Gut training 101 for endurance athletes
Your gut is trainable. If your event relies on fueling, your ability to take in carbs while you're moving is a performance variable, and just like the rest of your body, it adapts to training.
The endurance athletes who fuel the best are often the athletes who finish the strongest, but that doesn't start on race day. If you want to fuel better, perform better, and stop your stomach from being your limiting factor, here's where to start.
What Is Gut Training?
Gut training is the process of teaching your digestive system to tolerate and absorb more carbohydrate and fluid during exercise. Just like your muscles and cardiovascular system adapt to progressive overload in training, your digestive system adapts too, and it's a critical piece of the puzzle if you want to perform at your best in events lasting more than about 90 minutes.
Optimal intake usually isn't possible right out of the gate. Gut training means slowly increasing your tolerance to nutrition during exercise, and repeatedly practicing your race-day fueling strategy well before race day actually arrives.
How Your Gut Adapts
Over time, the goal is to teach your digestive system to empty food and fluid from the stomach faster, absorb more carbohydrate in the intestine, tolerate larger amounts of fuel with fewer GI symptoms, and keep digesting even as your intensity climbs. Under the hood, your body does this in part by ramping up the intestinal transporters that move carbohydrate into your bloodstream, SGLT1 for glucose and GLUT5 for fructose. The result is more total carbohydrate absorbed, meaning more of it reaches your working muscles instead of sitting in your gut.
Why It Matters
A stronger gut means better performance. Without gut training, even the most "perfect" race-day fueling plan can backfire. A trained gut gives you better carbohydrate absorption for sustained energy and a delayed time to fatigue, a decreased perception of effort, less risk of bloating, nausea, cramping, and GI distress, improved hydration, and the ability to consume more fuel exactly when your body needs it most.
A well-trained gut often means you can perform harder for longer and avoid hitting the wall late in your race.
Who Should Gut Train?
Anyone exercising or racing longer than about 90 minutes, not just elite and sub-elite athletes. That includes marathoners, cyclists, triathletes, trail and ultra runners, and anyone doing long training sessions or competing in multi-hour events. The longer the session or race, the more fueling determines performance.
It's not uncommon to consume hundreds of grams of carbohydrate over the course of an endurance event, and an untrained gut can't handle that. Take in more than your gut is ready for and you're dealing with bloating, nausea, cramping, and a heavy, sloshing stomach, which leads to missed fueling targets and an inevitable decline in performance. Take in too little and your glycogen stores drain, forcing your body to rely more heavily on fat oxidation, which produces energy at a much slower rate. Pace drops, perceived effort climbs, and you get the dreaded feeling of hitting the wall or bonking. Neither one cares how fit you are.
If your event relies on fueling during exercise, don't wait until race day to practice it.
How to Train Your Gut
Practice race nutrition during training. Treat your nutrition strategy like any other workout in your training plan, and start as soon as possible, ideally no less than 8 weeks before your goal race. If you're a Black Iron client, your coach can guide you through all of this.
Begin with the amount of carbohydrate you already tolerate, since that starting point is individual, and gradually increase your carbohydrate intake on long sessions weekly, or as tolerated. If you're truly starting from scratch, start practicing on shorter sessions. Around 90 minutes is where intra-fueling goes from "optional, nice to do for practice" to "must do," and it's worth practicing in both easy and higher-intensity workouts.
Fuel early and consistently during long sessions. A time schedule is typically more efficient for consistent intake, and small doses are usually tolerated better than big ones. Try using the nutrition alarm on your watch to stay on schedule.
Hydrate appropriately. Dehydration and sodium depletion will affect your ability to process and absorb calories.
Experiment up front to define your personal preferences, then practice with the same products, timing, and fluids you'll use on race day, and pay attention to how heat, duration, and effort or intensity impact your tolerance.
Train Your Gut Outside of Training, Too
Consume enough carbohydrate on a daily basis. This ensures your digestive system and gut microbiome are already adapted to processing and absorbing carbohydrate efficiently, laying the foundation for better intra-exercise fuel tolerance.
Don't save carbs just for your carb-load and race day. A misdirected "train low, race high" approach increases the risk of GI distress because your gut isn't used to handling high carbohydrate intake. Consistently fuel your hardest training sessions the way you plan to fuel your race.
The Takeaway
Train your gut before race day, not a few weeks before. No matter how fit you are, if you can't get nutrition in during long efforts, your performance will eventually suffer. Training your fuel tolerance is part of an endurance training plan, not separate from it.
Nothing new on race day. Practice your fueling as often as you practice your sport, and be patient and methodical, increasing carbs gradually rather than all at once. Your stomach adapts with consistent practice. Fitness isn't the only thing that needs training, and your gut tolerance just might be the missing piece to your next PR.
Your gut is trainable, and now you know it. Put in the effort and it stops being the thing that falls apart and starts being the thing that carries you home.
