How intermittent fasting impacts your gut health or microbiome

Here I go again, touting the benefits of intermittent fasting, and this time it’s about the microbiome in your gastrointestinal tract. The microbiome encompasses all microbes, including bacteria, viruses, and fungi that naturally live in and on the body. These microbes don’t sound like a big deal — just a bunch of germs — but they contribute to the health of the body in many important ways.

It is estimated that there are over 100 trillion microbes, mostly bacteria that live in the upper (small) and lower (large) intestine. We still have a lot to learn about these microscopic organisms, but among other things, they help mediate the inflammatory processes ongoing in the body. This important issue has captured center stage when it comes to discussing devastating diseases promoted by inflammation, including diabetes, heart disease, some forms of cancer, and even dementia. What’s more, these microbes can be a factor in obesity, which makes everything, including inflammation, worse.

Since these microbes that make up the microbiome are so helpful, we want them to be as healthy as possible. The key to their health is diversity, and the greater the diversity the greater the number of ongoing helpful functions. This raises the question: How can you promote diversity in your microbiome?

Recent research points to two helpful interventions, intermittent fasting and a calorically restricted diet. The microbiome works all the time and is especially active when you eat and afterward. When you either fast or eat less, you allow the microbiome to take a bit of a break. In turn, this can help the microbes to repopulate and enhance diversity. In a sense, it does what sleep does for the body.

Is intermittent fasting or a calorically restricted diet better?

So, which works better, intermittent fasting or a calorically restricted diet? A recent study published in the journal “Nutrients” (2023) by scientists from the University of Colorado Medical School found both approaches to be effective in promoting microbiome health and diversity. The diet required participants to cut their daily caloric intake by 34%. That’s quite a bit, but it wasn’t a crash diet. Crash diets slash caloric intake to a much greater extent, resulting in rapid with loss. Unfortunately, much of the weight that is lost is muscle mass, weight you don’t want to lose. What’s more, the odds of sticking to a crash diet over time are incredibly small, which is a good thing because it is so unhealthy.

This means the study examined two healthy alternatives and both were helpful. The tradeoff is, with intermittent fasting you don’t cut the amount of food you eat, but rather you restrict your food intake to a narrow window. In my experience, the odds of sticking to intermittent fasting after the two-to-four-week adjustment phase are much greater than adhering to a calorically restricted diet.

Why is intermittent fasting effective as a diet?

Why is intermittent fasting so effective as a dietary intervention? Several reasons, but I believe the main selling point is you aren’t forced to make choices about what or how much you eat, and there is no calorie counting. What’s more, when you deprive yourself of something you enjoy eating on an ongoing basis it tends to become a focal point of stress. It’s like, “Gee, I can’t wait till this diet is over and I can go back to enjoying a Big Mac and fries.”

Such foods are not healthy, of course, and should be avoided. But, if they are typical of what you eat daily and you are permitted with intermittent fasting to keep eating them, what are you giving up? Nothing, other than you need to squeeze these foods into an eating window of no more than 8 hours, like from noon to 8 p.m. (or whatever hours work for you), and if you do, there will be beneficial effects. So, as to the mind games we play, the odds are greatly increased that you will make it till noon or later without eating, knowing you get to eat what you want.

Now, with this said, a Big Mac and fries are not ideal. Far from it, but it shows how intermittent fasting can still be helpful even under less-than-ideal circumstances. This means even if you continue to consume the same amount and kinds of food and the same number of calories, intermittent fasting will help you lose some body fat. On the other hand, if you eat healthier foods and add in some exercise, the loss of body fat accelerates greatly.

How does intermittent fasting impact my glucose level?

This is a common question and it centers around your blood sugar (glucose) concentration. The body carefully regulates blood glucose concentration to make certain it doesn’t go too low. When crash dieting with an ongoing severe caloric restriction there is inadequate intake of carbohydrates to sustain your blood glucose at a healthy level. So, when your blood glucose level drops and the body wants to bring it right back up, it can’t because of your severe diet. This means you must break down muscle tissue into proteins and then the proteins into amino acids. Some of the amino acids are very similar to glucose and can easily be converted to glucose in the liver. Problem solved, except you lose muscle mass, not fat.

In contrast, when you fast intermittently, each time you eat you are consuming plenty of food, including carbs, and easily replenishing your stockpile of glucose (stored as glycogen) in the liver. Therefore, when your blood glucose level drops, instead of breaking down muscle, you simply mobilize glucose from storage in the liver and release it into the bloodstream.

All in all, other than being disciplined and “truly” fasting for 16 hours daily (nothing other than black coffee, unsweetened tea, or water), you are not restricting yourself during your 8-hour eating window. To enhance results, gradually choose healthier foods and add in some exercise. To accelerate change, gradually narrow your eating window from eight to six hours or less.

Reach Bryant Stamford, a professor of kinesiology and integrative physiology at Hanover College, at stamford@hanover.edu.

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