When it comes to exercise, there is definitely a happy medium that provides health benefits. Too little activity is associated with detrimental health effects, including a compromised immune system, decreased resistance to stress, and decreased resilience to circadian rhythms.
However, too much (too strenuous, too intense) activity also negatively impacts health, causing dysregulated cortisol, increased susceptibility to immune-related diseases and infection, and a leaky gut.
It actually shouldn’t come as a surprise that intense and strenuous activities cause gut problems. Up to half of all long-distance runners experience something called runner’s diarrhea (colloquially referred to as “runner’s runs,” “runner’s trots,” or “the gingerbread man”).
The symptoms include dizziness, nausea, stomach or intestinal cramps, vomiting, and diarrhea, which occur mainly while running. All these are symptoms of something more insidious happening in the gut.
While not all endurance athletes suffer overt symptoms, strenuous exercise does appear to increase intestinal permeability in everyone who indulges in exhaustive exercise, albeit to varying degrees. A variety of studies have documented increased intestinal permeability in athletes who reported no gastrointestinal symptoms. And one study showed that well-trained athletes who suffered from exercise-induced gastrointestinal symptoms experienced significantly more intestinal permeability after exercise than asymptomatic athletes.
Cortisol (Oh and CRH too) & Gut Health
The way that exercise increases intestinal permeability is multi-faceted. First, intense activity is a stress on the body and activates the Hypothalamus-Pituitary-Adrenal (HPA) Axis, i.e., the Stress Axis or the Fight-or-Flight Response. This causes the release of two important hormones that directly affect gut health.
The better known of these hormones is cortisol, the master stress hormone. This secretion of cortisol is largely responsible for the negative impact of intense activity on the immune system, however it can also directly affect the integrity of the gut barrier as well. Very high cortisol alters tight junction assembly, making the gut more permeable to low molecular weight substances and less permeable to high molecular weight substances. High cortisol further, decreases gut motility, decreases mucus production in the gut (by goblet cells), inhibits digestion (by reducing pancreatic enzyme secretion and gallbladder function), decreases intestinal blood flow and may even directly alter the gut microbiome.
Yep, cortisol is not your gut’s friend. But when it comes to activating the HPA Axis, there’s another upstream hormone that has an even more profound impact on the health of the gut: corticotropin-releasing hormone, or CRH. CRH is released by the hypothalamus in response to stress (regardless of whether the stress is a lion chasing you, a traffic jam when you’re late for an important meeting, or a 3-hour long marathon training run).
CRH signals to the pituitary gland to release another hormone which then signals to the adrenal glands to release everything they release, including cortisol. CRH not only increases the permeability of the intestinal barrier (which causes a leaky gut), but it also increases permeability of the lung barrier, the skin barrier, and the blood-brain barrier. It does this two ways:
- First, it activates mast cells (an immune system cell type that resides in most tissues and is a major producer of histamine, for example during allergic reactions) which then secrete a variety of substances (histamine, heparin, and cytokines) that trigger the opening of tight junctions.
- Second, CRH has a direct effect on the tight junctions by increasing the incorporation of a type of protein into the junction that causes those junctions to be more open (the tight junction protein is called claudin-2, and when claudin-2 is incorporated into tight junctions, epithelial barriers are more leaky, or permeable).
However, the stress response isn’t the only way that exercise causes a leaky gut.
Blood Flow & Leaky Gut
In order to prioritise blood flow to the heart and skeletal muscles during exercise, blood flow is diverted away from the gastrointestinal tract and other visceral organs (like the liver and spleen). This lack of sufficient blood flow results in what is called ischemic injury (injury that results from inadequate blood supply) to the gut, which disrupts the intestinal barrier and thus increases intestinal permeability (aka, the dreaded leaky gut).
Participating in strenuous and exhaustive exercise further stimulates the production of a class of proteins called heat shock proteins (so called because the first of this family of proteins was discovered to be induced by fevers). Heat shock proteins have a direct effect on tight junctions (by affecting the levels of two integral protein families called occludin and claudin), opening them up and causing a leaky gut. In fact, understanding how exercise directly impacts the formation of gut epithelial tight junctions is a robust field of study. If you want to get into geeky details, this review article is a great place to start: http://bit.ly/2zyVCSz.
Perhaps given all this, it’s no surprise that vigorous exercise is also associated with a condition called food-dependent, exercise-induced anaphylaxis, in which the exercise-induced increase in intestinal permeability facilitates the absorption of allergens from the gastrointestinal tract.
Sports Drinks & Ibuprofen Are Not Your Gut’s Friend
A few conditions can aggravate the increased intestinal permeability caused by strenuous exercise. One study showed that the use of ibuprofen, a nonsteroidal anti-inflammatory drug (NSAID), significantly exacerbated both intestinal permeability and intestinal damage caused by strenuous exercise in well-trained athletes (ironically, popping ibuprofen is a common practice for endurance athletes).
There is also a strong correlation between both food intake and the consumption of carbohydrate-dense, electrolyte-enhanced beverages and gastrointestinal symptoms in endurance athletes. Strenuous exercise inhibits gastric emptying (the movement of food from the stomach to the small intestine), which is then further inhibited as the concentration of carbohydrates and salt in the stomach increases, so sugary sports drinks can actually make the problem worse.
Of course, it goes without saying that dehydration also causes heightened symptoms. It remains unknown whether food and overly concentrated sport drinks actually increase intestinal permeability or simply magnify the symptoms felt by the athlete. Watch this space.
Heat & Exercise = Further Inflammation
Environmental conditions also have an impact. One study showed that a sixty-minute run in both hot (91°F or 33°C) and cool (72°F or 22°C) conditions caused increased intestinal permeability, but that the amount of endotoxin (bacterial protein from Gram-negative bacteria) detectable in the blood was much greater after strenuous exercise performed in hot conditions but not in cool conditions (http://bit.ly/2yxeVI5). This implies that strenuous exercise is more inflammatory if performed in the heat.
Probiotic treatment may also help protect the gut from increased permeability caused by strenuous exercise. One study showed that probiotic supplementation reduced the amount of proinflammatory cytokines in the blood after strenuous exercise in male athletes and decreased the amount of zonulin (a protein that opens up tight junctions between the gut epithelial cells, implicated in a few autoimmune diseases) detectable in the feces (http://bit.ly/2zEINpv).
Runners, cyclists, and triathletes have been studied for exercise-induced intestinal-barrier dysfunction. Although there have been no definitive studies on the connection between resistance training and intestinal permeability, it probably depends on the style of workout and the amount of rest time between sets. Certainly, high-intensity, short-rest workouts have been shown to increase cortisol secretion more than traditional resistance training.
Low Intensity V Higher Intensity Exercise
By contrast, regular exercise at a relatively low intensity may protect the gastrointestinal tract from becoming diseased. There is evidence that physical activity reduces the risk of colon cancer, gallstones, diverticulosis, and inflammatory bowel disease, which is yet another argument for increasing physical activity while avoiding strenuous exercise. Again, it’s the happy medium thing.
The Take Aways
So there you have it, it’s not all doom and gloom. Taking the stress out of training will go a long way in protecting the integrity of your mucosal lining and gastrointestinal health in general. This looks different for everybody but too many long and strenuous workouts crammed into an already busy schedule are going to place a strain and likely compromise overall gut health integrity. Limiting the amount of intensive workouts in hot weather, staying away from NSAIDS and ditching those gels and sports drinks will go a long way in protecting you from developing a leaky gut. As will taking a quality probiotic, eating for gut health (fermentable fibres), removing as many toxins as possible from your environment and treating any detected intestinal pathogens.
If you suspect a leaky gut may be the root cause of your digestive issues and performance issues then I thoroughly recommend a Poop test to dig a little deeper ;)
