Triathlon Aid Station Nutrition Planner

Carbohydrate absorption is limited by transport, not by appetite. Glucose crosses the intestinal wall through the SGLT1 transporter, which saturates at roughly 60g per hour. Eat more glucose than that and the excess simply sits in your gut, drawing water in and producing the bloating, nausea, and sloshing that long-course athletes know well.

The way past that ceiling is fructose, which uses a different transporter (GLUT5) entirely. Combining glucose and fructose lets you use both pathways at once, and the research on multiple transportable carbohydrates — the basis for the modern 60-90g guidance — showed meaningfully higher oxidation rates than glucose alone. This is why nearly every long-course sports nutrition product now advertises a glucose to fructose ratio.

Two practical consequences follow. First, hitting 90g per hour requires products with a mixed carbohydrate source; you cannot get there on maltodextrin alone. Second, the gut is trainable. Absorption capacity increases with repeated exposure, which means the athlete who has practised 90g per hour in training can use it, and the athlete who tries it for the first time on race day usually cannot.

The bike and the run are not equivalent fueling environments, and planning them identically is one of the most common errors this planner is meant to prevent. On the bike you are stable, your torso is relatively still, blood flow to the gut is higher because the effort is lower, and you can carry everything you need. On the run you are being jolted vertically, gut blood flow has dropped, and you can only carry what fits in a belt.

The practical rule is to front-load. Take the majority of your total carbohydrate on the bike, at the highest rate you have trained, and plan the run around smaller, more frequent doses — a gel every 20-25 minutes rather than large volumes. If you arrive at T2 already behind on fuel, the run is not where you will catch up.

Aid-station spacing is what turns this from theory into a plan. Knowing that run aid stations sit roughly every 2km lets you attach each dose to a physical landmark instead of relying on remembering a schedule at hour six, when arithmetic is the first thing to go.

Sweat contains sodium, and sweat rates vary enormously between athletes — both the volume and the salt concentration are largely individual. That makes generic sodium advice weak, but the failure modes are worth understanding.

Under-replacing sodium during a long hot race contributes to cramping and, at the extreme, to exercise-associated hyponatremia: dangerously diluted blood sodium. The counterintuitive part is that hyponatremia is usually caused by drinking too much plain water, not by sweating too much. Athletes who drink to a schedule rather than to thirst, in a long race, in the heat, are the classic case.

The ACSM position stand on exercise and fluid replacement is the standard reference and is cited on our methodology page. Its practical summary: drink largely to thirst, include sodium in long events, avoid losing more than about 2% of bodyweight, and do not force fluid beyond what thirst and sweat rate justify.

Everything this planner produces should be tested in training before it is trusted in a race. Use your longest ride to practise the exact products, the exact hourly rate, and the exact timing. A plan that works at 60% effort on a Sunday morning is not evidence that it works at race intensity in the heat.

Then simplify. The best race nutrition plans are the ones you can execute while tired and slightly stupid: a fixed number of items per hour, attached to landmarks you cannot miss. Complexity is the enemy at hour seven. If the plan needs a spreadsheet on race day, it is the wrong plan.

Finally, know what is on course. Most races publish which drink and gel brands will be at aid stations. Training on those products means you can carry less and still recover from dropping a bottle — and a dropped bottle should never be the thing that ends your race.