High Country Trail Shoe
Race Day Fueling Strategy
The most common reason endurance athletes don't finish races isn't fitness. It's their gut.
Research suggests that between 30 and 90 percent of endurance athletes experience gastrointestinal symptoms during competition, nausea, cramping, bloating, the particular misery of needing a bathroom three miles from the finish line. And the primary driver of that problem isn't bad luck or a sensitive stomach. It's a fueling strategy that was never actually tested.
The fix isn't complicated, but it does require deliberate training before race day. Here's how.
Why Your Gut Needs a Strategy Before Race Day
During sustained exercise, your body redirects blood flow away from your digestive system and toward working muscles. The result is a gut that's already working with reduced capacity at exactly the moment you're asking it to process gels, drinks, and whatever you grabbed off the aid table.
Add heat, fatigue, or intensity, and that reduced capacity shrinks further. The athletes who seem to have stomachs of steel on race day don't have different physiology. They've trained their guts to handle what race day requires, and they've practiced with the specific products and volumes they're planning to use.
A fueling strategy isn't something you design the week before your goal race. It's something you build and refine over months.
How Much Carbohydrate Do You Actually Need?
The current evidence-based guidelines for carbohydrate intake during endurance exercise are more aggressive than most athletes realize. For events lasting more than 60 minutes, carbohydrate intake of 30 to 60 grams per hour supports performance. For efforts lasting more than 2.5 hours, the upper end of the range extends to 90 grams per hour, and some elite athletes competing in ultra-distance events consume even more, though there is a ton of individual variability, and intake should match the demands of your event, like environmental factors and your output levels.
Most recreational athletes are taking in a fraction of that. A 2025 study examining actual race-day intake found that experienced endurance athletes consumed far less than the recommended 60 to 90 grams per hour, and, notably, overestimated how much they'd taken in. The gap between what you think you're eating and what you're actually eating tends to close when you practice your strategy in training rather than estimating on race day.
The practical target for most trail runners competing in events over two hours: 60 to 90 grams per hour, with a glucose-fructose blend.
Why the Glucose-Fructose Ratio Matters
Your gut uses different transporters to absorb different types of carbohydrates. The SGLT-1 transporter handles glucose and maxes out at roughly 60 grams per hour. Fructose is absorbed via a separate transporter (GLUT-5), which means combining glucose and fructose allows your gut to absorb more total carbohydrate without overwhelming a single pathway.
Products with a roughly 2:1 glucose-to-fructose ratio, or the increasingly popular 1:0.8 ratio in some newer formulations, can meaningfully increase the amount of carbohydrate your body actually absorbs and uses during exercise, rather than sitting undigested in your gut and causing problems.
When you're shopping for gels, chews, or drinks, check the carbohydrate source. Products using maltodextrin or glucose combined with fructose are better positioned for high-volume fueling than products relying on a single carbohydrate source. Most modern sports nutrition is in this camp, so you don’t have to think too hard about. However, if you plan on making your own, or fueling with aid station fruit, you’ll want to be a bit more intentional.
How to Train Your Gut
The concept of gut training is well-supported in the literature: your digestive system responds to progressive overload just like your muscles do. The protocol is straightforward in principle, though it requires patience in practice.
First, figure out what your baseline is. Most runners simply don’t realize what they’re actually taking in on runs. Log what you eat and drink on your long runs (I like to use the notes app on my phone and save gel wrappers for a more accurate accounting) so that you can figure out the exact point you’re starting from, and increase from there.
Start conservative. If you're currently taking in 30 grams of carbohydrate per hour on your long runs, start there. Get comfortable with that volume, including the products you plan to use on race day. Once your gut tolerates that amount without symptoms, increase gradually, add one gel or add 15 grams per hour, and repeat the process.
Practice on your longer sessions and on sessions that simulate race intensity. Your gut behaves differently at easy aerobic effort versus the sustained intensity of a race. If you’re racing a marathon, be sure you’ve practiced fueling at that intensity. Train it at conditions that match what you're preparing for.
Use the products you'll race with. Every gel, drink, and chew that goes into your body on race day should have been through multiple training runs first. Aid station nutrition is a wildcard; know your backup plans.
Fuel earlier than feels necessary. Waiting until you feel hungry or depleted on a long run is waiting too long. Glycogen depletion degrades your gut's function along with everything else. Start fueling 30-40 minutes in.
Stay hydrated. Dehydration impairs gastric emptying and compounds GI distress. The relationship between your hydration strategy and your fueling strategy is tightly linked; electrolyte-containing fluids alongside your carbohydrates support both.
How to Log and Refine What You're Learning
The athletes who show up to race day with a dialed strategy are the ones who treat their training runs like data collection. Keep a simple fueling log. After every long run or key session where you practiced your nutrition, write down:
What you used and how much per hour. Whether you hit GI symptoms and at what point. How you felt energetically late in the session. What you'd change.
Over weeks and months, patterns emerge. You'll learn which products sit well at higher intensities, what volume your gut can reliably handle, whether you tend to under-eat or over-eat on long efforts. That information is worth more than any generic recommendation.
Nothing New on Race Day
This principle is so foundational it has become a cliché, which means athletes still routinely ignore it. Nothing new on race day: not a new gel brand, not a new sports drink the race is distributing at aid stations, not a new combination of products you haven't tested together.
Your gut's capacity for adaptation is impressive. Its capacity for surprise on race day is not. Every product, volume, and timing pattern you plan to use should be practiced, ideally multiple times, under training conditions that resemble race day as closely as possible.
A race day fueling strategy is built, not decided. The target for most endurance athletes in efforts over two hours is 60 to 90 grams of carbohydrate per hour, using glucose-fructose blends to maximize absorption. The gut responds to training: start conservative, increase gradually, practice consistently, and log what you learn. The athletes who feel good late in a race are the ones who treated fueling as a trainable skill from the beginning of their build, not an afterthought in race week.
For more deep dives on nutrition, check out Your Diet Sucks, proud to partner with rabbit.