Race Strong: Gut Health for Endurance Athletes

Don’t let stomach issues slow you down on race day.

How many of you have experienced gut issues during a race? I distinctly remember getting terribly ill and having to stop at every portable restroom (there was one every mile) during the marathon of my first Ironman. I was miserable. Sadly, many of us have similar war stories. It is estimated that between 30-90% of people have struggled with gastrointestinal issues at some point in their lives. In fact, a study of long-distance triathletes competing in extreme conditions found that up to 93% of the athletes experienced gastrointestinal symptoms.1 That is a remarkably high statistic. So what gives?

Your race has just begun. Do you have a fueling plan to stay strong all day? [Photo courtesy of Pixabay].

Gut Problems?

In the world of endurance sports, there is a common belief that you should replace as many calories as physically possible with “easy to digest,” simple carbohydrates, typically by slamming gels and sugary drinks. This approach is supposed to help you spare your glycogen stores, allowing you to maintain a higher intensity for longer (read: race faster). However, this approach is attacking the problem from the wrong angle, and it is overloading our digestive system in already trying conditions.

Some studies suggest humans can safely absorb up to 360 calories (90g) of carbohydrates per hour, and should do so for events longer than 2.5 hours. This figure is highly optimistic, and is it really necessary to push the envelope?

During exercise your body delivers as much oxygen to your muscles as possible, which requires blood. The higher the intensity, the more blood you need to keep up with demand. This causes blood flow to your stomach to take a back seat, and leaves you in a less-than-optimal state for digestion. Add in factors like hot and humid conditions or the jarring impact from running, and your stomach becomes even less capable of handling a high consumption of carbohydrates and you end up with gut problems.

Flipping the Script

Instead of trying to keep up with carbohydrate demands that could wreak havoc on your insides, why not become as aerobically efficient as possible to burn more fat and less carbohydrates to race faster? This frees you up to take in less calories and gives your gut a break.

Carbohydrates are a small part of the total energy you naturally store in your body. Most average athletes store roughly enough calories from carbohydrates to last for 2-3 hours of moderately intense exercise. Conversely, there are typically 70,000-80,000 calories available from intramuscular and subcutaneous fat that can potentially be used for fuel. If you can train your body to tap into this fuel at higher intensities, you will spare your glycogen stores and lessen the stress you place on your gut.

aerobic efficiency graph

[Tables and graphs courtesy of Shawn Gerber]

How Do You Become Aerobically Efficient?

To train your body to become more “fuel efficient” and use fat at higher intensities, start by being mindful of how your food choices and timing affect your blood sugar and insulin levels.

When you eat, the pancreas releases insulin to process the incoming nutrients and help stabilize blood glucose levels. The amount that is released depends on the concentration and amount of glucose being dumped into the bloodstream. The more blood sugar your body has to balance, the more insulin is released in response. This is important for two primary reasons:

  1. It signals your body to prioritize glucose for fuel and increases glycogen storage. You have to either use it or store it to get it out of the blood and maintain homeostasis.
  2. In doing so, the use of fatty acids for fuel is diminished.

When you consistently prioritize carbohydrates in your diet and fueling plans, your body will put itself in a state that prioritizes glucose (carbohydrates) for fuel over fatty acids. That’s when you see graphs like the “poor” example above, where even at low intensities fat is not utilized well. In contrast, when you eat and train in a way that consistently signals the body to release, break down, and use fat for fuel, it will eventually lean in that direction as its natural state.

To get your body as aerobically efficient as possible, focus on three main areas: daily diet, workout fueling, and race fueling.

Daily Diet

Endurance training falls into two categories: Low-intensity workouts to build endurance, and high-intensity workouts to build strength, speed, and power. Low intensity periods are a gold mine for improving your aerobic efficiency. During these blocks your diet should consist mainly of lean protein, healthy fat, fruit, and vegetables.

When workouts become more intense, introduce modest amounts of whole-grain, low-glycemic starches to help your body to deal with the higher carbohydrate and caloric demands from your workouts. These more intense periods of training are when sports nutrition products like gels, blocks, or drinks play a role, as they help replenish and keep your glycogen stores topped off for the next intense workout or during your current workout.

Workout Fueling: The 3-Hour Rule

The 3-hour rule is a helpful guideline when thinking about how to fuel your workouts. As a rule of thumb, workouts less than three hours should focus primarily on hydration. These workouts should rely on the fuel you already have stored in your body.

Start thinking about fueling once your workouts hit the 3-hour mark. You don’t want to blow out your glycogen stores completely on these workouts, so it is prudent to have a fueling plan in place from the very beginning to sustain energy throughout the day.

Race Fueling: The 24/12 Rule

Carb loading is an infamous concept for endurance athletes. But as it turns out, the old-school protocol of binging on bagels, pasta, and breads for a week prior to racing is a bad idea. Recent research has shown that it only takes 24 hours to effectively top off glycogen stores to be optimally prepared for race day. This is where the 24/12 rule comes into play.

Rather than spend a week trying to get your body to overcompensate and fill your glycogen stores to the brim, you can do it in just one day. This goes a long way for maintaining all the hard work you put into becoming aerobically efficient in the first place, while still topping off to make sure your energy stores are full for race day.

After a competition effort, or even after a competition-type workout in your peak training weeks, refuel with carbohydrates in the twelve hours following the event before switching back to your standard diet of lean protein, fats, fruits, and vegetables.

Feel Better and Race Stronger

So what do you say? How about we improve on those horrendous statistics and go out and have great races this year? All it takes is a shift in perspective and simple tweaks to your diet and fueling plan.

Speaking of simple, there is plenty more “nerdy stuff” involved with this whole topic. If you are curious and want to learn more of the science behind becoming aerobically efficient, drop me a line in the comments. I’m always happy to dig in deeper.

You’ve dialed in your diet. Now dial in your training:

Strength Endurance for the Endurance Athlete


1. Jeukendrup AE, Vet-Joop K, Sturk A, et al. “Relationship between gastro-intestinal complaints and endotoxaemia, cytokine release and the acute-phase reaction during and after a long-distance triathlon in highly trained men,” Clinical Science (Lond) 98(2000):98:47–55.

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