As far as interventions go that prevent chronic disease, increased physical activity is king.  A recent article in Scientific American goes over a number of reasons why evidence indicates that human health is contingent upon adequate physical activity.  The article, titled Humans Evolved to Exercise, covers how our physiology seems to demand high levels of physical activity for optimal function.

The article was fantastic, and it motivated me to look at how the high level of physical activity that seems to prevent deterioration in brain function, mood, and musculoskeletal health does the same for the gut.  Turns out there’s quite a bit there to look at, with exercise having a number of beneficial effects on the microbiome, GI motility, autonomic function, and processing of endotoxin.

So let’s take a look at the many ways physical activity promotes good gut health.

Physical activity promotes GI motility

When most people think of GI motility, they think only about the contraction and relaxation of the smooth muscle cells that make up the intestine.  But there’s a lot more to GI motility than smooth muscle contraction/relaxation.  Skeletal muscles that are activated during heavy breathing and physical activity enclose the abdominal cavity and provide additional forces to move things along, so to speak.

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There are a couple of studies that indicate that physical activity is a critical component of GI motility.  A small pilot study in 2016 found that electrical stimulation of the transverse abdominis and external obliques improved GI transit time in people with MS who had chronic constipation.

Although it was a small study with only 4 people completing it, whole gut transit time was reduced on average by 38% and constipation scores improved in all participants. I’d venture to guess that it’s not so much the strengthening of these muscles as much as it’s the regular contraction mechanically promoting motility.

A fantastic review published last year took a look at non-GI symptoms that cluster with IBS and how they relate to functional impairments in muscles that make up the core.  These symptoms include changes in the coordination of the breathing muscles, pelvic pain, low back pain, TMJ dysfunction, chronic headaches, GERD, anxiety/depression, and widespread pain/inflammation.  Special focus is given to the diaphragm and how improper breathing as well as inadequate physical activity may play a role in IBS.

Physical activity improves autonomic balance

Regular physical activity improves autonomic nervous system function by promoting balance between the sympathetic and parasympathetic nervous systems.  Unfortunately there’s no real way to game or cheat this effect, it seems hardwired in to our physiology.  And we need to look at how physical activity and exercise are used separately to promote this effect.

Physical activity in the form of walking is probably the biggest ticket habit you can pick up that will improve autonomic balance, but form and technique certainly matter.  Your posture and exercise technique can make or break your success.  I use resistance training with clients to reverse some of the negative postural effects that come with being sedentary throughout the day.

Once good posture and technique is established, cardiovascular exercise is effective for improving autonomic balance provided proper breathing technique is used.  By virtue of its location, the diaphragm may play a role in stimulating the vagus nerve.  But it doesn’t do this in isolation, and strengthening the aforementioned core muscles that assist in breathing and GI motility may promote a synergistic effect.

However, for exercise to be truly beneficial, you have to progress at your own pace, especially if you’re dealing with some type of chronic issue.  Pushing yourself too hard or for too long will actually make autonomic imbalance worse, which is the opposite of what you’re trying to do.

Exercise increases the thermic effect of food/diet-induced thermogenesis

While most of the benefit to digestion comes with moving from a sedentary lifestyle to one with higher levels of general physical activity, exercise-induced improvements may take you even further.  A study looking at cardiorespiratory fitness(CRF) found that people with higher levels of CRF utilized more calories to process their food, especially after larger meals.

Part of this probably comes down to improving function of the autonomic nervous system as mentioned above.  But there’s also significant evidence that exercise alters the microbiome in a way that promotes the abundance of butyrate-producing bacteria.  We don’t know exactly how this works, but it’s believed that exposure to lactate from the blood increases the amount of monocarboxylate transporters(MCTs) on cells that make up the intestinal wall.

Since MCTs also transport short-chain fatty acids from the gut, this increase should also increase the amount of butyrate our cells can take in.  Increasing butyrate uptake in the epithelial cells that make up the gut increases their oxygen utilization, which is likely one of the reasons we see this increase in diet-induced thermogenesis in people with higher CRF.

Exercise decreases endotoxin levels in the blood

The gut performs a large number of functions that promote health.  One of these functions is as a barrier, preventing unwanted bacteria and bacterial components from entering the bloodstream and causing inflammation.  One of these bacterial components, called endotoxin, likely plays a role in increasing the risk for chronic disease by increasing systemic inflammation.

Keeping endotoxin out of the blood is important, and we have 2 ways of limiting the amount we’re exposed to.  First, having a strong intestinal barrier prevents endotoxin from gaining access through a “leaky gut”.  However, we absorb small amounts of endotoxin when we consume fat, so we also want to be adept at clearing it whenever we absorb it.

Exercise seems to play a role in doing both of these things.  First, by increasing butyrate-producing bacteria, exercise strengthens the intestinal barrier by increasing expression of the tight junction proteins that seal the gut.  Additionally, exercise improves endotoxin clearance and decreases the inflammatory response to it.  These 2 factors are the likely reasons why highly trained people have lower serum endotoxin levels than those who are sedentary.

Myokines and the gut

It should come as no surprise that exercise benefits the gut.  There’s a ton of data supporting exercise as a therapy for preventing colon cancer or preventing it from coming back.  What’s less clear is how exercise accomplishes this, but myokines released from muscle during exercise are an attractive candidate.

Myokines are signaling molecules released from exercising muscles that have effects throughout the body.  Irisin is a myokine that has garnered a lot of research interest due to its potential ability to convert energy storing white fat in to energy burning brown fat.  A recent study showed irisin increases bone density in mice.

A study published last year found that irisin restored normal lymphatic architecture in mice with inflammatory bowel disease(IBD).  As in humans, this mouse model of IBD presented with an increase in lymphatic vessel infiltration in the colon due to chronic inflammation.  Administering irisin restored the lymphatic architecture to that of healthy controls.

The cool thing about this study was that while the colon was the initial site of inflammation, chronic IBD eventually affected bone turnover, mimicking osteoporosis.  But don’t fret, the irisin treatment had the same beneficial effect in bone as it did the colon.  While this is a long way from directly showing that exercise-induced irisin has the same effect, it could certainly explain the anti-inflammatory and anti-colon cancer effects of exercise.

Conclusion

No matter how you cut it, regular physical activity and exercise are very beneficial to gut health.  Some of the mechanisms by which exercise does this include promoting GI motility, strengthening the muscles that buttress the intestines, improving autonomic balance, altering the microbiome, increasing diet-induced thermogenesis, decreasing serum endotoxin, and myokine induced inhibition of inflammation.

While this is a pretty good sized list, I’m certain this is just scratching the surface.