Meta description: Almost 90% of endurance runners deal with GI distress. Here's the exact gut-training protocol and race-day carb strategy to fix it — grams-per-hour targets, an 8-week build, and what to do when it still goes wrong.

You've done the long runs. You've hit your paces. And then, somewhere around mile 16 of your last marathon — or worse, in the middle of a 50K with no bathroom in sight — your stomach shut the whole thing down. Cramping, bloating, that specific urgency that makes you start scanning tree lines. If this has happened to you, you are not broken and you are not alone: studies putting the number at anywhere from 30% to nearly 90% of endurance athletes reporting GI symptoms during racing, depending on distance and intensity. It is, statistically, more likely to happen to you than not.

The good news is that "runner's trots" isn't random bad luck. It's a trainable, predictable physiological response, and the fix isn't "eat less" or "just deal with it" — it's a specific carbohydrate protocol combined with weeks of deliberate gut training before you ever toe the start line. This is that protocol, written the way I wish someone had handed it to me before my first blown-up long run.

Why Your Gut Shuts Down Mid-Race

When you run, your body redirects blood flow away from your digestive organs (splanchnic circulation) and toward working skeletal muscle and skin (for cooling). At easy paces this redirection is modest. At marathon effort or harder, blood flow to the gut can drop by 60-80%. Your intestines are essentially being asked to digest and absorb nutrients while running on a fraction of their normal blood supply.

Layer onto that the mechanical jostling of running (which doesn't happen in cycling or swimming, which is part of why runners report GI issues more than any other endurance sport), plus the osmotic load of concentrated sugar solutions sitting in your gut, and you have the recipe for cramping, bloating, nausea, and the diarrhea urgency that gives "runner's trots" its name.

The research is consistent on a few practical points:

• Foods high in fat, protein, fiber, or fermentable carbohydrates (the FODMAP group) in the 24-48 hours before a race are strongly associated with GI symptoms during it.
• Eating within 30 minutes of race start, especially something dense, increases the odds of bloating and urgency early.
• Dehydration makes GI symptoms worse, not better — being under-hydrated further compromises gut blood flow.
• Younger runners and male runners report GI symptoms at somewhat higher rates, but this is a near-universal endurance problem, not a niche one.

None of this means you should under-fuel to avoid the risk. Under-fueling a marathon or ultra creates a different, worse problem: glycogen depletion, the bonk, and a collapse in pace that no amount of "clean gut" will save you from. The goal isn't to eat less. It's to train your gut to handle more, so you can hit the carbohydrate targets that actually support fast, strong racing.

The Carb Number You're Actually Aiming For

For years the standard advice was 30-60g of carbohydrate per hour during racing. That number is outdated for anyone racing a marathon or longer at a serious effort. Current sports nutrition research, and the practice of elite marathon and ultra fields, has pushed the target considerably higher — largely because of "multiple transportable carbohydrate" sources.

Your gut has separate transporters for glucose and fructose. A glucose-only drink or gel caps out around 60g/hour of absorption because you're bottlenecking a single transporter (SGLT1). But combine glucose (or maltodextrin) with fructose in roughly a 2:1 or 1:0.8 ratio, and you open a second absorption pathway, allowing total carbohydrate oxidation rates of 90g/hour or more in trained guts.

These are targets to build toward, not numbers to attempt cold on race day. If your gut has never processed more than 40g/hour, showing up to your marathon and trying to hit 90g/hour is how you end up walking the last 10K clutching your stomach. This is where the training protocol comes in.

The 8-Week Gut Training Protocol

Gut training works the same way muscle training does: progressive overload, applied consistently, with recovery built in. You're teaching your intestines to upregulate carbohydrate transporters and tolerate volume under exercise stress. Do this alongside your normal long-run progression — it doesn't require extra training time, just intention during runs you're already doing.

Three rules make this protocol work:

1. Fuel by the clock, not by feel. By the time you feel like you need carbs, you're already behind. Set a repeating timer for every 15-20 minutes and take in a consistent, small amount rather than large boluses every 45 minutes. Smaller, more frequent doses reduce the osmotic hit to your gut at any one time.

2. Change one variable at a time. If you're testing a new gel brand, don't also change your hydration strategy or try a new breakfast the same week. When something goes wrong, you want to know exactly which variable caused it.

3. Train the exact race-day plan at least twice before race day. "Nothing new on race day" is a cliché because people ignore it constantly and pay for it. Your last two long runs before taper should be dress rehearsals: same wake-up time, same breakfast, same fuel brand, same approximate aid-station spacing.

What to Eat and When: The Full Race-Day Timeline

3 hours before: Eat a low-fiber, low-fat, familiar breakfast of 150-250g carbohydrate (oatmeal, white bread, banana, bagel). This is not the time for eggs, beans, or anything high in fiber — save the fiber for a normal training day, not race morning.

30 minutes before: A small top-up of 20-30g of fast carbohydrate (a gel or a few sports chews) with water tops off liver glycogen without adding a full stomach going into the start corral.

During the race: Hit your trained target (see the table above) in small, frequent doses. Pair concentrated gels with water — roughly 500ml of water per 100 calories of gel helps dilute the osmotic load and speeds gastric emptying, rather than sipping gels neat.

Hour 2 onward: Add sodium. As a rough starting point, 300-600mg of sodium per hour in hot conditions or for salty sweaters, delivered through electrolyte tabs, salt capsules, or a sports drink rather than plain water alone, helps offset the GI-slowing effect of hyponatremia and cramping.

Ultra distances, hour 4+: Pure sugar becomes harder to stomach as fatigue accumulates. This is where savory real food (broth, potatoes, pretzels, boiled eggs at aid stations) often outperforms another gel, both for calories and for psychological relief from sweetness fatigue.

Choosing Your Fuel Format: Gels vs. Chews vs. Drink Mix vs. Real Food

The carbohydrate target matters more than the delivery format, but format affects how easy it is to hit that target consistently, especially as a race goes long. Each has real tradeoffs worth testing in training rather than assuming one is "correct."

A pattern that works well for a lot of hybrid athletes: gels or drink mix as the backbone for anything marathon distance or faster, where precision and speed matter, shifting toward a gel-plus-real-food mix for anything 50K and beyond, where your stomach and your palate both need variety to keep taking in calories for six, ten, or twenty-plus hours.

Sodium and Electrolytes: The Fueling Variable Most Runners Get Wrong

Carbohydrate gets most of the attention, but sodium is arguably the more common cause of a race falling apart in hot or humid conditions, and it interacts directly with GI comfort. Low blood sodium (hyponatremia) slows gastric emptying, which backs up everything else you're trying to digest and makes nausea and bloating worse, not better.

Sweat sodium concentration varies enormously between individuals — from around 200mg/L for a "light" sweater to over 2,000mg/L for a heavy salty sweater (if you finish long runs with visible white salt lines on your shirt or hat, you're almost certainly on the higher end). A reasonable starting range for most runners in moderate conditions is 300-600mg of sodium per hour, trending toward the higher end in heat, at altitude, or if you know you're a heavy sweater. Salty sweaters sometimes need up to 1,000-1,500mg/hour in hot races, which usually requires combining electrolyte capsules with a sodium-containing drink mix rather than relying on gels alone, since most gels contain only 50-100mg of sodium each.

The practical test: if you're cramping late in long runs despite adequate carbohydrate and fluid, sodium is the first variable to adjust before you change anything else in your fueling plan.

Building Your Personal Race Nutrition Plan

Generic targets only get you so far — the point of the 8-week protocol is to convert them into a plan specific to your gut, your goal race, and your pace. Work through these five questions during your build:

1. What's your goal finish time? This tells you roughly how many total fueling windows you have and how many hours of sustained carb intake you need to plan for.

2. What carb rate did you land on during gut training? Not the textbook number — the number you actually tolerated well in week 7-8 of your build. That's your real race-day target, with maybe a 10% buffer cut for race-day adrenaline and heat.

3. What's your product, and how many units does that require per hour? If your gel is 25g of carbs and your target is 75g/hour, that's 3 gels an hour, or one roughly every 20 minutes. Write the exact timing into your race plan (or your watch's interval alerts) rather than trying to remember it under race stress.

4. What's the course's aid station spacing, and does your plan rely on it? If you're depending on course-provided sports drink, know exactly what it is (brand, carb concentration) and whether it's the same product you trained with. If not, plan to carry more of your own and treat the course drink as a backup, not your primary source.

5. What's your backup plan if your primary fuel goes wrong? Carry one product you haven't been relying on but know your stomach tolerates, in case your main gel or chew starts causing problems at hour three. This is cheap insurance against having to fully bonk before you adjust.

Common Mistakes That Wreck an Otherwise Good Race

Trying a new product on race morning. Race expos are full of free samples, and free samples are how good training blocks end in a porta-potty line at mile 8. Anything you put in your body on race day should have already been through at least two long runs.

Under-fueling out of fear. Some runners respond to a bad GI experience by cutting fueling back drastically the next race. This trades one problem for a worse one — glycogen depletion and a hard bonk in the last quarter of the race. The fix for GI distress is gut training, not starvation.

Ignoring hydration as a GI variable. Both under- and over-hydrating cause stomach problems. Weigh yourself before and after a couple of long training runs in race-like conditions to estimate your sweat rate, then aim to replace roughly that volume rather than guessing.

Skipping the caffeine test run. Caffeinated gels and chews are a legitimate performance tool (research generally supports 3-6mg/kg bodyweight spread across a race), but caffeine also speeds gut motility for some people. Test it in training, not for the first time on race day.

Eating high-fat or high-fiber the night before. That "carb-loading" pasta dinner with a cream sauce and a big salad is doing the opposite of what you want. Keep the pre-race dinner simple, familiar, and moderate in fiber and fat.

When It Still Goes Wrong: In-Race Troubleshooting

Even with a trained gut, things can go sideways — heat, nerves, and race-day adrenaline all affect digestion differently than a Saturday training run. A few practical fixes if symptoms hit mid-race:

Slow down for 3-5 minutes. Dropping intensity restores some splanchnic blood flow and can settle a cramping gut faster than pushing through it. Switch to plain water for one aid station if a gel just went down badly — skip one dose rather than stopping fueling entirely. Walk through an aid station rather than grabbing cups on the run if bloating is building; the brief change in torso position and breathing helps some runners more than any nutritional fix. And if you feel true urgency, take the bathroom break. A 90-second stop is a much smaller time loss than five miles of survival-mode shuffling with a cramping gut.

Special Situations

Heat racing: Hot conditions reduce gut blood flow further on top of the exercise effect, which is part of why GI issues spike in summer races. Consider trimming your carb target by 10-15% and increasing fluid and sodium priority when racing in heat — and if you're building a summer race block, it's worth reading how heat adaptation training changes your whole approach, not just fueling.

Racing around your menstrual cycle: Some female athletes report increased GI sensitivity in the days before and during menstruation, related to prostaglandin activity affecting gut motility. If this is a pattern for you, it's worth tracking alongside your training log so you're not caught off guard on a race that happens to fall at that point in your cycle.

Lifters new to endurance racing: If you came to running from a strength background, you may be used to eating very little around training sessions under an hour. That habit doesn't transfer to marathon and ultra distances — the calorie and carbohydrate demands of a 3+ hour effort are categorically different from an hour in the gym, and under-fueling because "I never used to need this much" is one of the most common mistakes lifters-turned-runners make.

Practicing the Protocol Without Race-Day Variables

One underrated way to nail down your fueling plan: run your gut-training long runs somewhere you control every variable, including immediate bathroom access. A lot of hybrid athletes do their dress-rehearsal fueling runs on a home treadmill specifically because it removes the two biggest confounders — unpredictable weather and distance from a bathroom — while you're testing exactly how your gut responds to a new gel or a higher carb rate. If you're building toward a marathon or ultra and don't already have that option at home, it's worth a look at what separates a curved manual treadmill from a motorized one for this kind of controlled fueling practice; either works, but the tradeoffs are different depending on whether you're testing pace-specific fueling or just want reliable mileage regardless of conditions.

Putting It Together

Gut training isn't glamorous and it doesn't show up in a training log the way a tempo run does, but it's one of the highest-leverage things you can do in the 8 weeks before a goal race. The runner who trained their gut to handle 80g/hour and executes their fueling plan on autopilot has a real advantage over the runner who's stronger and fitter but bonks or blows up their stomach at mile 20 because they never rehearsed it.

Treat your fueling plan with the same seriousness as your pacing plan. Write it down. Practice it. Adjust it based on what your long runs tell you. By race week, taking in 80-90g of carbohydrate an hour should feel as routine as tying your shoes — not something you're troubleshooting for the first time at mile 18.

If you're deep in a training block right now, this pairs directly with our 12-week marathon training plan for lifters and our broader guide to fueling for strength and cardio — that piece covers daily nutrition to support both training styles; this one is specifically about the 3-4 hours of race day itself.

Frequently Asked Questions

How many carbs per hour should I eat during a marathon?
Current research and elite practice support 60-90g/hour for most trained runners at marathon effort, using a mix of glucose and fructose sources (check gel/drink labels for "2:1 maltodextrin:fructose" or similar). Beginners or anyone who hasn't gut-trained should start lower, around 30-45g/hour, and build up over successive long runs.

Why do I get diarrhea when I run long distances?
"Runner's trots" is caused primarily by reduced blood flow to the gut during exercise combined with mechanical jostling, made worse by high-fat, high-fiber, or high-protein foods eaten close to the run, and by concentrated sugar taken without enough water. It's extremely common and highly trainable.

Can gut training actually be trained, or is it just genetics?
It's trainable. Studies show the gut can upregulate carbohydrate transporters and improve tolerance to higher intake rates with repeated exposure over weeks, similar to how muscles adapt to progressive load.

Is it normal to need the bathroom mid-run every time?
Occasional urgency is common and not usually a medical concern, but if it happens on nearly every long run despite following gut-training principles, it's worth ruling out FODMAP sensitivity, iron supplement timing, or an underlying GI condition with a doctor before assuming it's purely a training fix.