Fasting for Gut Health: Does It Work?

Fasting has been linked to benefits like lower inflammation, better blood sugar and fewer markers of disease, but its effects on gut health aren’t as clear. Popular methods like intermittent fasting and time-restricted eating appear to influence the microbiome and gut immunity in different ways, some of which may do more harm than good.

How fasting affects the gut environment

Extreme fasts that include only water or juice eliminate the fiber that feeds the beneficial microbes responsible for keeping pathogens in check, and their numbers decrease. Meanwhile, microbes that feed on substances like the protective mucus lining in the gut begin to multiply. These changes can increase the risk of bacterial imbalances and leave your gut vulnerable to damage.

But intermittent fasting—which alternates between eating a normal amount of food and fasting for a set period—appears to positively affect the microbiome. Some studies show that this less extreme approach to fasting for gut health may increase the number and types of microbes in the gut, measures known as richness and diversity.

The shift has the potential to be either beneficial or harmful. Higher numbers of probiotic bacteria lead to increased production of short-chain fatty acids, byproducts of fiber breakdown that may reduce inflammation, influence immune cell development and modulate immune responses. But some studies show increases in pro-inflammatory microbes, including some types found in higher concentrations in the guts of people with conditions like irritable bowel syndrome (IBS) and inflammatory bowel disease (IBD).

Prolonged fasting may lead to more fundamental changes in the tissue of the gut itself. Long periods of abstinence or significant caloric restriction can put the body into a starvation state and trigger an increase in gut cell turnover and cell death. Fasts can also trigger autophagy, the process by which the body breaks down damaged cells and recycles the proteins for other uses. The boost in activity could be the body’s way of creating an optimal environment for nutrient absorption when the fast is broken.

Whether this is beneficial or not may depend on the length of the fast. Fasting-mimicking diets that restrict calories on only a few days per month appear to reduce inflammation and increase intestinal stem cells to promote tissue regeneration. But one study in mice, which go into starvation states faster than humans, showed that a 24-hour water fast caused gut tissue to atrophy and reduced the number of immune cells in the intestines.

Should you fast for gut health?

Because the research is inconclusive, going on a fast or adopting a fasting dietary pattern may not be the best option for improving gut health. Whether it may provide other benefits depends on factors like age, activity level and health status; even less extreme fasts may pose risks or have unpleasant side effects.

Considerations when planning a fast

If you do decide to fast, start by familiarizing yourself with the options so you can choose an approach that makes the most sense for your lifestyle. Common fasting diets include:

• Intermittent fasting, which includes alternate periods of normal eating and lengthy fasting. Some people fast for a full 24 hours every other day or fast only one or two days per week while eating normally the rest of the time.
• Time-restricted eating, where you eat a normal amount of calories during a specific window of time each day and fast the rest of the time. “Eating windows” are typically between six and 12 hours long.
• Occasional fasting, sometimes called a detox or reset, involves reducing your calorie intake for a short period every few months or a couple of times per year to give your gut a rest.

Whatever plan you choose, follow a consistent routine to maintain adequate nutrient intake, and center your meals around foods like fruits, vegetables, lean proteins and healthy fats to feed your gut microbiome and promote an anti-inflammatory environment.

Fasting precautions and side effects

Even well-planned fasts can have side effects that include:

• Dizziness
• Headaches
• Nausea
• Weakness or shaking
• Mood issues
• Insomnia
• Fatigue

Staying hydrated and avoiding high-intensity exercise during your fasting windows may help minimize some symptoms. But side effects that interfere with daily life or become severe could indicate that fasting isn’t a safe option for you.

Who shouldn’t fast for gut health (or any other reason)?

Total fasts and diets that include periods of fasting aren’t advisable if you:

• Have a history of eating disorders or restrictive eating patterns
• Are underweight
• Have an active infection
• Have low blood pressure
• Are pregnant, breastfeeding or trying to become pregnant
• Have amenorrhea or a history of hormonal imbalances
• Are taking prescription medications
• Are under 18 or over 75

Women may need to be particularly mindful of fasting’s potential effects. Some research suggests that following intermittent fasting regimens with fasting windows longer than 12 hours may reduce estrogen and progesterone levels. Fasting may also affect levels of DHEA, a hormone related to fertility.

If you’re having trouble with gut health and aren’t sure whether a fast might help, discuss your options with a doctor, nutritionist or functional medicine professional who can monitor you during the process and address any concerns you may have.