GI Distress in Ultras: A Troubleshooting Guide

The carbs-per-hour guide covers how to prevent GI distress - gut training, pacing, product choice. This one is for when you did most of that right and your stomach turns anyway, because it will, for most ultra runners, in most long races, at some point.

Studies of ultra runners put the share who experience at least some GI symptoms during a race anywhere from a third to the large majority, depending on distance, heat, and how the question is asked. It is one of the most commonly cited reasons for slowing dramatically or dropping in 100-mile races. The runners who handle it well aren't the ones who never get it - they're the ones who know what to do in the first five minutes.

The fastest fix is pace, not product

Above roughly 75-80% of maximum effort, blood is shunted away from the gut toward working muscle, and both gastric emptying and absorption drop sharply. This is why a runner who feels nauseous rarely fixes it by switching gel flavours - the gel was never the problem. The intensity was.

The first move, almost regardless of symptom, is to slow to a hike. Not because walking is virtuous, but because it is the single fastest way to restore blood flow to a gut that has been starved of it. Ten minutes of hiking resolves more stomach problems than any product decision you could make in the same window.

Symptom by symptom

Different complaints have different likely causes, and the fix isn't identical across them.

Vomiting is often a reset, not a crisis

Vomiting once mid-race, followed by feeling noticeably better, is a common enough pattern that most experienced ultra runners have a story about it. It isn't pleasant and it isn't the goal, but a single vomiting episode that clears an overloaded stomach and is followed by improvement is different from persistent vomiting that keeps you from taking in anything.

After vomiting once: rest for a few minutes, sip small amounts of plain water, and wait until you feel stable before eating again. Restart fueling at roughly half your normal rate rather than resuming at full rate immediately - the gut that just emptied itself is not ready for what caused the problem in the first place.

If vomiting repeats or you can't keep even small sips down after 20-30 minutes, that's a different situation - dehydration and further GI and electrolyte disruption compound quickly, and it's worth flagging to aid station staff or medical support rather than pushing on alone.

When it isn't ordinary GI distress

This is the part worth knowing before you're the one deciding, tired and nauseous, at kilometre 60.

Early exercise-associated hyponatremia - dangerously low blood sodium, almost always from drinking more fluid than you're losing - produces nausea, bloating, and headache that look almost identical to ordinary GI overload. The hydration and sodium guide covers this in full, but the short version that matters here: if nausea comes with confusion, a headache that keeps building rather than easing, or puffy fingers and a tight watch strap after steady drinking, stop drinking fluid and get help. Drinking more, which is the instinctive response to feeling unwell, is the wrong move if this is what's actually happening.

Heat illness can present with nausea too, alongside disproportionate fatigue, stopped sweating, or confusion. If you're in hot conditions and the picture includes any of those, treat it as a heat emergency and cool aggressively rather than treating it as a stomach problem.

The practical rule: ordinary GI distress responds to slowing down and eases within 20-30 minutes. If it isn't easing, or if anything on top of the nausea looks neurological - confusion, disorientation, a headache that's getting worse - stop assuming it's your stomach.

The in-race protocol

A simple sequence to run through, roughly in order:

1. Slow to a hike immediately. Don't wait to see if it passes at pace.
2. Stop or halve your carbohydrate intake for the next 15-20 minutes.
3. Sip small amounts of plain water, or flat cola if reflux or a sugar-concentration issue seems likely - the small amount of caffeine and the flat fizz settle some runners's stomachs well, though it's worth having tested this in training rather than trying it cold.
4. Reassess after 15-20 minutes. Most ordinary GI distress meaningfully improves in this window.
5. Restart fueling at roughly half your prior rate, with a bland, low-fat, low-fibre product, and build back toward your target rate over the following hour rather than jumping straight back to full rate.
6. If it isn't improving, or something looks more serious than a stomach problem, get eyes on you at the next aid station rather than continuing to self-manage.

On anti-diarrhoeal medication

Some experienced ultra runners carry loperamide (Imodium) for race-day diarrhoea, and it does work for a meaningful share of them. It also isn't something to start using for the first time on race day - it can mask a more serious underlying problem, interacts with how your gut handles fluid and electrolytes, and like every other product on this list, needs to be tested in training first if you're going to use it at all. Talk to a doctor about whether it's appropriate for you before race day, not during it.

I had an athlete two seasons ago whose stomach shut down completely at hour five of a 50 miler she was fit enough to run comfortably. Her instinct was to keep forcing gels down because "you need calories" - which is broadly true and exactly wrong in the moment. We'd gone through this scenario in advance, so she knew the sequence: hike, stop fueling, sip water, wait twenty minutes. She lost twelve minutes standing at that decision point and finished the race with a stronger last third than most of the field, having eaten almost nothing between kilometre 45 and 55. The runners who post-mortem a bad stomach day usually find the mistake wasn't the gel that triggered it - it was refusing to stop and reset once it had.

The mistakes

1. Trying to fix a pace problem with a product change. If the cause is intensity, no amount of gel-swapping helps until you slow down.
2. Force-feeding through nausea. More carbohydrate on top of a gut that's already struggling usually makes it worse, not better.
3. Stopping fueling entirely for hours after one bad patch. A measured restart at half rate beats either extreme.
4. Chugging large amounts of water to "flush" a bad stomach. This is closer to the hyponatremia risk profile than it is to a fix.
5. Ignoring symptoms that look neurological. Confusion or a worsening headache alongside nausea is not routine GI distress until proven otherwise.
6. Trying a new remedy - a different antacid, an anti-diarrhoeal, a new electrolyte product - for the first time mid-race, because the current plan isn't working.

Prevention is still the better investment than troubleshooting - the gut training protocol is where to start well before race week. If GI distress is a recurring pattern across your races rather than a one-off bad day, that's a pacing and fueling diagnosis 1:1 coaching can dig into properly.