Research suggests up to 70% of endurance athletes experience GI distress during racing. Nausea, cramping, urgent bathroom stops, vomiting at the run-to-finish transition — these are not random bad luck. They have identifiable causes and, more importantly, preventable patterns. If your race has ever been derailed by your stomach, this is what you need to know.

Why the gut struggles during exercise

During sustained exercise at race intensity, blood is redirected from the digestive system to working muscles. Splanchnic blood flow — the blood supply to the gut — can drop by 60–70% during hard effort. The digestive system does not shut down, but it slows significantly. Gastric emptying takes longer, intestinal absorption is reduced, and the gut lining becomes more permeable (a phenomenon sometimes called leaky gut during exercise).

This is a normal physiological response. The problem is when you ask your gut to process foods or volumes it is not equipped to handle under these conditions.

The most common causes of race-day GI distress

1. Taking in too much, too fast

The biggest driver of nausea and vomiting during long-course triathlon is concentrated carbohydrate entering a gut that cannot process it fast enough. Taking multiple gels in close succession, using high-concentration sports drinks, or front-loading all your fueling in the first hour of the bike creates a bolus of sugar that the gut cannot absorb — and it comes back up.

The fix: spread fueling evenly across the event. One gel every 25–30 minutes is far easier to absorb than two gels every hour.

2. Training fasted, racing fueled

Athletes who do most of their long training runs and rides in a fasted or low-carb state are not training their gut to handle race-level carbohydrate input. The intestinal transporters that absorb glucose and fructose are upregulated through repeated exposure — they get more efficient when you practice using them. An athlete who trains fasted and then tries to absorb 80 grams of carbohydrate per hour on race day is asking their gut to do something it has never practiced.

3. Unfamiliar products on race day

Race day is not the time to try the gel brand at the aid station you have never used in training. Different gels have different carbohydrate formulations, concentrations, and flavour intensities. What your gut has learned to tolerate through training is specific. Using a different product — especially at higher intensities than training — is a common cause of mid-race GI shutdown.

4. Dehydration and concentrated solutions

When you are dehydrated, the gut pulls water from tissues to dilute hyperosmotic solutions in the intestine. Gels taken without enough fluid create this effect. The result: osmotic diarrhoea. The standard guidance is 150–200 ml of water with each gel. In heat, this becomes even more important.

5. High-fibre or high-fat food too close to race start

Fibre and fat slow gastric emptying. Eating a meal high in either within 2 hours of the start leaves partially digested food sitting in your stomach when the gun goes off. In a full Ironman, this means the first hour of the bike can be deeply uncomfortable as the body tries to manage two competing demands at once.

How to train your gut

Gut training is the deliberate practice of consuming race-level carbohydrates during training to adapt the digestive system to race conditions. Here is an 8-week protocol:

  • Weeks 1–2: Consume 40–50 g carbohydrate per hour on all long sessions. Use the same products and timing you plan for race day. Do not train fasted on long sessions during this block.
  • Weeks 3–4: Push to 60 g per hour. Note any symptoms — mild discomfort is normal and adaptive, sharp pain or vomiting is a signal to reduce volume.
  • Weeks 5–6: Target 70–80 g per hour. At this stage, make sure you are using a 2:1 glucose-to-fructose ratio product. Single- sugar products hit their ceiling around 60 g/hr regardless of gut training.
  • Weeks 7–8: Race simulation. Execute your full race-day fueling protocol — including pre-race meal timing — on your last long brick or long run. This is a rehearsal, not an experiment.

Race-day management if it goes wrong

Even with a solid plan, GI distress can arrive. Here is how to manage it without DNF-ing:

Nausea on the bike: Back off concentration. Switch from gels to dilute sports drink. Small, frequent sips rather than large volumes. Sit up slightly to reduce abdominal compression.

Bloating and cramping: Skip the next fueling window. Take only water for 15–20 minutes. If you are in the run, slow to a jog and take small sips of cola — the sodium and caffeine often help reset the gut.

Vomiting: If you can keep moving, do. After vomiting, wait 10–15 minutes before attempting to fuel again. Start with water, then small amounts of dilute carbohydrate.

Urgent stops: Do not fight them. The time cost of a bathroom stop is far lower than the performance cost of continuing in pain or the medical cost of ignoring a serious signal.

The case for a personalised plan

Your GI history matters. Athletes with previous hyponatremia, those who are salty sweaters, athletes with FODMAP sensitivity, and those who have had GI DNFs all need protocol adjustments that generic advice cannot provide. A plan built from your gut history, your sweat rate, your product preferences, and your target pace gives your stomach the best possible chance of holding up when it matters.