Stomach Strong

Four tips to “train your gut” for your big endurance event

It’s a sad fact of physiology that our bodies can expend energy faster than they can absorb it. Because of this, if you hammer during a long endurance event, there’s a solid chance that you’ll bonk, meaning you’ll run out of blood sugar and glycogen.

To make matters worse, your body doesn’t really like digesting food during hard efforts. It doesn’t understand why you’re running, swimming, or cycling; it just knows that you’re busting butt—an action historically reserved for fleeing from danger—so it focuses on the task at hand. Digestion suffers, leaving you with a funny tummy.

All this means that if you’re doing a marathon, gravel grinder, or Ironman, well… sad for you. But you’re not alone. Gut issues are common in endurance athletes. A 2019 study in the journal Sports demonstrated that most ultrarunners are trainwrecks when it comes to gastrointestinal issues. However, being a distance runner myself, I can attest to the fact that it’s all worth it for the finisher medal, t-shirt, and occasional space blanket.

While there’s no way to completely escape this annoying evolutionary wrinkle, you can train your gut to absorb as much fuel as possible during the event without causing you to cramp, crap, or tap out. Here’s how.

But first, some math…

Oversimply put, your primary sources of fuel are fat and carbohydrates. Body fat stores are useful for less intense efforts, but the harder you push, the more you need carbs.

In your body, carbs manifest as blood glucose and glycogen, a back-up carb stored in your muscles, liver, and brain. Assuming your glycogen stores are at 100 percent and assuming you go full tilt, you have about a 90 minutes supply. After that, you need to lean into eating or drinking for energy. (Although you should have been feeding the whole time in order to get ahead of the bonk.)

Generally speaking, a person can burn about 240 grams of carbohydrates per hour, or roughly 1000 calories. But realistically, in a multi-hour race, most of us probably only burn about 600-700 calories per hour, since amateur badassery has its limits.

Until recently, science thought that your gut could absorb 60 to 90 grams of carbs per hour, or about 240-360 calories. Recent studies have shown trained athletes can handle up to 120 grams per hour, or 480 calories—provided they consume the right combo of sugars.

Either way, it’s mission critical to absorb as many carbs as possible during your event to stave off the inevitable as long as possible.

…and a nutrition lesson.

Here’s the part of the digestive process that you need to know for these tips to make sense. You eat food. It travels to your stomach. A bunch of digesting happens there, then it dumps down to your small intestine via “gastric emptying.”

In the intestine, little buddies called glucose transporters shuttle carbohydrates (in the form of sugar) through the intestinal wall into your blood. Sodium-Dependent Glucose Transporters (SGLT)-1 move glucose. Incongruously-named Glucose Transporters (GLUT)-5 move fructose.

(Personally, I would have gone with Fructose Transporter (FRUT)-5, but what do I know?)

Then a bunch of additional digesting happens and you poop, but that’s not important for this article. Now, the tips.

Tip One: Have a feeding plan.

When you train your body, you need an exercise plan. When you train your gut, you need a feeding plan.

Know what you’re going to eat leading up to your event. I recommend something called “glycogen supercompensation,” which is a fancier version of carb loading. (Glycogen supercompensation is a whopper of a topic and I’m aiming for the whole brevity thing today, so we’ll cover it next week.)

Know what you’re going to eat during your event. if you use gels, gummies, or carby liquids, take advantage of your multiple glucose transporters, SGLT-1 and GLFRUT-5, by feeding on supplements with a roughly even split of glucose (aka dextrose) and fructose. This way, you’ll absorb more sugar because you’ll have more transporters working for you.

While I’m normally a “real foods” advocate, dietary fiber and fat don’t provide much benefit during single-day events from a physiological perspective, so consider experimenting with the aforementioned during-race supplements.

Bonus pro tip: Don’t fill up right before a hard mid-race effort, like a big hill. I know it’s tempting because you want to crush that monster climb up the road, but gastric emptying slows at around 74% VO2 max, so the moment you dial it up, that Shot Block in your belly ain’t goin’ nowhere but back up again.

Tip Two: Train with a full stomach.

It sounds gross, but you get used to it—which is the point. I know I just told you not to eat right before a hard effort, but during-training nutrition has different rules. It’s okay to operate under less-than-ideal circumstances because you’re forcing adaptation—in this case, to improve gastric emptying.

Tip Three: Overdo the carbs during training.

Again, this will help force adaptation. It also increases your absorption capacity and helps you learn exactly how much sugar you can tolerate per hour.

Tip Four: Simulate race conditions and race feeding plan.

Obviously, this will help you finetune your plan. Also, factors like heat, effort, and fiber or fat intake can impact digestion, so it’s important to prepare your gut for these potential blockers.

You don’t want to turn your entire training schedule into an epic food challenge, so space these tips out a little. Sports physiologist and carbohydrate evangelist Asker Jeukendrup recommends “gut training” once a week for 5 to 10 weeks.

So, give it a try and report back. I’d like to hear how it goes!

P.S. If you dig today’s article, please share it. And if you have a topic you’d like to discuss, post it in the comments section. I’m hungry for questions!