Why Do Gels Make You Feel Sick? How to Train Your Gut for Race Day

You're 14 miles into your marathon. Heart rate is manageable. Legs feel strong. You tear open a gel, squeeze it down, and within ten minutes your stomach is churning. By mile 17 you're looking for a portaloo instead of chasing your PB.

Studies show that 30–50% of endurance athletes experience gastrointestinal distress during races. And the cruel irony is that the fuel you need to avoid bonking is often the same thing making you feel awful.

Here's the key thing most runners don't realise: gel nausea is not a fixed trait. Your gut is trainable — just like your cardiovascular system. With the right approach, you can teach your digestive system to handle race-day fueling without the rebellion.

What actually happens when you take a gel mid-run

When you run, your body redirects blood flow away from your digestive system and toward your working muscles. During intense or prolonged exercise, blood flow to the gut can drop by up to 80%. That leaves your stomach operating at reduced capacity — right when you're asking it to process a concentrated hit of sugar.

A typical gel contains 20–30 g of carbohydrate in a small, viscous package. When that hits a blood-deprived stomach, several things can go wrong:

Delayed gastric emptying. Your stomach can't process the gel as fast as it normally would. The result: fullness, bloating, or nausea.

Osmotic imbalance. Concentrated carbohydrate solutions draw water into the gut through osmosis, causing cramping, bloating, and in some cases, diarrhoea. Thickening agents in many gels make this worse.

Fructose overload. Some gels use high amounts of fructose, which is absorbed through a different intestinal transporter than glucose — and it's slower. If fructose content exceeds your gut's absorption capacity, it ferments in the lower intestine, causing gas, cramping, and urgency.

Dehydration amplifies everything. If you're already behind on fluids, gut function is further compromised. Taking a gel without adequate water is one of the most common triggers for GI distress.

The 5 most common gel mistakes

1. Taking your first gel too late. Starting fueling early — within the first 30–45 minutes — is far easier on your stomach because blood flow to the gut hasn't yet been drastically reduced.

2. Swallowing a gel without water. Without water, gels sit in your stomach as a thick, hypertonic mass that draws fluid in and slows gastric emptying. Always take a gel with 150–200 ml of water. If you can't get water at the next aid station, wait until you can.

3. Trying a new gel on race day. Your gut adapts to specific ingredients, concentrations, and textures. A gel you've never tried before introduces unknown variables. If your race provides a specific brand at aid stations, train with that exact product.

4. Taking too much at once. Some athletes do better with half a gel every 20–25 minutes — the same total carbohydrate in smaller doses that are easier to absorb.

5. Ignoring caffeine content. Many gels contain 25–50 mg of caffeine, which stimulates gut motility — helpful for some, problematic for others. Save caffeinated gels for the second half of the race.

Your gut is trainable — the science

A 2025 systematic review in the Journal of the International Society of Sports Nutrition confirmed that gut training protocols lasting just 2 weeks (10 sessions over 14 days) produced 60–63% reductions in GI symptoms compared to baseline.

What happens during gut training:

• Gastric emptying speeds up. Your stomach learns to process carbohydrate solutions faster during exercise.
• Intestinal absorption improves. The transporters in your small intestine upregulate — meaning you can absorb more carbohydrate per hour without overflow.
• Gut motility stabilises. The gut-brain signalling that triggers nausea and urgency becomes less reactive.

The 2-week gut training protocol

Designed for runners 3–8 weeks out from a race. Start at least 2 weeks before your event during your final training block.

Week 1: Introduction phase

Sessions 1–2 (easy runs, 45–60 min): One gel at the 30-minute mark with 200 ml of water. Use the exact gel brand and flavour you plan to use on race day. Note symptoms on a 1–10 scale.

Sessions 3–4 (moderate runs, 60–75 min): One gel at 25 minutes, a second at 50 minutes, each with water. If symptoms are above 5/10, reduce to half-gel portions.

Session 5 (long run, 90+ min): Practise your intended race-day fueling pattern at your planned frequency. This is a dress rehearsal.

Week 2: Adaptation phase

Sessions 6–7 (easy to moderate, 45–60 min): Repeat the dual-gel protocol from sessions 3–4. Symptoms should be lower than week 1.

Sessions 8–9 (moderate to tempo, 60–75 min): Increase to your target race-day carbohydrate intake per hour (30–60 g/hr for most runners, up to 90 g/hr for experienced athletes using glucose-fructose combinations). Test your upper limit.

Session 10 (long run or race simulation, 90–120 min): Full race-day nutrition rehearsal. Same gels, same timing, same hydration. This gives you the data to refine your plan.

If tracking this manually sounds like a lot, Sweatr builds your fueling plan automatically based on your wearable data — training load, pace, and conditions — so your gut isn't overloaded. It takes the guesswork out of exactly this process.

Alternative fueling options

If you've done the gut training and gels still don't agree with you:

Dates and dried fruit: 20–25 g of carbs per serving. Gentler on the stomach, contain some electrolytes.

Honey packets: 17 g of carbs per tablespoon. Similar profile to a gel but often better tolerated because of the natural glucose-fructose ratio.

Chews and blocks: Some athletes find chewing helps regulate intake speed and reduces the "dump" effect of liquid gel hitting an empty stomach.

Liquid carbohydrate drinks: If you can't tolerate any solid or semi-solid fuel, mixing maltodextrin powder into your water bottle gives you a steady drip of fuel without the concentrated hit.

Test these alternatives during training, not on race day.

Race-day checklist

Race morning: Eat your tested pre-race meal 2.5–3 hours before the start. Sip on an electrolyte drink in the 60–90 minutes before the gun. Do not try anything new.

During the race: Start fueling within the first 30–45 minutes. Take every gel with 150–200 ml of water. Use smaller, more frequent doses. Save caffeinated gels for the second half.

If things go wrong: Switch to water and electrolytes only for 15–20 minutes. Try a different fuel form (chews, banana, flat cola). Reduce pace — even 10–15 seconds per mile reduces gut stress significantly.

The missing link: personalised fueling

Your fueling needs depend on your sweat rate, body weight, pace, the temperature on race day, and how much you've trained your gut. Generic advice can't account for you.

Sweatr calculates your personalised fueling plan using your Apple Watch or Garmin data — training load, estimated sweat rate, and race-day conditions — telling you exactly what to consume and when, down to the specific gel and the specific minute.

If you've ever finished a race wondering whether you fuelled right, Sweatr gives you the answer before the race even starts.