Ultramarathon GI Distress Is Not a Reason to Stop Eating

The wheels come off at hour eight. Or hour twenty. Or somewhere in the back third of a hundred miler when the gel you've been choking down for six hours starts tasting like motor oil and your stomach files a formal complaint.

Here is what most people do next: they stop eating.

Ultramarathon GI distress gets treated as a verdict instead of a symptom. The stomach said no, so the plan is over, and the rest of the race becomes a slow negotiation with a body that has nothing coming in. This is the most common self-inflicted wound in long-distance racing, and it is almost always the wrong call.

Late race fueling is genuinely one of the hardest problems in endurance nutrition, and it's hard for a structural reason: you cannot recreate the conditions in a lab or in a standard long run. Most athletes only experience deep late-race fatigue a handful of times across a career. That makes every exposure precious. It also makes the problem feel impossible to train for.

It isn't impossible. But it requires something more specific than "just practice your nutrition."

What causes GI distress in ultramarathons?

Ultramarathon GI distress is not one condition. It's a stack of overlapping insults that happen to converge in the same hour for most people, which is why single-cause explanations always feel slightly wrong.

Here is what's happening at once.

Blood flow has shifted away from your digestive system toward working muscles and skin. This is called splanchnic hypoperfusion, and it scales with how hard you're going and how hot it is. Less blood to the gut means slower gastric emptying, so food and fluid sit in your stomach longer than they should.

Mechanical jostling is grinding on your GI tract. This is a running-specific problem. It's part of why cyclists can slam so many carbs per hour.

Your gut has been absorbing continuously for hours and some of that carbohydrate isn't making it across. Unabsorbed carbohydrate reaching the large intestine ferments, which produces bloating, cramping, and, you know, urgency.

Low glycogen can also cause nausea. So does the accumulated stress of hours of hard effort, elevated core temperature, and disrupted sleep.

In a survey of 272 finishers of the Western States 100, 96% reported gastrointestinal symptoms during the race, with nausea reported by roughly 60% (Stuempfle & Hoffman, 2015). So you're in good company here.

How to train your gut for late race fueling

Late race conditions are better approximated by training fueling at high fatigue, in heat, and at race effort. Back-to-back long days. A fueled workout the day after a hard interval session. Practicing intake on hot afternoons instead of cool mornings. Running the back half of a long effort at goal race intensity instead of cruising it home.

The point is not to suffer for its own sake. The point is to put your gut under realistic stress while you still have bailout options. That's how you find out what your stomach actually does at hour five instead of guessing.

Get specific. Use exactly the products you plan to use on race day, in the amounts and at the intervals you plan to use them. Match the terrain and conditions where you can, because a steep course, a hot course, and a very cold course stress your system in different ways.

There's real evidence this works. Repeated carbohydrate exposure before and during training reduces gut discomfort, upper GI symptoms, and carbohydrate malabsorption, with measurable changes appearing after roughly two weeks of progressive exposure (Costa et al., 2017). Your tolerance is trainable.

And just generally being more fit works too. The fitter you are, the more stress your body can handle while functioning at a given percentage of total capacity for longer. A fitter athlete running the same pace is working at a lower percentage of their maximum, which means less blood diverted away from the gut at any given speed. Getting faster is, indirectly, a fueling intervention.

Why do gels stop tasting good during an ultra?

Somewhere around hour six, gels stop working. Not chemically. Hedonically. My beloved margarita flavored hydration mix takes a villainous turn.

This has a name in the food science literature: sensory-specific satiety. Repeated exposure to one food drops its rated pleasantness specifically, which is why you can be finished with dinner and still want dessert. The effect lasts a couple of hours and it extends to foods with similar sensory properties.

Taste preference also shifts during exercise itself. In one trial, cyclists exercising in the heat preferred sweeter, higher-carbohydrate drinks early and saltier ones as the session went on. That was 27 recreationally active participants over 60 minutes in a hot room, rating small samples, so it's suggestive rather than definitive. But it lines up with what every crew chief already knows. The broth request is coming.

If broth sounds good and gel doesn't, you have a flavor problem. If nothing sounds good, including cold things, salty things, and plain water, that's not your palate. That's your gut, and it needs a different fix.

How to fix nausea during an ultramarathon

When your stomach turns, you have a small number of real options. Work through them in this order.

Drop the effort. This is the highest-leverage move and the most underused. Gut blood flow is inversely related to intensity, so backing off lets perfusion partially return. Hike the climb. Walk five minutes. Take a full aid station stop instead of a rolling one. Most athletes will try six flavors of gel before they will try slowing down, which is exactly backwards.

Dilute what you're taking. Highly concentrated carbohydrate solutions empty more slowly and are associated with more symptoms. Beverages above roughly 500 mOsm/L have been linked to increased GI complaints. Chase gels with more water. Cut your drink mix. Fewer grams per swallow, more swallows.

Cut the dose, not the total. If 60 grams in one aid station stop is coming back up, take 15 grams every fifteen minutes. Smaller, more frequent, more liquid. Many people find cold fluids go down when nothing else will, and cold has the added benefit of helping with heat load.

Cool down. Heat drives blood to your skin and away from your gut, and core temperature independently produces nausea. Ice in a bandana, cold water over your head and neck, a wet buff, ice in your hands. This is a fueling intervention even though it doesn't look like one.

Notice what is not on this list: stopping fueling.

Why stopping fueling makes ultramarathon GI distress worse

This is the whole point of the post, so here it is directly.

When you stop eating because you feel sick, you get hypoglycemic, which is itself nauseating. You get slower, which extends the total time your gut spends under stress. You lose the ability to make good decisions, which is when people talk themselves into dropping.

There's observational data here, too. In a small study of runners at a 161-km race, those who avoided GI distress had consumed more fluid and more fat before symptoms ever appeared than those who developed it. Fifteen runners is not a large sample and correlation isn't causation, but the temporal ordering is suggestive: lower intake preceded symptoms rather than following them.

Meanwhile, nausea and vomiting show up repeatedly in the literature as leading reasons runners drop from 161-km races. "I had to stop fueling" is not a neutral outcome. It's the opening move of a documented failure pattern.

The correct response to a bad stomach is almost never zero. It's less, slower, colder, more dilute, and at a lower effort. Don't let perfect be the enemy of good enough, even if good enough is eating tater tots out of a dirty ziploc at mile 80.

How to build a late race fueling plan with contingencies

Some of this will always be trial and error across race cycles. The best you can do is maximize the data from each rare exposure and reduce variables one at a time.

A useful protocol has three pieces.

A primary plan. Written out. Carbohydrate grams per hour, fluid volume, sodium, and specific named products at specific intervals. Not "eat every 30 minutes."

A first contingency, for when sweet tolerance dies. The savory, cold, and liquid pivot. Broth, pretzels, boiled potatoes, rice balls, miso, salted watermelon. Note that this is a category change, not a flavor change, because satiety generalizes across similar sensory profiles. Rotating gel flavors will not save you.

A second contingency, for when GI distress hits. Lower volume, simpler carbohydrate, more dilute, slower intake, a walking reset, and aggressive cooling.

Identify both pivots before the race. Put the products in your drop bags. Tell your crew exactly what to hand you when you say "I can't do another gel," and tell them the correct response to "I feel sick" is a smaller, colder, more diluted option rather than sympathy and an empty hand.

Then rehearse the pivot. If the plan is gels through hour six and broth and potatoes after, you need to have actually run on broth and potatoes.

You will get better data each time, refine the protocol, and slowly compress the failure window.

We're covering late race fueling and the gut in depth on the podcast. Subscribe so you don't miss it.

The bottom line on ultramarathon GI distress

Ultramarathon GI distress affects the overwhelming majority of runners in long events, so experiencing it is not evidence that you did something wrong. Quitting your fuel plan in response usually is.

Most late-race nausea comes from some combination of effort, heat, dehydration, concentration, and accumulated gut stress, and every one of those has a mid-race adjustment attached to it. Slow down, cool off, dilute, and cut your portion size before you cut your intake to zero. Plan a savory pivot in advance and rehearse it, because flavor fatigue is predictable and switching gel flavors won't fix it. And know that nausea plus a headache plus heavy drinking is a different problem that requires the opposite response.

Something is almost always better than nothing. Get comfortable being an athlete who eats badly rather than one who stops.

If you want to bring your own race-day disaster to the group and get Kylee's take on what to do differently next time, the weekly nutrition thread on Patreon is where that happens.

References

Costa, R. J. S., Miall, A., Khoo, A., Rauch, C., Snipe, R., Camões-Costa, V., & Gibson, P. (2017). Gut-training: The impact of two weeks repetitive gut-challenge during exercise on gastrointestinal status, glucose availability, fuel kinetics, and running performance. Applied Physiology, Nutrition, and Metabolism, 42(5), 547–557. https://doi.org/10.1139/apnm-2016-0453

Hoffman, M. D., & Fogard, K. (2011). Factors related to successful completion of a 161-km ultramarathon. International Journal of Sports Physiology and Performance, 6(1), 25–37.

Rehrer, N. J., van Kemenade, M., Meester, W., Brouns, F., & Saris, W. H. (1992). Gastrointestinal complaints in relation to dietary intake in triathletes. International Journal of Sport Nutrition, 2(1), 48–59.

Rolls, B. J., Rolls, E. T., Rowe, E. A., & Sweeney, K. (1981). Sensory specific satiety in man. Physiology & Behavior, 27(1), 137–142.

Stuempfle, K. J., & Hoffman, M. D. (2015). Gastrointestinal distress is common during a 161-km ultramarathon. Journal of Sports Sciences, 33(17), 1814–1821.

Stuempfle, K. J., Hoffman, M. D., & Hew-Butler, T. (2013). Association of gastrointestinal distress in ultramarathoners with race diet. International Journal of Sport Nutrition and Exercise Metabolism, 23(2), 103–109.