Caffeine Can Kickstart Your Workout but Beware Its Effect on Your Gut

The performance benefits of the world’s favorite chemical could be washed out if it causes trouble in your gut.

Photo: Getty Images

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Caffeine is perhaps the most widely used mind-altering substance in the world, though unlike some of humanity’s other favorite psychoactive chemicals, caffeine is neither illegal nor demonized. Indeed, most people have very positive views of caffeine-containing foods and beverages. (How many “I need my coffee” memes and GIFs are there? I’m too unmotivated to count, but I’ll venture to guess thousands.)

Like the general public, many athletes also love their coffee, energy drinks, and caffeine. According to one study that looked at caffeine in urine samples analyzed at the Spanish Anti-Doping Laboratory, roughly three-quarters of over 20,000 samples received over a 5-year period contained traces of caffeine. Interestingly, endurance athletes were found to have the highest levels, suggesting they’re more likely to ingest larger amounts of caffeine before or during competition than other types of athletes.

As with any nutritional strategy, though, caffeine use comes with some potential downsides. These drawbacks, to a large extent, depend on dosage and the situation, and they vary quite a bit between people. The body organs affected by caffeine are many in number, so we’re going to narrow our lens to focus on the gut, my area of expertise. Whether you’re a crazed coffee enthusiast or just the occasional caffeine consumer, you’ll hopefully find something useful to take away from this overview of how caffeine impacts digestive function and gut symptoms.

Caffeine’s Effects on the Gut

Anecdotally speaking, most coffee drinkers know that having a cup of joe in the morning can open the metaphorical freeway that is their bowels. Perhaps up to one-third of people believe that coffee triggers an urge to poo for them, and some experiments confirm that drinking coffee promotes activity in the distal gut (i.e., colon and rectum), at least in a subset of people. Part of this effect is due to caffeine, though even decaffeinated java can spur motor activity in the colon, so coffee’s poo-inducing effects aren’t solely due to its caffeine content. Still, studies in humans and other animals (hound dogs, for example) show that caffeine impacts large intestine activity and that larger doses produce more profound changes. Furthermore, diarrhea and loose stools are one of the documented side effects of ingesting toxic (extremely high) amounts of caffeine.

Caffeine can undoubtedly influence the occurrence of lower-gut symptoms, but it’s also capable of provoking problems that affect the upper half of your digestive tract. Of any upper-gut problems that an athlete can experience before or during competition, nausea is unquestionably one of the most troublesome. Nausea tends to be more prevalent with super-intense, relatively brief exercise bouts (think a 400-meter dash) or extremely prolonged exercise (ultramarathons).

The underlying origins of nausea differ depending on the situation, but high-dose caffeine ingestion is one possible culprit, particularly when it comes to nausea that arises with high-intensity anaerobic exercise. These brief bouts of super-intense activity cause loads of catecholamines (a.k.a. adrenaline and noradrenaline) to be released by the body, and these hormones can exacerbate nausea via their effects on the brain and nervous system. Caffeine, as it turns out, is one of the things that amplifies the body’s secretion of catecholamines (particularly adrenaline), which, in theory, could provoke greater feelings of nausea in some athletes.

It should be noted that most people don’t experience much, if any, nausea after ingesting moderate caffeine doses (e.g., 3-5 milligrams per kilogram of body mass, or about 2-4 cups’ worth of brewed coffee for the average person). In addition, many studies have shown caffeine to be performance-enhancing across a broad range of athletic endeavors, which further supports the notion that its gut-related side effects are a major problem for only a minority of athletes. That said, there are a few things athletes can do to minimize the likelihood that caffeine will cause them digestive troubles before and during competition.

  • Avoid taking caffeine in high doses (>7 mg per kg of body mass) or with other stimulants. One study found that using a dose of 9 mg per kg of body mass led to gut problems in 31% of exercising subjects versus only 8% when lower doses were used. In addition, research has shown that mixing caffeine with other stimulants can elicit nausea during high-intensity exercise.
  • If you’re typically a nervous Nellie before or during competition, consider skipping the caffeine or using just a little. It’s worth keeping in mind that nearly all the research supporting caffeine’s performance benefits has been done using simulated competition, and I highly doubt the testing done in these situations was as stress-provoking as real-life competition.
  • Instead of ingesting your brew just before competition, down your coffee 60-90 minutes earlier. Coffee’s colon-stimulating properties probably peak over the initial 30 minutes after ingestion.
  • Make sure you aren’t totally fasted when taking caffeine before competition. Your body responds to fasting by dialing up the release of catecholamines and other stress hormones, which, as I’ve already mentioned, can worsen perceptions of nausea. In basic terms, taking loads of caffeine on an empty stomach before a grueling workout or competition can be a perfect recipe for being stricken with nausea.
cup of coffee on saucer on green wood table
Photo: Shutterstock

Daily Caffeine Use and Tolerance

The above discussion of caffeine’s effects on the gut is largely based on studies that have acutely administered caffeine, oftentimes after making people avoid caffeine for a day or two. You might be wondering, then, whether we can extrapolate those findings to situations where people habitually consume caffeinated beverages and foods every day. Unfortunately, there aren’t many studies that have specifically looked at the gastrointestinal implications of chronic caffeine consumption.

The findings of one recent investigation, however, do seem to confirm that daily caffeine ingestion leads to more digestive problems in some people. For the study, published in 2020, investigators had 11 participants consume caffeine (3 mg per kg of body mass or about 2 cups of coffee) or a placebo for 20 days. After their first treatment period, the participants were then reassigned to the opposite treatment for another 20-day period. The researchers were interested in seeing whether the participants would develop a tolerance to caffeine and whether the potential negative impacts of its consumption (blood pressure elevations, nervousness, insomnia, etc.) would wear off over time. Interestingly, caffeine’s blood pressure-raising effects seemed to diminish somewhat after about 10 days, while its propensity to cause nervousness did not seem to dissipate over the 20-day period.

The researchers also asked participants about their “gastrointestinal distress” levels, though they didn’t elaborate on what exactly that phrase meant. Gut distress was assessed on a 1-to-10-point scale every 2-3 days, and overall, average distress levels were likely higher in the caffeine condition than in the placebo condition at several points throughout the 20 days, including as far as 15 days into the supplementation period. Notably, average ratings of gut distress during the caffeine condition were typically between 2 and 3 (in comparison to 1-1.5 with the placebo), although there were clearly some subjects who reported ratings of greater than 5. This re-enforces the idea that although most people experience only mild GI disturbances from caffeine ingestion, a subset occasionally deals with moderate-to-severe gut troubles.

Final Thoughts

Decades of research verifies that caffeine can be used as a performance-booster, so long as one considers the factors that increase the likelihood of negative effects such as gut distress. At high doses, caffeine can trigger nausea, urges to go número dos, and perhaps other symptoms like reflux and abdominal cramping.

As with many nutritional strategies, the responses to caffeine ingestion can vary tremendously between individuals, so trial and error is crucial for athletes who want to determine how to get the most out of their caffeine use. One important yet undiscussed factor that may influence your response to caffeine is your genetics, which is a topic that will be covered in a follow-up article. In the name of science, I decided to have my genes tested for important caffeine metabolism pathways. Here are the results and an overview of a burgeoning research field on the genetics of caffeine responses.

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