Everyone’s probably tried intermittent fasting at least once in their life. Whether it’s a New Year’s Resolution or just a sensible way to lose weight, intermittent fasting provides a great framework for weight loss and longevity. The idea is simple: restrict when you can eat, so that your body spends more time in a fasted state, which may improve how energy is processed and used.

The benefits? Intermittent fasting promises everything from longer lifespans and better blood sugar control to weight loss and clearer thinking.

But is this true? And is intermittent fasting good for gut health and your microbiome? Or is it doing more harm than it’s worth? We’ll also cover the best gut health test for monitoring your gut microbiome.

What Is Intermittent Fasting, Exactly?

Intermittent fasting isn’t about what you eat, it’s about when you eat. Instead of counting calories all day long, you alternate between periods of eating and not eating.

Some of the most common approaches include:

• 16:8: You fast for 16 hours and eat within an 8-hour window. It’s often something like 10 am to 6 pm or midday to 8 pm.

• 5:2: You eat normally five days a week, then cut calories sharply on two non-consecutive days.

• Time-restricted eating: A broader term for limiting daily eating to a set window, whether that’s 12 hours or 8.

The key thing? There’s no single “right” way to do it. Some people thrive on fasting, others don’t, and small tweaks often matter more than strict rules.

How Your Gut Works (Quick Refresher)

Most people think they understand their gut. Food goes in, a combination of acid and enzymes digests it, and then the nutrients are absorbed into your body (the rest is excreted as waste).

There’s just one missing key ingredient: your gut microbiome.

That’s the community of trillions of bacteria, viruses, fungi, and archaea living in your gut. Together, your gut microbiome helps digest your food, produces critical micronutrients, defends against harmful bacteria, and even helps influence inflammation.

This microbiome thrives on what you eat. Ideally, a healthy diet is rich in lots of different plant species. If that’s not present, it can alter the microbiome composition, potentially causing havoc.

How Might Intermittent Fasting Affect Gut Health

You might think intermittent fasting would harm your gut health. Aren’t you starving the gut bacteria and other species?

Well, no.

Preliminary research suggests that intermittent fasting improves the composition of your gut microbiome, strengthens your gut barrier, and provides benefits for your entire body.

There are many reasons fasting has been part of human cultures for thousands of years. And periods of rest from constant eating appear to support metabolic and digestive health.

What Does the Research Actually Say?

Is this just hearsay? No, there’s a growing body of evidence that supports intermittent fasting as a tool for supporting your gut.

Here are some of the studies:

• During Ramadan, Muslims fast for around 16 hours a day (similar to the 16/8 method). One study monitored healthy men, measuring the gut microbiomes at the beginning and end. By the end of the month, higher levels of Lachnospiraceae were observed. This is a beneficial bacterium linked to lower cancer and inflammatory bowel disease risks.

• Another study in young men following the 16/8 method found a significant increase in microbiome diversity, especially Prevotellaceae and Bacteroids. Additionally, markers of obesity and metabolic health also improved.

The problem? Well, in the first study, the gut microbiomes returned to normal after daily fasting ended. That casts doubt on whether there are any long-term benefits if the routine is stopped.

When Intermittent Fasting Might Not Help Your Gut

There is a caveat.

Mild fasting (like 16/8 or 5/2) generally appears to be good for your gut. But if you’re over-restricting calories, it can push your body into a stress response. Higher cortisol levels (a stress hormone) will negatively affect your gut bacteria.

It’s also a matter of what you eat when you can. If you focus on fast food and ultra-processed foods, you’re unlikely to see any beneficial effects. Plus, fasting isn’t recommended for people with any active gut diseases.

How to Try Intermittent Fasting Without Harming Your Gut

Want to try intermittent fasting without the downsides? Try these tips:

• Start slowly with a shorter fasting window before jumping into 16:8 or 5:2.

• Prioritise fibre-rich foods such as vegetables, beans, whole grains, and fruit at meals.

• Stay hydrated - water and herbal teas help digestion during fasting periods.

• Avoid binge-style eating when your window opens; steady, balanced meals work best.

If you’re unsure whether fasting is right for you, or if you think there’s a problem with your gut microbiome, The Functional Gut Clinic can help. Our specialists use advanced testing and personalised plans to uncover what’s really happening in your gut and guide you toward lasting improvements.

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