A New Study Asks: Does Training Your Gut Work?

You have most certainly heard the saying “nutrition is the fourth discipline of triathlon” at some point in your journey as a multisport athlete. While it may sound a bit cliché, there’s a reason this phrase is repeated so frequently. Getting your nutrition right takes knowledge, practice and repetition, just like we build when practice our swim, bike, and run. But you know how to train your swim, bike, and run – how about training your gut?

How your gut processes carbs during exercise

Nutrition is the one area where athletes continue to struggle to get things right. As race distance and duration gets longer, the importance of nutrition increases while the potential for mistakes grows exponentially. In long-course triathlon, where athletes exert themselves at a high level of intensity over a long period of time, nutrition is usually centered around carbohydrates, the preferred fuel of cells functioning aerobically at that level. Our bodies can only store a limited amount of carbohydrate (in the form of glycogen), so it is imperative that after about an hour or so that we start ingesting more to provide substrate for our cells to convert to energy in order to keep us going.

But how much carbohydrate should a triathlete ingest per hour? That’s where things get complicated. There are limitations on how much we can consume, how much we can pass from the stomach to the intestines, and how much the intestines can absorb. Most age group athletes can only take around 50-60 g/hour (200-240 calories). The rate-limiting part of this alchemy of food to energy appears to be how much can be absorbed from the intestines.

Training your gut to take in more carbs

In the 2000s, the concept of training the gut to be able to cope with exercise was introduced to endurance athletes. The idea was that by repetitively exposing the gut to race-day conditions we could improve tolerance for larger amounts of food in the stomach during exercise, enhance gastric emptying, improve intestinal absorption of nutrients and potentially improve the utilization of substrates at the cellular level.

There is certainly some appeal to this notion. We push harder in swim, bike, and run training to be able to make gains there – so why not push our guts with increasing loads of carbohydrate?

But there’s a catch. While gut training has been bandied about as a viable strategy by coaches and nutritionists alike, there are no established protocols about how it should be done, nor has much good science been done to delineate if it actually works. That’s why a recent article published in Sports Medicine is of interest in this regard. It is a systematic literature review with an attempt to answer the question: Does gut training provide any real performance benefits?

What the science says about gut training

The authors of the above-mentioned review found 48 studies that looked at gut training and its impact on various physiologic and performance metrics.

Exercise-induced GI distress

The first of these was gut integrity. It is well understood that intensive and prolonged endurance exercise compromises the integrity of the gut. With ongoing exercise, blood flow is diverted away from the gastrointestinal system. This can result in some degree of injury to the intestines, with varying degree of symptoms. The main issue is that gut permeability tends to increase, leading to fluid retention in the lumen of the intestines and exercise-induced gastrointestinal distress. The authors reported that based on the published literature, gut training had no effect whatsoever on this process.

Rate of gastric emptying

A second metric evaluated with gastric function. Specifically the authors wanted to know if gut training had any impact on increasing the rate of gastric emptying (GER). Numerous studies have evaluated this question and unfortunately, none demonstrated any benefit in this regard. So the authors concluded that again, gut training does not facilitate improvements in GER.

Exercise-associated gastrointestinal symptoms

One area where gut training did seem to show benefit was in reducing exercise-associated gastrointestinal symptoms (Ex-GIS). This is a very common issue among triathletes and has major implications for performance outcomes. First, because Ex-GIS can limit ongoing nutritional intake, making it hard to keep your fuel tank topped off. It can also decrease exercise tolerance – it’s hard to run when you’re nauseous or feeling an intense urge to use the bathroom. There are various risk factors and contributors to the development of Ex-GIS, but in some of the studies pooled for the systematic literature review, gut training was shown to be helpful in reducing the incidence of Ex-GIS.

Exercise performance

The final metric that the authors assessed related to exercise performance. Gut training, after all, is most useful to athletes if it translates to measurable benefits in their performance. The results here were mixed. In some studies, the authors found that gut training was effective in improving glucose availability in the bloodstream of athletes. This was suggested to be a result of either improved absorption of carbohydrates from the intestine or decreased malabsorption. This is an area of some controversy. Many coaches and exercise physiologists strongly believe and advocate for the idea that we can train our gut to absorb higher quantities of sugar per hour by increasing the amount of a type of sugar transporter on the cells of the intestine. The evidence to support this is very limited, and indeed as the authors in this review noted, many other studies did not show any such increases.

With respect to actual exercise performance, few studies showed any significant benefits and those that did often linked those improvements to a related decrease in Ex-GIS suggesting that it was the lower amount of Ex-GIS that led to the performance improvement.

Final notes

In the end, this paper suggests that gut training does have a role but it may not be the one that has been suggested previously. Rather than training our guts to absorb carbohydrates more efficiently, training our guts seems to be more effective in allowing us to tolerate more nutrition for longer periods, because of the associated decrease in Ex-GIS. This in turn may be related to decreased malabsorption of sugars in the intestine and leads to improved sugar availability. Together, these subjective and objective improvements may result in improved performance.

Want to give it a try for yourself? Check out Training Your Gut to Absorb More Carbs.

Dr. Jeffrey Sankoff is a Denver, Colorado-based emergency room physician, who  produces the “TriDoc Podcast.” Dr. Sankoff is also a triathlete himself and a USAT- and Ironman-certified coach.