In recent years, intermittent fasting has gained popularity in the health and wellness world. It’s been praised for its ability to help you lose weight, increase energy, improve mental clarity, and even help you live longer. These are some big claims – so, what does the science say?

First, what exactly is intermittent fasting?

The idea is simply to alternate between periods of eating and fasting, and there are several ways to do this. One popular method is the 16/8 protocol, where you eat all your meals within an 8-hour window (for example, 10 am to 6 pm) and fast the rest of the day. Another approach is the 5:2 method, where you restrict your caloric intake for 2 days of the week and eat a regular, healthy diet for the other 5 days. Then there’s alternate-day fasting, where you fast (or consume a restricted number of calories) every other day. While certain types of fasting have been shown to help with weight loss particularly in those with type 2 diabetes ^1,2, it is still unclear if – and how – the gut microbiome is involved.

Does timing really matter?

Most of the studies we have for the effect of intermittent fasting on the gut microbiome come from mice. These studies show several potential benefits, including increased microbial diversity, reduced inflammation, and increased production of beneficial microbial compounds known as short chain fatty acids (SCFAs)^3,4. Intermittent fasting also improved cognitive function and increased levels of tryptophan (a precursor for important hormones and neurotransmitters) in the blood, suggesting the gut-brain axis may be involved in mediating the effects of fasting³.

Another area of interest is the influence of the microbiome on our circadian rhythm – the internal clock that helps regulate your natural sleep and feeding cycles. It turns out your gut microbiome has its own circadian clock; studies show that the composition and function of the microbiome fluctuates throughout the day, including changes in the production of SCFAs^5-7. These SCFAs alter the expression of ‘circadian clock’ genes in the liver, which play an important role in the regulation of sleep, metabolism, and behaviour. While certain things can disrupt this rhythm (e.g., high-fat diets or jetlag), time-restricted feeding is thought to restore normal circadian patterns but so far this has only been shown in mice^5,6. Together, these studies have started to give us clues into the effect of fasting on the gut microbiome.

What we do know in humans

Despite compelling evidence in animal studies, we still have a way to go to know how intermittent fasting affects the human microbiome, and in turn, our overall health. Some of the best insights we currently have come from studies of individuals participating in Ramadan^8-12. In the Muslim holy month of Ramadan, individuals fast from dawn to sunset.

Ramadan fasting practices have been shown to have many health benefits, including weight loss, reduced cholesterol and improved blood sugar levels¹³.

Some studies have found that after Ramadan, individuals had increased levels of beneficial gut bacteria such as Akkermansia, Faecalibacterium and Roseburia^8,9. Fasting also increased microbial richness (number of different species in the microbiome) and diversity (how diverse and evenly spread out species are), and increased levels of the beneficial SCFA, butyrate^8,9,12 which are all generally associated with a healthy gut microbiome. However, it’s also important to note that Ramadan had different effects on different ethnic groups¹⁰ and age groups¹², meaning that while fasting might benefit some people, it might not work for everyone.

Is fasting all the same?

As mentioned before, there are several different types of fasting. When looking at the 16:8 protocol, one study found that individuals who fasted had higher microbial richness compared to those who followed their usual eating patterns¹⁴. However, a different group of participants on the same fasting protocol lost weight, but there was no impact on the gut microbiome¹⁵.

Another study looked at the impact of a longer fast on the gut microbiome and overall health. Individuals with metabolic disease were divided into two groups: both were put on a healthy Mediterranean-style diet for 3 months, but one group fasted for five days (300-350 kcal/day) before beginning the diet¹⁶. The fasting-diet combination led to weight loss, reduced blood pressure, and altered immune response. However, there was no significant change in gut microbiome diversity, and changes observed in the types and amounts of bacteria present reverted back to normal after the period of fasting was over. Importantly, individual responses to fasting were very different, and researchers could actually predict who would respond better to the diet intervention based on their gut microbiome at the start of the study.

So, is intermittent fasting good for the gut? While there is evidence for its role in weight loss, the research just isn’t there yet to back up the claims that intermittent fasting can improve your gut microbiome health.

Even more, the human studies we do have confirm what we already know – everyone has a unique microbiome, so everyone will likely respond to fasting or other interventions differently.

Before you try it

We do know that what we eat, rather than when, has a drastic effect on the microbes living in our gut. A diet high in fibre provides fuel for these beneficial microorganisms, and in turn, they provide us with substances that benefit our metabolism, immune system, and even mental health. When considering how a fasting diet might impact your microbiome you might like to consider how it would fit into your lifestyle and impact the types of foods you consume. For many, limiting late-night snacks can be a positive choice because generally, these aren’t the types of foods known for their health benefits. Skipping breakfast, on the other hand, could naturally cut out gut-healthy foods like whole grain oats, yoghurt, or fruit. No matter what time of the day, incorporating more plant-based, whole foods into your diet can help improve the health of your gut microbiome.

It is important to remember that intermittent fasting is not for everyone and can be associated with some risks. It is not recommended for certain people, such as children, pregnant or lactating women or those at risk of an eating disorder¹⁷. If you are interested, talk to your healthcare professional to see if intermittent fasting is right for you. If you choose to trial a new diet a great way to assess the impact on your microbiome is to use Insight™ gut microbiome analysis to measure your microbiome before and after the diet change. You can easily compare your report insights side-by-side and measure the evolution of your gut microbiome over time. As always, it is recommended that you do this under the guidance of a qualified healthcare professional.

About the Author

Dr Kaylyn Tousignant

Kaylyn is a Scientific Writer at Microba with a passion for making science engaging and accessible to the general public. She has a PhD in Biomedical Sciences from QUT, with a research focus on cancer biology and cell metabolism. Kaylyn’s current work aims to further our understanding of how the gut microbiome influences various aspects of health and disease.