Why Do Energy Gels Upset Your Stomach?

Reviews of exhausting endurance events report 30 to 50% of participants experiencing at least one gastrointestinal symptom. This is common, not a personal failing.

Why do gels make me feel sick?

Three mechanisms stack. The gel is highly concentrated, your gut is receiving far less blood than usual, and if it uses one sugar you have saturated one absorption route. Concentrated fuel sitting in an under-perfused gut is what you feel as bloating, sloshing and nausea.

Mechanism one: concentration

Gastric emptying slows in proportion to how concentrated the meal is. A typical gel is around 25 g of carbohydrate in 30 to 40 ml, which is a syrup.

Osmolality is the measure that matters. Drinks above roughly 500 mOsm/L are associated with more reported symptoms. A neat gel is well past that, which is why "take with water" is a mechanism instruction rather than a suggestion.

Mechanism two: blood flow

During exercise your body redirects blood to working muscle and skin. Splanchnic blood flow starts dropping sharply around 70 to 80% of VO2max and can fall by up to 80% at maximal effort.

So the gut is asked to process a concentrated sugar load exactly when it has least capacity. That is why the same gel is fine on Sunday and turns on you at race pace, and why heat makes everything worse.

Mechanism three: the saturated door

Glucose-only products cap at the SGLT1 ceiling of about 60 g an hour. Anything past that stays in the intestine, pulls water in osmotically, and ferments.

What actually helps

Dilute it. Water with every concentrated dose, every time.

Use two pathways. Check the label for fructose. Maltodextrin and glucose alone means one door.

Match intake to intensity. 90 g an hour is an Ironman number, not a 10K number.

Spread the load. Smaller, more frequent doses arrive gentler than one large hit every 45 minutes.

Train the gut. It adapts. A high-carbohydrate intake increases SGLT1 density within days to two weeks, and tolerance improves across roughly six to ten weeks of weekly high-carbohydrate sessions.

Are energy chews easier on your stomach than gels?

The research does not single out format. What a chew changes is control of the dose: a strip of six is 48 g if you take all of it and 24 g if you take three, so you decide how much arrives and when. A gel is a single unit, and once it is open the dose is set.

I will be straight though. Concentration, intensity, heat and total hourly load are the variables the research keeps returning to, and format sits alongside them rather than above them. Plenty of people race five hours on gels perfectly happily. Whatever you use, rehearse it in training months out, not the week before.

Tom