Intermittent Fasting and the Gut Microbiome

What Do We Know So Far?

Survey data suggests that, in the eyes of my fellow dietitians, intermittent fasting [IF] has surpassed the ketogenic diet as the number one “diet trend” among their clients.

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.

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 out of 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 Bacteroide 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 to be both abundant and beneficial and their increase in number may explain the parallel increase in the presence of butyrate, which is among the best characterized SCFAs and may improve immune health and functioning.

A similarly conducted study published in the same year out of 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 also tend to change during Ramadan which could contribute to the observed microbiome alterations. 

With methodology in mind, I did encounter a more scientifically robust placebo controlled trial that looked at the effects of a 2x weekly fasting regimen whereby participants consumed 75% less calories than normal twice weekly over an 8 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”. 

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 is certainly not yet near sufficient to make bold claims regarding fasting and gut health.