Intermittent Fasting and Gut Health
By now, I think almost everyone has at least heard about Intermittent Fasting. It’s often scattered in amongst health conversations on topics like plant-based burgers, keto or ancestral diets and morning rituals.
But it’s also a conversation you may have had with a naturopath or alternative healthcare practitioner if you suffer, or have suffered, with chronic gut issues.
When I mentored with a local naturopath she often encouraged her patients with digestive dysfunction to experiment with the practice of fasting to improve gut health and microbiome balance.
There are lots of reasons to engage in intermittent fasting, but in this blog I wanted to specifically address how fasting impacts gastrointestinal health.
I’ll define what it actually means to be in a “fasted state”, what intermittent fasting is (and what it isn’t), and then get into what science tells us about how fasting can improve gut functioning.
Fed versus Fasted States
When you eat food, your body breaks it down into small molecules, and those molecules enter your blood.
Your body then releases insulin to transport nutrients for use or storage.
While your body is digesting food, you’re considered to be in a “fed” state. Depending on the size and composition of your meal this can last between 3-6 hours.
Once your body finishes processing the food, your insulin levels then drop to baseline levels. This is then known as the “fasted” state.
It’s important to keep in mind that just because you’re hungry does not mean you are fasted. Even if you are a little hungry or you feel like your stomach is empty, you might not be in a fasted state.
For example, if you eat a large lunch around noon and then start getting hungry around 4 or 5 pm, that doesn’t mean you’re in a fasted state.
But if you have your last meal of the day around 7 pm and then get up around 5 am, you’re in a fasted state.
What Is Intermittent Fasting?
In the world of intermittent fasting, “fasting” doesn’t refer to not eating for days on end. Instead, it refers to the practice of abstaining from meals for a set period. It’s a dietary strategy in which you refrain from eating for a specific pre-determined length of time.
Intermittent fasting (IF for short) starting gaining popularity alongside the dietary practices of ancestral or Paleolithic eating. It’s an attempt to return to a pre-agricultural practice of eating when food is available and not eating when it isn’t.
Fasting is an ancient fail-safe designed to boost our body’s resilience during conditions of starvation. Our bodies are literally designed for “feast and famine”.
But most people don’t use IF to build resilience to starvation (an outdated concern in today’s overfed society) but for the other health benefits the practice can provide, such as autophagy, a process of enhanced “cellular recycling” akin to spring cleaning.
Fasting also turns on powerful anti-inflammatory healing mechanisms within the body. We experience a sharp increase in our bodies production of antioxidants and we optimize our immune system.
Fasting can also (depending upon the context) help with weight loss and body composition efforts.
I’m not going to go into detail on all the health benefits of intermittent fasting here. But I do want to break down three ways IF is commonly practiced and briefly explain each one.
In no particular order:
Time-restricted feeding (TRF)
Alternate day fasting (ADF)
Prolonged fasting (24-36 hours)
Time-restricted feeding involves daily fasting, while limiting feeding to certain hours of the day. This is commonly know as fed and unfed windows of time. It’s pretty straightforward, there are 24 hours in a day so perhaps you fast for 16, and eat for 8.
Or a more aggressive approach might be eating for 6 and fasting for 18.
A temperate approach would be eating for 10 and fasting for 14.
Alternate day fasting on the other hand, involves more planning. It requires you to know how many calories you put in your body on a daily basis as well.
Here’s my understanding of how it works: on one day you consume 25% of your normal caloric intake. So if we use the standard 2000 calorie diet as an example, that looks like 500 calories a day. Ouch.
But on the second day you actually eat an additional 25% of your calories or 2500 calories. This pattern continues on an alternate day schedule, hence where it gets its name.
Finally, you can fast on a once weekly schedule for 24 hours or more. This means going an entire day without food. In my experience this option is less desirable but equally as effective as the other strategies. Basically you go a full 24 hours without eating.
I knowalsp want to mention that while I know IF is trendy, and the internet is saturated with information on how do do it, fasting really should not really be attempted without PROPER SUPERVISION from a health coach, registered dietitian or other medical professional.
So, Can Fasting Improve Gut Health?
In order to understand how intermittent fasting may be helpful for those suffering with digestive issues, let’s talk about what happens in the gut in between meals.
Let’s start by discussing gut motility, the coordinated contraction of gut muscles that move your food from the mouth all the way down to the colon.
Motility is controlled by the nervous system in the gut. This nervous system is known as the enteric nervous system (ENS) and is why the gut has been called the “second brain.”
Proper motility is crucial for the movement of food, waste and bacteria through the digestive tract. In the most basic sense a disruption in motility can either cause things to stop or slow down (such as constipation) or speed up (such as diarrhea).
A major feature of gut motility is whats known as the migrating motor complex (MMC). This is a series of contractions that occur in the stomach and small intestine to push out any residual food particles or bacteria.
Think of the MMC as a cleaning service, that works in between meals to tidy the stomach and small intestine.
It comes along after you eat and sweeps away undigested food particles to the large intestine through a series of contractions that happen in three phases.
What you need to know about the MMC is that this process- this very essential process- can only happen between meals. The moment you put food in your mouth, it stops.
Basically the intestines have two modes: digesting your food or cleaning up afterwards.
Here’s why this matters: the cleaning wave happens approximately every 90-120 minutes, assuming you’re not eating. And it takes this cleaning crew about two hours to move from the stomach to the ileum (the final section of the small intestine).
If you’re constantly eating, or eating every two hours, guess what. Your MMC can’t come in and properly do it’s job.
This cleaning process does a number of beneficial things, but most importantly it sweeps bacteria out of the small intestine, sending it back home to the colon.
The MMC also cleans up any lingering food particles and anything else that shouldn’t be in there.
This is why an absent, impaired or dysfunctional MMC has been linked to digestive issues including SIBO (small intestinal bacteria overgrowth). If bacteria are not swept out of the small intestine regularly and properly it creates an overpopulation where there shouldn’t be one.
This is why fasting is often recommended for digestive health. It’s one way to support the MMC function.
Gut hormones are also really important for intestinal motility. Two of these hormones are motilin and ghrelin. Aptly named, motilin is a hormone released during the fasted state that increases intestinal motility via the MMC.
Ghrelin is released when we’re hungry and has a particular impact on gastric (stomach) motility and appetite.
The growling of your stomach isn’t always just a sign you’re hungry. It’s a sign that your internal vacuum cleaner is up and running.
Its the sound of gut motility in action.
The same hormones that stimulate appetite also stimulate the intestines to clear out. After all, you need to make room for the incoming food. Intermittent fasting taps into this system. It does this by stimulating hunger and the subsequent release of gut hormones.
How Fasting Impacts the Gut
Now that you know the MMC function is crucial to intestinal health, I can start by saying fasting can help in the case of conditions like SIBO (small intestinal bacterial overgrowth)
Although there are many causes of SIBO, it often results after damage to the MMC occurs.
If the MMC doesn’t function properly it impairs the body’s ability to clear out bacteria from the small intestine. This leads to an overgrowth of bacteria and stubborn symptoms like gas, bloating, diarrhea and/or constipation.
One popular strategy of SIBO treatment simply involves lengthening the time between meals. For example allowing 4-5 hours between meals versus 2-3 can theoretically improve MMC function. Intermittent fasting of course, can also help.
SIBO often overlaps with IBS (Irritable Bowel Syndrome). One of several ways intermittent fasting might improve symptoms is by providing what’s called “bowel rest.” A diet low in fermentable carbohydrates (AKA a Low FODMAPs diet) is a well known treatment for IBS. It works by reducing the amount of carbohydrates that bacteria can feed on and ferment. This results in a reduction in gas and water in the bowels, which lessens pain and reduces diarrhea.
In a similar manner, fasting can accomplish the same thing. It provides a much needed break for overly-sensitive gut nerves which often contribute to symptoms in gut disorders.
IF can help reduce intestinal permeability or leaky gut.
Intestinal permeability refers to the passage of contents from the intestines into the rest of the body. The gut barrier is highly selective in controlling what goes out of and comes into the intestines.
When the gut barrier (intestinal barrier) is disrupted due to injuries, infections, or microbial toxins, the intestinal contents can leak into the bloodstream. Hence the name “leaky gut.”
This condition can potentially increase the risk of many diseases and autoimmune conditions like IBD and diabetes.
A 2017 study found that fasting can help reduce gut permeability and possibly cut down the risk of these disorders.
Researchers of the study believe that extended fasting or bowel rest, could reduce gut permeability and systemic inflammation. Note that long-term systemic inflammation can cause cancer, heart disease, diabetes, asthma, and swollen joints.
IF also boosts Intestinal function. Aging decreases intestinal function by decreasing the number of intestinal stem cells (ISC).
Moreover, aging also impairs the function of intestinal stem cells.
ISC are early forms of cells that continuously divide and form new types of specialized cells in the intestines. When an injury occurs in the intestines, ISC can help repair it by dividing into the types of cells that have been injured.
A 2018 study on mice found that fasting for 24 hours enhanced their intestinal stem cell function. They concluded that fasting could be a novel approach to helping intestinal cell regeneration and maintaining gut health.
There is also some pretty significant science backing up the positive impact fasting can have on the microbiome.
First, fasting can improve the population of certain species of bacteria that are associated with positive health outcomes. This has been a consistent finding in studies looking at the impact of fasting on the microbiome.
In fact, just recently intermittent fasting was put to the test in a clinical trial. The researchers wanted to see if intermittent fasting would influence the growth of beneficial bacteria in the gut.
They found two species were significantly increased: Akkermansia mucinophila and Bacteroides fragiles. These two species are known to have significant health-promoting effects. Akkermansia for example, helps maintain the health of the intestinal barrier.
Fasting also improves the balance of bacteria in the gut. For optimal gut health, there has to be a balance between different types of bacteria.
When a person’s gut loses this balance it’s called dysbiosis. That’s when certain types of bacteria can overgrow and lead to serious complications.
IF has shown promising results in helping maintain gut balance by keeping different types of gut bacteria in check.
In a 2016 study, rats that fasted 72 hours every 2 weeks for 18 weeks had improved gut balance. Their gut had more Helicobacteraceae and less Lactobacillus, Roseburia, Erysipelotrichaceae, and Ralstonia. In addition, the fasting rats had significantly lower levels of cholesterol in the bloodstream.
Also, what’s called the Firmicutes:Bacteroides ratio, a well known microbiome marker for overall health is improved with intermittent fasting. Obesity and metabolic syndrome are associated with less favorable ratios.
IF also boosts bacterial diversity in the microbiome and diversity is important in gut health.
Bacterial diversity measures how many different types of bacteria are present in the intestines and how evenly they are distributed.
Generally, higher bacterial diversity gut balance while lower diversity indicates dysbiosis.
In a recent study, a team of researchers led by Francesca Cignarella, Ph.D. (Neuroimmunology Scientist at Washington University in St. Louis) found that IF significantly increased bacterial diversity in mice after four weeks of IF.
So if fasting is “so” healthy, we all should be fasting right?
Is Fasting Ideal for Everyone?
In short, no. There are drawbacks to fasting and not everyone should do it. Just as with any dietary approach, it's all about the individual and the context.
I'll go through some situations in which fasting might not work optimally nor be ideal.
For starters, individuals prone to dysglycemia or irregular blood sugar levels will usually struggle with fasting. Similarly, Type 1 and 2 diabetics absolutely need to be closely monitored by a professional to avoid life-threatening complications like hypoglycemia.
It’s also been my experience that for those with moderate to severe IBD it likely isn’t optimal either. While there’s research showing that intermittent fasting can benefit people in the general population, and those with IBS and dysbiosis, studies in people with Crohn’s and Colitis are lacking.
Lack of research aside, I also believe that people with Crohn’s/Colitis shouldn’t try IF, partly because they may already be deficient in certain nutrients or underweight. Some studies have suggested that fasting in general — though not IF specifically — can lead to nutrient deficiencies, including protein. This is what an analysis published in October 2020 in the journal Nutrients found.
Limiting the amount of time you eat each day — or even the number of days per week — also limits the number of opportunities you have to get calories, vitamins, and minerals into your body which is not a good idea for anyone who is underweight or who struggles with nutrient deficiencies due to digestive flareups.
Also individuals with a history of disordered eating patterns or an eating disorder should not attempt intermittent fasting as the practice of depriving yourself from food can exasperate an unhealthy relationship with food. I’ve found that many people who have a history of binging i particular respond terribly to IF.
It’s also worth mentioning that many individuals will respond poorly to the “stress” (yes fasting is a physiologically perceived stressor) of fasting.
This is especially true of women who tend to generally be more stress responsive then men. It’s also true of individuals who already have a lot of stress present in their lives, such as people with high stress jobs, or anyone with an HPA (hypothalamic-pituitary-adrenal-thyroid) problem.
A stress response to fasting could manifest as:
Low resilience to stress
Frequent infections and immune challenges
Significant hair thinning, falling out or very dry skin
IF works because it stresses your body into adaptation. But if you’re already under a tremendous amount of stress then there’s no more room for adaptation.
This can cause some of the symptoms I mentioned above.
It can generally cause people to feel lousy and defeated because they didn’t respond like “everyone else” but the truth is, just because IF is popular, or trendy, or even effective in certain contexts doesn’t mean it is going to work optimally for everyone.
If these instances apply to you, it may be necessary to support those systems first before considering intermittent fasting or it might make them worse.
In addition to the stress response symptoms I mentioned above, I would encourage you to watch out for these potential side effects:
Binge eating leading to weight gain
Disordered eating or heightened obsessiveness with food
Worsening of sleep patterns
Dangerous blood sugar swings (hypoglycemia)
Changes in electrolytes (if not hydrating properly)
Constipation – yeah, it can possibly worsen digestive symptoms too.... all about context!
Low energy or fatigue
If you do decide to give fasting a try I’d encourage you to do so with professional guidance, and to start modestly, perhaps with a simple 12 hour unfed window. If you eat your last meal of the day at 7pm, simply eat breakfast the following morning around 7am.
A less aggressive method of fasting like this is usually the best place to start to assess how fasting impacts you. Then, listen to your body and pay attention to how you respond.