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.

Sound familiar? You're not alone. 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.

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

This guide explains exactly why gels cause problems and gives you a practical protocol to fix it before your next race.

What Actually Happens When You Take a Gel Mid-Run

To understand why gels cause problems, you need to know what's happening inside your body during hard exercise.

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 and intestines operating at reduced capacity — right when you're asking them to process a concentrated hit of sugar.

A typical energy gel contains 20–30g 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 gel sits there, creating a feeling of fullness, bloating, or nausea.

Osmotic imbalance. Concentrated carbohydrate solutions draw water into the gut through osmosis. This can cause cramping, bloating, and in some cases, diarrhoea. The 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. If the 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, your gut function is further compromised. Taking a gel without adequate water to dilute it is one of the most common triggers for GI distress.

None of this means gels are bad. It means your gut wasn't ready for the demand you placed on it.

The 5 Most Common Gel Mistakes

Before we get to the training protocol, let's rule out the basics. Many athletes experience GI distress not because their gut can't handle gels, but because they're making avoidable mistakes.

1. Taking your first gel too late

If you wait until mile 10 or 12 to take your first gel, you're already behind on fuel. Your blood sugar is dropping, stress hormones are rising, and your gut is increasingly compromised. 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

Gels are concentrated carbohydrate solutions. Without water, they sit in your stomach as a thick, hypertonic mass that draws fluid in and slows gastric emptying. Always take a gel with 150–200ml 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 — even if it's a "better" brand — introduces unknown variables. If your race provides a specific gel at aid stations, train with that exact product.

4. Taking too much at once

The standard advice of "one gel every 45 minutes" doesn't account for individual variation. Some athletes do better with half a gel every 20–25 minutes — the same total carbohydrate, but in smaller doses that are easier to absorb. Spreading your intake across smaller, more frequent doses keeps the load on your gut manageable.

5. Ignoring caffeine content

Many gels contain 25–50mg of caffeine. Caffeine stimulates gut motility — helpful for some athletes, problematic for others. If you're using caffeinated gels, know how your stomach responds. Save them for the second half of the race when you need the boost, not the first gel when your stomach is least prepared for surprises.

Your Gut Is Trainable — Here's the Science

The breakthrough insight from recent sports nutrition research is that the gut adapts to the demands you place on it — just like any other system in your body.

A 2025 systematic review published 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. Athletes who practised consuming carbohydrates during training were able to tolerate significantly more fuel during races without distress.

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 (SGLT1 for glucose, GLUT5 for fructose) 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 to the presence of carbohydrate during exercise.

The practical takeaway: if you train your gut systematically in the weeks before your race, you can dramatically reduce the chance of GI distress on race day.

The 2-Week Gut Training Protocol

This protocol is designed for runners and triathletes preparing for a race that's 3–8 weeks away. Start this at least 2 weeks before your event — ideally during your final training block before the taper.

Week 1: Introduction Phase

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

Sessions 3–4 (moderate runs, 60–75 min): Take one gel at 25 minutes and a second at 50 minutes, each with water. You're now introducing your gut to the rhythm of repeated fueling during exercise. If symptoms are above a 5/10, reduce to half-gel portions.

Session 5 (long run, 90+ min): Practise your intended race-day fueling pattern. If your plan is one gel every 30 minutes, follow that schedule. Carry water or plan your route around water fountains. This is a dress rehearsal.

Week 2: Adaptation Phase

Sessions 6–7 (easy to moderate runs, 45–60 min): Repeat the dual-gel protocol from sessions 3–4. You should notice symptoms are lower than week 1. If they're not, reduce the concentration — dilute the gel in a handheld bottle instead of taking it straight.

Sessions 8–9 (moderate to tempo runs, 60–75 min): Increase to your target race-day carbohydrate intake per hour (30–60g/hr for most runners, up to 90g/hr for experienced athletes using glucose-fructose combinations). This is where you 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. Take note of everything: what worked, what didn't, when symptoms appeared. This run gives you the data to refine your plan.

What to Track

For each session, note:

• Which gel and flavour you used
• How much water you took with it
• Time into the run when you fueled
• GI symptom score (1–10)
• Energy levels and perceived effort

This data is gold. It tells you exactly what your gut can handle and where the threshold is.

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

Beyond Gels: Alternative Fueling Options

If you've done the gut training and gels still don't agree with you, you have options. Research shows that whole foods and natural alternatives can be just as effective for recreational athletes.

Dates and dried fruit: 20–25g of carbs per serving. Easy to carry, gentler on the stomach for many athletes, and they contain some electrolytes.

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

Banana pieces: 25–30g of carbs per banana. Harder to carry, but if your race has them at aid stations, they're a reliable option.

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

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

The key is testing these alternatives during training, not on race day. The gut training protocol above works with any fuel source — gels, chews, real food, or liquid carbs.

Putting It All Together: Your Race-Day Fueling Checklist

Here's a summary checklist for race day, built on everything above:

The week before:

• Confirm your fueling plan: which gels, how many, and at what intervals
• Check the race's aid station map — know where water is available
• Do one final fueling rehearsal on your last long run

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 150–200ml of water
• Use smaller, more frequent doses rather than large, infrequent ones
• If nausea hits, slow your pace slightly, sip water, and wait 10 minutes before your next gel
• 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 from an aid station, flat cola)
• Reduce pace — even 10–15 seconds per mile reduces gut stress significantly

The Missing Link: Personalised Fueling

Everything in this guide is built on averages and general protocols. But the truth is, your fueling needs are personal. They depend on your sweat rate, your body weight, your pace, the temperature on race day, and how much you've trained your gut to handle.

That's the problem with generic advice — it can't account for you.

Sweatr calculates your personalised fueling plan using your Apple Watch or Garmin data. It factors in your training load, your estimated sweat rate, and your race-day conditions to tell you exactly what to consume and when — down to the specific gel and the specific minute. No spreadsheets. No guesswork.

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