“Training your gut” is a phrase coined in the sports nutrition world to describe decreasing exercise-induced gastrointestinal (GI) stress. The often-overlooked point of discussion is that digestion primarily takes place under resting conditions when there is no blood flow diversion away from the gut. When exercise comes into the equation—with the added increased stress of race day exercise—there is a different environment to consider: a hot, hypoxic, low blood flow gut in which hormones, peptides, and nervous activity are all shifted. These changes affect GI motility, absorption, and secretion, resulting in maldigestion, changes in small intestinal transit, and subsequent poor appetite leading to improper fluid and food intake. This is primarily why the prevalence of GI distress is so prominent in long-distance athletes (and it is also worth noting that there is a higher incidence in women than men).

There is a vast amount of clinical research to suggest the gut is highly adaptable, especially when it comes to accommodating changes in diet (e.g. a diet high in fructose will cause an epigenetic change to increase the expression of the GLUT5 gene, the primary transporter of fructose in the small intestines). This new area of nutrition research (investigating changes in carbohydrate-related gene expression induced by sugar consumption) is why the concept of training your gut is gaining traction in sports science research. But does it work? There is some evidence to show that a two-week repetitive gut challenge during exercise can help. One research group conducted trials in young male endurance athletes to improve the absorption and tolerance of glucose: fructose mixed carbohydrates (the typical formula of sports gels and drinks).

In the first study, there was a comparison of the same amount of carbohydrate from gel versus food versus placebo, and after two weeks, there was a reduction of GI symptoms with food and less with the gel (but still an improvement over the placebo group). The second study built on the first, looking to increase the amount of carbohydrate (in the form of gels) tolerated during a three-hour performance test. The experimental group participants trained for two weeks between performance tests by having 30g of carbohydrates every 20 minutes during a 60-minute moderately-paced run five days each week. The end result? The experimental group still had GI issues, but they experienced a 44% reduction in gut discomfort and a significant reduction in total symptoms. This contributed to running further in their second performance test (after the two-week gut training).  The senior author did note that dehydration exacerbates all GI symptoms, thus hydration is a critical factor in performance (as it allows for greater blood volume, therefore less blood flow diversion from the gut).

So how should you go about training your gut? Thus far, the evidence indicates a two- to three-week period of increasing the amount of carbohydrate you consume during exercise should help reduce performance-hindering GI symptoms.  As with any nutrition, phase it in during training and test out your new plan before race day. It’s also important, too, to not neglect your hydration as this is a significant factor to help control GI distress.

When looking at gut training, remember that the gut is not an athletic organ in the sense that it adapts to increased exercise-induced physiological stress. However, adequate training leads to a less dramatic decrease of GI blood flow at sub-maximal exercise intensities and is important in the prevention of GI symptoms.