How fasting can and can't improve gut health

IIf you spend a lot of time online, you may have noticed that parts of the internet have caught starvation fever. Online message boards are full of posts touting the benefits of time-restricted eating and more intermittent fasting approaches which include the absence of caloric foods or drinks for an extended period of time – from 12 hours to several days. These online recommendations have helped popularize intermittent fasting and often contain two common-sense rationalizations: First, that human beings evolved in environments where food was scarce and meals were sporadic; and second, that the relatively recent shift to almost 24/7 eating is detrimental to our gut and metabolic health.

Digging the internet for accurate information, 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 nuggets can be easy to find. Much of the published peer-reviewed research on intermittent fasting makes the same claims you’ll find on these Reddit message boards. “Until recently, food availability has been unpredictable for humans,” the authors wrote in a 2021 review. 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 intermittent periods of food scarcity.” They say fasting regimes can provide a period of “gut rest” that can lead to several significant health benefits, including improved gut microbial diversity, gut barrier function, and immune function.

The past decade has seen an explosion in research related to fasting. (According to Google Scholar, the last five years alone contain nearly 150,000 articles that discuss or mention fasting.) While this work has helped establish links between intermittent fasting and weight loss and other benefits, it’s still unclear when (or whether) fasting can help heal a sick gut. “I would still consider the evidence to be moderate,” says Dr. Emeron Meyer, professor of medicine and founder of the Goodman Luskin Microbiome Center at UCLA. “[Fasting] seems like a reasonable way to maintain metabolic health or restore metabolic health, but it’s not a miracle cure.

When it comes to intestinal conditions like inflammatory bowel disease (IBD), he says the research is either lacking or inconclusive. In his view, researchers have found that fasting during Ramadan – a month-long religious period when people do not eat or drink between sunrise and sunset – can significantly “remodel” the bacterial communities in the gut in beneficial and healthy ways. However, among people with IBD, studies of fasting during Ramadan have also found that a person’s gut symptoms may worsen.

While it’s too early to tout fasting plans as a panacea for gut-related ailments, experts say there’s still 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 can restore the gut

For a series of 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 gastrointestinal tract. (Ramadan is mentioned frequently in published research because it provides a real-world opportunity for experts to study the effects of a 12- or 16-hour fast, which is what many popular intermittent fasting diets advocate.) “We really wanted to know what intermittent fasting affects the body,” says Dr. Michael Pepelenbosch, a member of this research team and 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 as a whole, not just Ramadan, you see that some types of bacteria increase.

For example, he says that intermittent fasting pumps out the gut population of a family of bacteria called Lachnospiraceae. “In the gut, bacteria are constantly fighting for ecological space,” he explains. Unlike some other intestinal microorganisms, Lachnospiraceae can survive happily in an empty gastrointestinal tract. “They can live off the mucus that the gut produces itself, so they can compete with other bacteria in the fasted state.” 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 gut dysfunction. Butyrate also improves gut barrier function, says Pepelenbosch. This is potentially a very big deal. Poor barrier function (sometimes called “leaky gut”) is a hallmark of common gastrointestinal diseases, including inflammatory bowel disease. If intermittent fasting can reduce inflammation and also help normalize the walls of the gastrointestinal tract, these changes could have major therapeutic implications.

Lachnospiraceae is just one of several types of beneficial bacteria that research has linked to fasting plans. But at this stage there are still many gaps in the science. Pepelenbosch says that the guts of people with gut disorders don’t seem to respond to fasting in quite the same way as the guts of people without those health problems. “In sick people, we see the same changes in the microbiome, but they are not as clear as in healthy volunteers,” he says. “So now we’re actually trying to figure out what’s going on there.”

Healthy changes in the microbiome aren’t the only possible benefits researchers have linked to intermittent fasting. UCLA’s Meyer mentions a phenomenon called the migrating motor complex. “It’s rarely mentioned in fasting articles today, 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 wash the contents of the gut, including its bacteria, down into the colon. “It’s this 90-minute repetitive contractile wave that goes down the gut, and its force is comparable to a nutcracker,” he says. Essentially, this motor complex behaves like a street cleaning crew tidying up after a parade. It ensures that the bowels are cleansed and cleansed between meals, through 90-minute repetitive cycles that fasting allows to become more frequent. It also helps to rebalance the microbial populations of the gut so that more of them reside in the colon and lower areas of the gastrointestinal tract. “But it stops the moment you take a bite – it shuts off immediately,” he says.

Meier says that modern eating habits — so-called “grazing,” or constant eating throughout the day — leave little time for the migrating motor complex to do its job. “This function is shifted to when we sleep, but even that is disrupted because many people wake up in the middle of the night and eat something,” he says. “So those longer periods of time where we’re cleansing and rebalancing our gut so that we have a normal distribution of bacteria and a normal population density — that’s seriously disrupted by these lifestyle changes.”

Ideally, Meyer says, people could (for the most part) stick to the kind of time-restricted eating program that allows a full 12 to 14 hours each day for the motor complex to work. “If you don’t eat, that motor complex will happen between meals, and you get that 12 to 2 p.m. window at night when the digestive system is empty,” he explains. In other words, sticking to three meals a day and avoiding snacking between meals (or nighttime snacks) may be enough. But again, it’s not clear whether this kind of feeding schedule can reverse gut damage or treat existing dysfunction.

Read more: The truth about fasting and type 2 diabetes

More potential benefits

Another possible benefit of fasting involves a biological process called “autophagy.” During autophagy, old or damaged cells die and are cleared from the body. Some researchers have called it a helpful housekeeping mechanism, and it occurs naturally when the body goes without energy (calories) for an extended period of time. There is some expert speculation, based mostly on evidence in laboratory and animal studies, that autophagy may help strengthen the gut or counteract the types of barrier problems seen in people with IBD. But these improvements have yet to be demonstrated in real human clinical trials.

Meanwhile, some experts have found that fasting can help recalibrate the gut’s metabolic rhythms in beneficial ways. “By changing the timing of the diet, it will really change the activity of the
microbiome and this can have downstream health impacts,” says Dr. Eran Elinav, Principal Investigator of the Host-Microbiome Interactions Research Group at the Weizmann Institute of Science in Israel.

Some of Elinav’s work, including an influential 2016 journal article cell, showed that the gut microbiome undergoes day-night shifts that are influenced by a person’s dietary schedule, and this leads to changing patterns of 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. This is likely to have health implications, although exactly what they are remains murky.

Read more: What we know about leaky gut syndrome

Fasting gets you nowhere

It’s clear that when you eat, including how often you eat, matters for your gut health. But the devil is in the details. At this stage, it is not clear how intermittent fasting can be used to help people with gut-related disorders or metabolic diseases.

“For a condition like IBD, it’s important to distinguish between what you do during an attack and what you do to prevent the next attack,” Meyer points out. Research on people observing Ramadan suggests that, at least during an attack, fasting can help a person’s IBD symptoms worse. Figuring out whether fasting can also lead to long-term improvements is just one of many questions that need to be answered.

Although much remains unknown, experts say that common approaches to fasting appear to be safe for most people. Time-restricted eating, for example, involves cramming all your calories for the day into one six- to eight-hour eating window. Even among people with metabolic diseases such as type 2 diabetes, research shows that this form of fasting is safe as long as the person is not taking blood sugar medication.

However, there just isn’t much work on intermittent fasting as a treatment for gut problems. In addition, there is very little research on more extreme forms of fasting, such as plans that involve calorie-free conditions for several days in a row. These diets can be therapeutic, but they can also 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 Pepelenbosch. “But generally speaking, increasing the interval between caloric intakes is a good thing for you. The body is not designed to eat all day.

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