more signs of inflammation in their colons. One inflammatory molecule was 27 per cent higher in the fasted mice, for instance. The animals on a restricted diet also had significantly shorter colon lengths and lower body weight 鈥� both indicators of gut inflammation.
Further experiments revealed intermittent fasting inhibited the rodents鈥� gut bacteria from producing indoleacrylic acid, a molecule that helps regulate immune responses in the gut. When the team supplemented the diets of a separate group of fasting mice with this molecule, it alleviated their intestinal inflammation.
These results suggest intermittent fasting can influence the gut microbiome in ways that aren鈥檛 always conducive to health, says at Stanford University in California, who wasn鈥檛 involved in the study.
However, at the Salk Institute for Biological Studies in California has 鈥渟ome serious concerns鈥� with the study, namely its small sample size, omission of dietary information and the fact that it used 5-week-old mice. 鈥淭his is akin to putting a toddler on an 8-hour feeding without mentioning what and how much food was given,鈥� he says.
Still, the results could explain why some animals don鈥檛 always benefit from intermittent fasting. In a 2024 study, Thaiss and his colleagues found various fasting regimens of most 鈥� but not all 鈥� mice. 鈥淲e had a few mice that developed complications that actually died earlier, despite the fact that they鈥檙e on an allegedly beneficial intervention,鈥� he says. 鈥淚 think this [most recent] study is providing one possible mechanism by which this might be the case.鈥�
Part of the challenge in disentangling how intermittent fasting impacts gut health is that most of the research is conducted in rodents, which have different metabolisms and eating behaviours than humans. 鈥淲hen you fast a mouse overnight, they lose a pretty significant portion of their body weight, which is, of course, not the case in humans,鈥� says Thaiss.
Another challenge is that we still don鈥檛 know what a healthy gut microbiome looks like, as 鈥渉ealthy鈥� depends on each context. For instance, a microbe that is usually considered helpful might not be when fasting, says Thaiss. Under this scenario, normally harmless microbes might start competing with the body for nutrients and become inflammatory. This makes it difficult to discern whether changes in the gut microbiome from fasting are beneficial or harmful.
What鈥檚 more, there is little consistency between study results. A 2024 review of eight studies in people found that while intermittent fasting seems to , changes in microbiota composition varied between studies, with some even showing opposing effects.
However, of the limited evidence available, most suggests intermittent fasting is beneficial for gut health. Yet 鈥渉ow much this can be generalised to a wider population is, I think, an open question鈥�, says Thaiss.
