Intermittent Fasting, Gut Bacteria, and Digestive Health
Survey data suggested that, in the eyes of my fellow dietitians, intermittent fasting (IF) had surpassed the ketogenic diet as one of the leading diet trends among their clients. Although it no longer tops the list in 2026, intermittent fasting continues to attract significant attention from both researchers and the public.
While IF is technically an umbrella term covering various forms of time-restricted eating protocols, the most commonly implemented and well-studied form of fasting is generally the stereotypical 16:8 regimen.
In addition to being the most practical form for most people to implement, it is also the easiest to study in part because it is quite accurately replicated during the month of Ramadan, where a significant amount of preliminary human data around fasting and health outcomes tends to stem from.
While I've personally written extensively on various facets of IF, today's article represents my first look at the data around its impact on the gut microbiome.
Given the growing interest in gut health and the role the microbiome plays in digestion, immune function, and overall wellness, this is an area that has generated significant scientific attention in recent years.
Suffice to say, I'm as excited to be writing this piece as I hope you guys are to be reading it, so let's get right to the good stuff.
IF & The Microbiome
A 2022 paper published in Frontiers in Nutrition compiled the limited number of human studies looking at IF and the microbiome, many of which were conducted during Ramadan, which generally entails a 12–18-hour daily fast over a period of a month.
This is great news for the everyday IF enthusiast who is overwhelmingly likely to employ IF in a similar capacity, so let's take a look at the findings of a few specific papers.
A 2021 study out of The International Journal of Clinical Practice looked at the microbiome profiles of participants before and after 30 days of Ramadan fasting and found that Bacteroides abundance increased, as did the abundance of the short-chain fatty acid (SCFA) butyrate.
Bacteroides are a genus of carbohydrate-metabolizing bacteria that are generally considered both abundant and beneficial. Their increase in number may explain the parallel increase in the presence of butyrate, which is among the best-characterized SCFAs and may support immune function, gut barrier integrity, and digestive health.
A similarly conducted study published in the same year in The American Journal of Clinical Nutrition looked at two cohorts' worth of Ramadan fasting data and found that butyrate-producing bacteria from the Lachnospiraceae family increased in abundance over a one-month period, with levels dropping back to normal following cessation of fasting.
Now, there is certainly something of a trend forming here, but it must be said that neither study above was close to methodologically flawless, given a lack of a control group, and also considering that dietary habits may tend to change during Ramadan, which could contribute to the observed microbiome alterations.
A More Controlled Look at Fasting and Gut Health
With methodology in mind, I did encounter a more scientifically robust placebo-controlled trial that looked at the effects of a twice-weekly fasting regimen whereby participants consumed 75% fewer calories than normal on two days each week over an eight-week period.
In this particular trial, the authors noted a similar increase in SCFAs to the other studies discussed and observed that, once again, a family of butyrate-producing bacteria experienced the biggest increase in abundance after the fasting period in what the authors described as "favorable alterations in the composition of the gut microbiota."
What makes this finding particularly interesting is that butyrate serves as a primary fuel source for the cells lining the colon. Research suggests it may help strengthen the gut barrier, regulate inflammation, and support communication between the gut microbiome and the immune system.
What Could This Mean for Digestive Health?
While researchers are still uncovering the exact relationship between fasting and the microbiome, these early findings suggest that intermittent fasting may influence gut bacteria in ways that could potentially support digestive health.
Because alterations in the gut microbiome have been linked to conditions such as IBS, inflammatory bowel disease, obesity, and metabolic disorders, researchers are particularly interested in interventions that may encourage greater microbial diversity and beneficial bacterial populations.
However, it is important to recognize that fasting is not appropriate for everyone. Individuals with a history of eating disorders, certain medical conditions, pregnancy, or those taking specific medications should consult their healthcare provider before beginning any fasting regimen.
My Thoughts On The Findings
Fascinating, but not fully formed.
Whenever I assemble content on intermittent fasting, I'm generally left with the prevailing sentiment that there's "something there," but we just don't have enough robust evidence to make definitive claims about it—yet.
I think that general pattern applies here today, where we are left with a limited number of very new studies that show some promise of a positive interaction between fasting and the microbiome, but the quality and quantity of evidence are certainly not yet sufficient to make bold claims regarding fasting and gut health.
For now, intermittent fasting appears to be an intriguing area of research with potential implications for the microbiome. Whether those microbiome changes ultimately translate into meaningful improvements in digestive symptoms or long-term health outcomes remains a question future studies will need to answer.
FAQs
Does intermittent fasting improve the gut microbiome?
Current research suggests intermittent fasting may increase certain beneficial bacteria and short-chain fatty acids such as butyrate. However, more human studies are needed before researchers can confidently recommend fasting specifically to improve gut health.
What is butyrate, and why is it important?
Butyrate is a short-chain fatty acid produced when gut bacteria ferment dietary fiber. It serves as a major energy source for colon cells and may help support gut barrier function, immune health, and inflammation regulation.
Can intermittent fasting help with IBS?
The evidence is currently limited. Some individuals with IBS report symptom improvements, while others find fasting worsens symptoms. Responses are highly individualized, and fasting should not replace established IBS management strategies.
Is a 16:8 fasting schedule the most studied approach?
Yes. Among time-restricted eating protocols, the 16:8 method is one of the most commonly practiced and studied forms of intermittent fasting.
Are microbiome changes permanent?
Not necessarily. Several studies suggest that some microbiome changes observed during fasting may return to baseline once normal eating patterns resume.
References
1Nutrition Trends for 2026 Show Gut Health, Clean Eating and Functional Foods Shaping Consumer Choices. Pollock Public Relations. (2026, March). https://lpollockpr.com/trends/
2Silva, Y. P., Bernardi, A., & Frozza, R. L. (2020). The Role of Short-Chain Fatty Acids From Gut Microbiota in Gut-Brain Communication. Frontiers in Endocrinology. https://doi.org/10.3389/fendo.2020.00025
3Mousavi, S. N., Rayyani, E., Heshmati, J., Tavasolian, R., & Rahimlou, M. (2022). Effects of Ramadan and Non-ramadan Intermittent Fasting on Gut Microbiome. *Frontiers in Nutrition*. https://doi.org/10.3389/fnut.2022.860575
4Su, J., Wang, Y., Zhang, X., Ma, M., Xie, Z., Pan, Q., Ma, Z., & Peppelenbosch, M. P. (2021). Remodeling of the gut microbiome during Ramadan-associated intermittent fasting. *The American Journal of Clinical Nutrition*. https://doi.org/10.1093/ajcn/nqaa388
5Guo, Y., Luo, S., Ye, Y., Yin, S., Fan, J., & Xia, M. (2020). Intermittent Fasting Improves Cardiometabolic Risk Factors and Alters Gut Microbiota in Metabolic Syndrome Patients. *The Journal of Clinical Endocrinology & Metabolism*. https://doi.org/10.1210/clinem/dgaa644
6Mohammadzadeh, A., Roshanravan, N., Mesri Alamdari, N., Safaiyan, A., Mosharkesh, E., Hadi, A., Barati, M., & Ostadrahimi, A. (2021). The interplay between fasting, gut microbiota, and lipid profile. *International Journal of Clinical Practice*. https://doi.org/10.1111/ijcp.14591
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