Does fasting improve gut health? What to know

IIf you spend a lot of time online, you may have noticed that some parts of the internet have been hit with fasting fever. Online message boards are full of posts touting the benefits of time-restricted eating and other forms of intermittent fasting, which involve going without high-calorie food or drink for extended periods of time, from 12 hours to several days. These online testimonials have helped popularize intermittent fasting and often have two common-sense rationales: one, that humans evolved in environments where food was scarce and meals were sporadic; and two, that the relatively recent shift to almost 24/7 eating has been disastrous for our gut and metabolic health.

Finding accurate information online, especially when it comes to diet, can feel like panning for gold. You have to sift through a lot of junk to find something of value. But this is one case where the pieces can be easy to find. Many of the published peer-reviewed studies on intermittent fasting make the same claims you’ll find on these Reddit message boards. “Until recently, food availability has been unpredictable for humans,” the authors of the 2021 review paper wrote in the journal. American Journal of Physiology. “Knowledge of early human evolution and data from recent studies of hunter-gatherer societies suggest that humans evolved in environments with alternating periods of food scarcity.” They say that fasting regimens can provide a period of gut rest that can lead to a number of important health benefits, including improved gut microbial diversity, gut barrier function, and immune function.

In the past decade, there has been an explosion of scientific research related to fasting. (According to Google Scholar, the past five years alone include nearly 150,000 articles that discuss or mention fasting.) While this work has helped establish links between intermittent fasting and weight loss, as well as other benefits, it’s not yet clear when (or if) fasting can help. to fix a sick gut. “I would still consider the evidence to be moderate,” says Dr. Emeran Mayer, professor of medicine and founding director of the Goodman Luskin Microbiome Center at the University of California, Los Angeles. “[Fasting] seems like a reasonable way to maintain or restore metabolic health, but it’s not a panacea.

When it comes to gut diseases like inflammatory bowel disease (IBD), he says research is either lacking or inconclusive. According to him, researchers have found that the fasting of Ramadan – a month-long religious period in which people do not eat or drink between sunrise and sunset – can significantly reshape gut bacterial communities in beneficial and healthy ways. However, among people with IBD, studies of Ramadan fasting have found that a person’s gut symptoms can worsen.

While it’s too early to talk about fasting plans as a panacea for gut-related disorders, experts say there’s reason to hope that these approaches could emerge as a form of treatment. Clearly, some radical, and perhaps radically beneficial, things happen when you give your body a break from food.

How fasting could improve the gut

For several recent studies, a team of researchers based in the Netherlands and China investigated the effects of Ramadan-style intermittent fasting on the gut microbiome, the billions of bacteria that live in the human digestive tract. (Ramadan figures heavily in published studies because it gives experts a real-world opportunity to study the effects of the 12- or 16-hour fasts that many popular intermittent fasting diets advocate.) “We really wanted to know what intermittent fasting does to the body,” says a member of this research team and Dr. Maikel Peppelenbosch, professor of gastroenterology at the Erasmus University Medical Center in the Netherlands. “Overall, we’ve seen that intermittent fasting changes the microbiome very clearly, and we think some of the changes are beneficial. If you look at fasting in general, not just Ramadan, you see that certain types of bacteria increase.

For example, he says, intermittent fasting pumps up a family of bacteria in the gut, the so-called Lachnospiraceae. “In the gut, bacteria are constantly fighting for ecological space,” he explains. Unlike some other gut micro-organisms, Lachnospiraceae can survive happily in an empty digestive tract. “They can live off the mucus that the gut makes itself, so they can compete with other bacteria in the empty stomach.” Lachnospiraceae produces a short-chain fatty acid called butyrate, which appears to be critical for gut health. Butyrate sends anti-inflammatory signals to the immune system, which can help reduce pain and other symptoms of bowel dysfunction. Butyrate also improves gut barrier function, says Peppelenbosch. This is potentially a very big deal. Poor barrier function (sometimes called “leaky gut”) is a hallmark of common gastrointestinal conditions, including inflammatory bowel disease. If intermittent fasting can slow down inflammation and also help normalize the walls of the digestive tract, these changes can have a big therapeutic effect.

Lachnospiraceae is just one of the many beneficial bacteria that research has linked to fasting plans. But at this point, there are still many gaps in the science. Peppelenbosch says that the intestines of people with intestinal disorders do not respond to fasting in exactly the same way as the intestines of people without such health problems. “In sick people, we see the same changes in the microbiome, but it’s not as clear as in healthy volunteers,” he says. “So we’re now really trying to figure out what’s going on there.”

Healthy microbiome shifts aren’t the only potential benefits researchers have linked to intermittent fasting. UCLA’s Mayer mentions a phenomenon called the wandering motor complex. “It’s rarely mentioned in fasting articles these days, but when I was a junior faculty member, it was one of the hottest discoveries in gastroenterology research,” he says. The migrating motor complex refers to repeated cycles of powerful contractions that sweep the contents of the gut, including its bacteria, into the colon. “It’s this 90-minute repetitive contractile wave that slides down the gut, and it’s comparable in strength to a nutcracker,” he says. Basically, this motor complex acts like a street cleaning crew cleaning up after a parade. It ensures bowel cleansing and cleansing between meals through repeated 90-minute cycles, which fasting allows to make more frequent. It also helps to balance the populations of gut microbes so that more of them live in the colon and lower parts of the digestive tract. “But it stops the minute you bite—it shuts off immediately,” he says.

Mayer says that modern eating habits—so-called “grazing,” or eating continuously throughout the day—leave little time for the itinerant motor complex to perform its tasks. “This function has remained until we sleep, but even that has been disrupted because many people wake up in the middle of the night and snack,” he says. “So those longer periods of time where we’re cleaning up our gut and rebalancing it so that we have a normal bacterial distribution and a normal population density, these lifestyle changes are seriously disrupted.”

According to Mayer, people could (mostly) follow such a time-restricted eating program that allows the motor complex to work for 12 to 14 hours each day. “If you didn’t snack, there would be this motor complex between meals and there would be this 12- to 14-hour window at night where the digestive system was empty,” he explains. In other words, three meals a day and avoiding snacks between meals (or nighttime snacks) may be sufficient. But again, it is not clear whether such a dietary schedule can reverse intestinal damage or treat existing dysfunctions.

Read more: The truth about fasting and type 2 diabetes

More potential benefits

Another potential benefit of fasting involves a biological process called autophagy. During autophagy, old or damaged cells die and are removed by the body. Some scientists have called it a useful housekeeping mechanism and it occurs naturally when the body goes without energy (calories) for an extended period of time. Experts have speculated, based mostly on evidence from laboratory and animal studies, that autophagy may help strengthen the gut or combat barrier problems in people with IBD. However, these improvements have yet to be demonstrated in real-world clinical trials involving humans.

Meanwhile, some experts have found that fasting can help recalibrate the gut’s metabolic rhythms in beneficial ways. “By changing the timing of the feeding, it really changes the activity as well
microbiome and may have downstream effects on health,” says Dr. Eran Elinav, lead researcher of the Host-Microbiome Interaction Research Group at the Weizmann Institute of Science in Israel.

Some of Elinavi’s work, including an influential 2016 journal article Chamber, have shown that there are day-night shifts in the gut microbiome that are influenced by a person’s eating schedule, leading to changes in metabolite production, gene expression, and other important elements of gut health. “If you change the timing of the diet, you can reverse the circadian activity of the microbiome,” he says. It likely has health implications, though exactly what those are remains murky.

Read more: What we know about leaky gut syndrome

Fasting will get you nowhere

It’s clear that what you eat, including how often you eat, affects your gut health. But the devil is in the details. It is currently unclear how intermittent fasting can be used to help people with gut-related disorders or metabolic diseases.

“With a condition like IBD, it’s important to distinguish between what you do during a flare-up and what you do to prevent the next flare-up,” Mayer points out. Studies of people observing Ramadan show that, at least during flare-ups, fasting can make a person’s IBD symptoms worse. Calculating whether fasting can lead to longer-term improvements is just one of many questions that need to be answered.

Although there are still many unknowns, experts say that common approaches to fasting appear to be safe for most people. For example, time-restricted eating involves cramming calories throughout the day into one six- to eight-hour eating window. Research shows that even among people with metabolic diseases such as type 2 diabetes, such fasting is safe as long as the person is not taking blood sugar-lowering medications.

That said, there just isn’t much work to be done on intermittent fasting as a treatment for gut problems. There is also very little research into more extreme forms of fasting, such as plans that involve going without calories for several days in a row. These diets can prove to be therapeutic, but they can also prove to be dangerous. If you are considering any of these approaches, talk to your healthcare provider first.

“We really need much better studies to compare all the different fasting protocols,” says Peppelenbosch. “But in general, widening your caloric intake is good for you. The body isn’t designed to eat all day.

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