As someone who has a PhD in exercise physiology and works with elite athletes, Andy King knows more about how the body responds to prolonged exercise than most people. This extra knowledge, however, didn’t make the piercing pain he was suffering from any less severe. King had just started the run leg of the Ironman 70.3 Staffordshire in England when he noticed an intensifying discomfort in his abdomen. While he had occasionally dealt with side stitches during training, the intensity of this pain was on a whole new level. The pain got so bad that King thought he would need to drop out of the race, but he somehow persisted and finished, albeit much slower than he had originally hoped for.
Although it’s impossible to know exactly what led to King’s woes, it’s a good bet that unusually warm conditions on race day played a role. Unsurprisingly, most endurance races are scheduled to avoid the hottest months of the year. Hosting a race in oppressive heat (say late July in the Northern hemisphere) is basically asking for bad things to happen to some participants.
The 2017 Ironman 70.3 Staffordshire—the race King participated in—was held on June 18. Conditions in England in mid-June are usually ideal for racing, with temperatures peaking in the mid-60s. Unfortunately for King and the other race participants, although the temperature in the early morning reflected classic conditions (low-to-mid 50s), thermometers hit 80℉ by noon, and the temperature eventually topped out at around 84℉ that afternoon.
Similar events like the Ironman World Championship in Hawaii, for example, often see temperatures that exceed 90℉ with stifling humidity, so by no means were conditions in Staffordshire extreme. Still, unexpected heat can pose a major challenge to athletes when they aren’t acclimatized to it. In this article, we’ll take a closer look at how the human machine responds to exercising in the heat and what that means for your gut function in particular. We’ll then review some of the tactics you can employ to hopefully beat the heat, or at least minimize your chances of being stricken with heat-induced gut distress.
How Your Body Responds to Exercise in the Heat
We all know that being in sweltering environments can make you sweat profusely. Particularly during exercise, sweating is your body’s attempt to rid itself of potentially dangerous levels of heat. The typical mechanisms of releasing body heat (radiating it off, for example) that work well on cooler days start to falter when air temperatures approach body temperature. As a result, your body must rely more on sweating (and evaporation of said sweat) to cool itself. This is precisely why you sweat more copiously on hot summer days than in the dead of winter.
Swampy, humid conditions can accelerate these fluid losses because it is much harder for sweat to evaporate from your skin when atmospheric air is already full of water vapor. In one study, sweat rates went from about 1.4 liters per hour in 40% relative humidity to nearly 1.8 liters per hour in 80% relative humidity conditions when men bicycled at a moderate intensity. Long story short, sweat rates during exercise swell with both air temperature and humidity, which places you at greater risk for experiencing large body fluid losses.
Blood Flow to the Gut is Critical
What do heat-induced sweat losses have to do with the gut? Well, when you have less body fluid available during exercise (particularly in your vascular system), it can start to compromise the flow of precious blood, oxygen, and nutrients to the gastrointestinal (GI) tract. If less oxygen and nutrients are delivered to your gut, its function can become impaired. Take for example what can happen to the intestinal barrier—the gateway between the gut and your bloodstream—when you exercise in the heat. Your intestinal cells are normally bound tightly together, which prevents potentially harmful substances from passing into your body. However, with heat stress during exercise, these tight junctions start to loosen, which lets invading molecules trespass into your blood.
Bacteria are some of the more consequential of these trespassers. For convenience, scientists usually measure the amount of lipopolysaccharide in the blood to evaluate the extent of bacterial transfer into the body. Lipopolysaccharide is a fancy scientific term for an endotoxin that is a major component of the walls of certain bacteria. Your body identifies these endotoxins and releases inflammatory molecules to help keep the invasion at bay. This is generally a good thing because you don’t want foreign invaders spreading unchecked in your body, but a notable byproduct of this surge in inflammatory molecules is the potential for disruptions in organ function and premature fatigue. Furthermore, the rise in endotoxins in the blood during prolonged exercise is correlated with the onset of gut distress, particularly nausea.
Another way that blood flow influences gut function is through changes in motility, the contractions and relaxations that control the movement of foodstuffs through your GI tract. During super-intense exercise, blood is diverted to the working muscles (say, the legs) as oxygen requirements there surge, leaving the gut in a comparatively blood- and oxygen-poor state. This relative oxygen deprivation in the gut may be further exacerbated in the heat and with dehydration. Ultimately, waning blood flow to the gut can manifest as disruptions in gut motility—in particular, reductions in esophagus and stomach muscle activity tone. If you’ve ever felt as if food was sitting like a brick in your stomach midway through a long run on a hot day, some of these alterations in gut motility were probably responsible.
Exercising in the Heat Dramatically Worsens Gut Problems
The changes that occur to your gut when you exercise in the heat sharply raise your chances of experiencing digestive difficulties. One of the best examples of this comes from a 2018 study published in the European Journal of Applied Physiology, which examined the incidence of gut troubles when runners exercised in different climates. In one situation, participants slogged away on a treadmill for two hours in temperate conditions (72℉), while on another occasion they ran at the same intensity in the heat (96℉). The overall incidence of upper gut symptoms (bloating, belching, heartburn, reflux, etc.) was 90% in the sweltry condition compared to only 40% in the milder environment. Likewise, lower intestinal issues (urge to poo, abdominal pain, etc.) were more common in the hot condition (70% vs. 30%). Perhaps most notably, the rate of nausea was four times as high in the hot condition (40% vs. 10%).
These experimental data also line up with what’s been seen in studies of endurance racers in the real world. The 2013 edition of the Western States 100-Mile Endurance Run, for instance, was one of the hottest on record, with temperatures peaking at 102℉. In these scorching conditions, six out of every 10 racers reported having nausea during the race.
Beat the Heat to Help Your Gut
By now it should be clear as day that balmy conditions can make for a troublesome gut. With a mild case of gut upset, an athlete may simply need to slow down to ease their problems; in more severe cases, however, they may end up dropping from a race.
Beyond modifying your pace, what are your options for easing these heat-induced bowel aggravations? Though there is no fool-proof method, here are several strategies that you can try to ward off gut woes in the heat.
- Get used to it. Habituating yourself to functioning in the heat is probably the #1 place to start. If you’re unaccustomed to heat stress, gut blood flow reductions will be more pronounced and could exacerbate GI symptoms. Those of us residing in hotter geographical locations may already be well acclimatized, but others living in cooler environments will need to find a way to create an effective heat training stimulus. This could include exercising in heavier clothing, resting in a warm environment (sauna), or immersing yourself in warm-to-hot water after a regular workout. Ideally, exercise sessions that aim to induce heat acclimatization should last at least 60 minutes, though you may need to progressively work up to this duration if you’re unaccustomed to the heat. The timeline of physiological and performance improvements with these sorts of protocols varies, with some changes taking place within a few days and others taking up to two weeks. Practical tips for developing an effective and safe heat acclimatization program are available from the Gatorade Sports Science Institute.
- Cool yourself down. A review of studies published in the Scandinavian Journal of Medicine and Science in Sports and Exercise found that both internal (cold water or ice slurry ingestion) and external (cooling vest, ice packs, etc.) strategies can be effective at lowering body temperature and enhancing performance in the heat. As it relates to the gut, one study failed to find any differences in digestive symptoms when runners ingested water at different temperatures (cold, cool, and lukewarm) while exercising for two hours in 95℉ conditions. Still, there were some trends suggesting slightly fewer gut problems when the cold water was ingested, and it’s possible that the sample was just too small (12 subjects) and the exercise task was too short to adequately detect a benefit among noise in the data.
- Drink enough (but not too much) to prevent dehydration. Sweating is amplified when it’s hot and humid, and this can ultimately quicken the dehydration process during exercise. Remember, less blood flow to the gut from said dehydration can put you on a path toward gut dysfunction. These potential impairments become more likely when you lose more than 3% or 4% of your body mass from sweating. The implication here is that you don’t need to replace 100% of your sweat losses during exercise itself, as some amount of fluid loss is to be expected. Instead, you might target something like a 50% to 75% replacement rate during prolonged exercise (over 2 hours), depending on your gut comfort level. Use an online sweat rate calculator to find your typical fluid loss. For relatively short bouts of exercise (less than 2 hours), simply relying on your thirst will be adequate in most cases.
- Try a probiotic (maybe). Probiotics are live microorganisms that confer health and metabolic benefits when administered to a host (in this case, humans). Studies on the supposed benefits of probiotics in athletes are mixed, but at least one has found promising results as it relates to exercising in the heat. Ten male runners completed a 10-week supplementation period with either a probiotic or placebo. The probiotic was mixture of nine strains, most coming from the Lactobacillus and Bifidobacteria genuses. After 10 weeks, the men ran to fatigue at a set intensity in thermally challenging conditions. Then, they switched to the other treatment for another 10 weeks and repeated the procedures. Notably, they were able to run at a high intensity for several minutes longer (37:44 vs. 33:00) after supplementing with the probiotic than the placebo. Furthermore, their blood contained fewer endotoxins after probiotic supplementation. Before you get too excited, it’s worth noting that gut symptoms weren’t different between the treatments, and another study failed to find similar benefits when endurance-trained men ingested a Lactobacillus casei probiotic for one week before running for two hours in the heat.