Why Do Gels Make You Feel Sick? The Science Behind Mid-Race GI Distress

You did everything right. You picked a well-reviewed gel, tested it on a long run, and felt fine. Then on race day, somewhere around mile 16, the nausea hit. You slowed down. You stopped eating. You bonked at mile 22.

Sound familiar? You are not alone. Studies estimate that 30–50% of endurance athletes experience gastrointestinal distress during racing, and energy gels are the most frequently blamed culprit. But the gel itself is rarely the whole problem.

The real issue is a mismatch between what your gut can handle and what you're asking it to do — and that mismatch is almost always worse on race day than in training.

Here's why, and how to fix it.

The three reasons gels make you sick

1. Osmolarity: your gel is pulling water the wrong way

Every gel has an osmolarity — a measure of how concentrated its sugars are relative to your blood. When you swallow a gel with high osmolarity, your small intestine pulls water in from surrounding tissue to dilute it before absorption. That fluid shift is what causes the bloating, cramping, and urgent need to find a portaloo.

Traditional gels built on maltodextrin tend to have moderate-to-high osmolarity. Newer hydrogel formulations (like Maurten) use a gel matrix that passes through the stomach at low osmolarity and breaks down in the small intestine. Isotonic gels (like SiS GO) are pre-diluted to match blood concentration. Both reduce the fluid shift problem, but neither is magic — they still need the right timing and volume.

The practical takeaway: if a gel makes you sick, the first question is not "which brand?" but "what's the osmolarity, and am I taking it with enough water?"

2. Blood flow: your gut shuts down when you race hard

During high-intensity exercise, your body redirects blood away from your digestive system toward working muscles and skin (for cooling). At race pace — which is harder than most training runs — splanchnic blood flow can drop by up to 80%.

That means your gut's ability to absorb carbohydrate plummets at the exact moment you need it most.

This is why gels that feel fine on an easy long run at 5:30/km pace cause problems at 4:45/km race pace. Your gut had plenty of blood flow during training. On race day, it didn't.

The mismatch is predictable, and it's fixable — but only if you practise at the right intensity. More on that below.

3. Carb type mismatch: single-source vs. dual-source

Your small intestine absorbs glucose and fructose through different transport pathways. Glucose (and its polymer, maltodextrin) uses the SGLT1 transporter, which maxes out at roughly 60g per hour. Fructose uses the GLUT5 transporter independently.

A gel that contains only maltodextrin saturates the SGLT1 pathway and leaves carbohydrate sitting unabsorbed in your gut. That unabsorbed sugar ferments, produces gas, and causes distress.

Dual-source gels — those with a roughly 2:1 ratio of maltodextrin to fructose — use both pathways simultaneously, pushing absorption rates up to 90–100g per hour in trained guts. If you're taking in 60g+ of carbs per hour (as most marathon fueling plans now recommend), single-source gels are almost guaranteed to cause problems.

Check the ingredients label. If the only carbohydrate source is maltodextrin or glucose, and you're eating more than one gel every 30–40 minutes, that's likely your issue.

Why race day is always worse than training

Even if you've tested gels in training, race day introduces three compounding factors:

Higher intensity. Most marathon long runs are done at 60–75% of VO2max. Race pace pushes closer to 80–85%. That intensity difference is enough to meaningfully reduce gut blood flow.

Adrenaline and nerves. Pre-race anxiety triggers a sympathetic nervous system response that slows gastric emptying before the gun even goes off. Add a too-big race morning breakfast eaten too close to the start, and your stomach is already behind.

Heat and dehydration. If race day is warmer than your training conditions, your body sends more blood to the skin for cooling, further reducing gut perfusion. Dehydration compounds the problem by reducing blood volume overall.

This is why "I tested it in training and it was fine" is not a reliable predictor of race-day tolerance. You need to test at race intensity, in race conditions.

The fix: a gut training protocol that actually works

The good news is that your gut is trainable. Research shows that consistent carbohydrate intake during exercise increases the expression of intestinal transport proteins (SGLT1 and GLUT5), improves gastric emptying rates, and reduces perceived GI symptoms — in as little as two weeks.

Here's how to do it:

Week 1–2: Build the baseline. On two runs per week (one easy, one at tempo effort), practise taking 30g of carbohydrate every 30 minutes. Use the gel brand and flavour you plan to race with. Take each gel with 150–200ml of water.

Week 3–4: Increase to race rate. Move to 45–60g per hour on your long run and one mid-week session. At least one of these sessions should include 20–30 minutes at your goal race pace. This is where most athletes skip — they only practise at easy effort, which doesn't stress the gut the same way.

Final 2 weeks before race: Lock in. Your final long run should use the exact fueling plan you'll follow on race day — same gels, same timing, same fluid intake, at race-effort intensity for the middle miles.

If you still get nausea at 60g/hour, try these adjustments in order:

1. Switch from a traditional gel to an isotonic or hydrogel formulation
2. Switch from a single-source to a dual-source carbohydrate gel
3. Reduce to 45g/hour and increase gradually over the following training cycle
4. Take gels with plain water only — never with sports drink, which adds osmotic load

Your gel timing is probably wrong too

Most gel timing advice says "take one every 30–45 minutes." That's a starting point, but it ignores finishing time.

A 3-hour marathoner takes 5–6 gels over the race. A 4:30 marathoner needs 7–9. The slower runner is on their feet longer, burning more total energy, but most generic plans give both runners the same number. The slower marathoner is the one more likely to under-fuel and bonk.

Your gel timing should also account for the course. Take gels 5–10 minutes before major aid stations so you can chase them with water. Avoid taking gels on steep uphills when blood flow is already redirected to your legs. And plan your last gel by mile 22 at the latest — past mile 24, GI risk outweighs the energy benefit.

Stop guessing — let your data build the plan

The fundamental problem with generic gel advice is that it's generic. Your carbohydrate needs per hour depend on your body weight, your pace, your sweat rate, and your individual gut tolerance. The right gel, the right amount, and the right timing are different for every runner.

Sweatr calculates your personal carbohydrate and fluid targets using your Apple Watch, Garmin, or Strava data — your actual training load, your estimated sweat rate, and your goal race pace. It maps those targets to real products you can buy, tells you when to take each one, and adapts the plan as your training changes.

No more guessing whether you need 45g or 75g per hour. No more discovering on race day that your gut can't handle the plan you found on a blog.

Your watch already has the data. Sweatr turns it into a fueling plan that works for your body — not the average body.