The Benefits of Fasting for a Healthy Gut

When you eat has a big effect on your microbiome. Discover the meal schedule that helps your good bugs thrive.

There are trillions of microorganisms living in your intestines, helping to break down your food and make nutrients, like B vitamins and vitamin K. Fasting can be one of the many ways to improve overall gut health by helping these microorganisms thrive. There are different types of fasting that work for different people and lifestyles. Learn more about fasting and how it can help your gut.

The Benefits of a Healthy Gut

First, let's talk about why a healthy gut is so important.

When the organisms in your gut digest dietary fiber, they create compounds that researchers are finding are critical to muscle function and disease prevention. Your gut bugs even affect your emotions and cognition, sending signals between the brain and the digestive system.

"We really rely on them and need them," Suzanne Devkota, PhD, director of microbiome research at the F. Widjaja Foundation Inflammatory Bowel and Immunobiology Research Institute at Cedars-Sinai in Los Angeles, told Health. "They play a fundamental nutritional role for us."

Feed your microbes the right stuff, the wisdom goes, and the balance of good bugs to bad will stay in check, giving you major health benefits. When the gut has a healthy balance of microbes, it reduces the chance of developing many types of chronic conditions, including:

  • Heart disease
  • Diabetes
  • Asthma
  • Chronic kidney disease
  • Rheumatoid arthritis
  • Irritable bowel syndrome
  • Mental health conditions

But it turns out, it's not just what you eat that impacts the delicate balance of microorganisms in your gut. Research is finding that when you eat (and perhaps more importantly, when you don't eat) affects your microbiome in a big way.

Use Intermittent Fasting

Intermittent fasting can be done in a variety of ways. Two examples are:

  • Time-restricted feeding: Restrict your eating to a certain number of hours each day, fasting the rest of the time.
  • Weekly intermittent fasting: Restrict what you eat on two to three nonconsecutive days per week.

Research suggests that both methods may lead to weight loss and improved metabolic function, which in turn may lower the risk of diabetes and cancer. However, many of the studies have been done on rodents and the few studies done with humans have had mixed results, so much more research is needed to determine if these fasting approaches are effective.

Note: People who are pregnant or breastfeeding, and those who have a history of disordered eating, should not try any form of fasting.

Changing Gut Microbiome

Intermittent fasting can also change the makeup of your microbiome. "Your gut bacteria are very responsive to the presence and absence of food," explained Devkota. "When you remove food, the microbiome shifts in composition."

There's a rapid expansion of a particular bacteria—Akkermansia muciniphila—that is associated with positive health markers, like decreased intestinal inflammation and a healthier gut barrier. Your gut barrier is the lining of the intestines. It lets nutrients pass through but blocks bacteria and other bugs.

Animal studies show that the microbiome has its own circadian rhythm and is constantly cycling between different populations, Amir Zarrinpar, MD, PhD, an assistant professor of gastroenterology at the University of California San Diego School of Medicine who studies the gut microbiome, told Health.

Dr. Zarrinpar hypothesized that when humans are asleep and not eating, one set of bacteria may thrive. When you wake up and start to eat, other bacteria may bloom and take over. The cycle repeats every 24 hours but can get thrown off when you eat off-schedule or consume an unhealthy diet. Time-restricted feeding can reinforce and help reinstate those naturally occurring fluctuations, according to a study published in Trends in Endocrinology & Metabolism and coauthored by Dr. Zarrinpar.

Types of Fasting

There are several intermittent fasting patterns, but two common types are:

  • 16:8 time-restricted feeding, where you fast for 16 hours and eat during the remaining eight hours
  • 5:2 weekly fasting, where you eat normally for five days of the week and severely restrict food calories for two nonconsecutive days of the week

5:2 Fasting

There are pros and cons to each. Devkota likes the 5:2 plan and explained that the blooming of beneficial bacteria just starts to kick in after 16 to 18 hours of fasting in studies conducted with mice. For humans, you'd have to fast longer to see the effect, Devkota believed.

"If you want to see changes at the cellular or microbiome level, you have to push the fasting period longer," said Devkota. However, Devkota explained that the fasting days don't have to be completely devoid of calories. Most plans recommend a 70% to 75% reduction in calories on fasting days, but even reducing 60% could make a difference, said Devkota.

Keep in mind that studies done with animals don't always translate to the same results in humans, so more research is needed to confirm these same positive changes in gut bacteria in humans.

16:8 Fasting

Other experts suggest the 16:8 protocol, where you prolong your overnight fast each day by eating breakfast slightly later and finishing your last meal in the early evening, so your eating window is just eight hours. But even a less intense version shows beneficial health effects, Dorothy Sears, PhD, professor of medicine at the University of California San Diego School of Medicine, told Health. Sears suggests 12 to 14 hours of fasting overnight, with your eating window ending between 5 and 8 p.m.

Don't restrict your calorie intake during the eating periods. Most of your gut bugs need food to survive—not to mention you need food for energy and other functions—said Devkota. When your body enters starvation mode—like if you fast for too long—you'll decrease the diversity of bacteria in your gut. Gut diversity is why Devkota recommended making sure your fasting is "truly intermittent" and that you don't fast for two days in a row.

Eating by the Clock

A fasting regimen on a 24-hour cycle—like a 16:8 or 14:10 breakdown—allows you to align your fasting with your natural sleep/wake cycles. Sears said this is important because circadian rhythms regulate nutrient processing.

For example, insulin is most effective in the morning and midday; in the evening and overnight, insulin secretion decreases.

"If you eat a snack at night, the insulin you secrete to process it isn't going to function as well as if you ate that same food for breakfast," said Sears.

Since your insulin response—which is responsible for shuttling sugar from your bloodstream into your cells—is dampened in the evening, your blood sugar will stay higher for longer, explained Sears. If high blood sugar happens repeatedly, it can increase your risk for cardiovascular disease, type 2 diabetes, and stroke.

By eating breakfast a touch later (say, 8 a.m. or 9 a.m.) and moving dinner a bit earlier (finishing around 6 p.m.), you can align your food intake with your circadian rhythm and still get that prolonged overnight fast without too much extra effort.

A Quick Review

A healthy gut is associated with many health benefits and preventing chronic conditions. The different types of fasting can be one of many ways to improve gut health. Intermittent fasting can take several forms including specific weekly day fasting and fasting based on the number of hours each day. The variety of fasting schedules can accommodate different preferences and schedules to achieve your best overall health. Before starting a fasting approach, talk with a healthcare provider to determine what is the best approach for you.

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