How to fuel for 8 to 17 hours without stomach trouble: carbohydrates, sodium, and the real hyponatremia risk of long-distance racing.
The question that shapes your long-distance day more than any watt on the bike: "How do I eat and drink for 8 to 17 hours without my stomach rebelling?" The honest answer: on long-distance, nutrition itself becomes a discipline — on a sprint distance one gel is enough, here your fueling decides finishing or not.
The reason lies in duration: at around 60 g of carbohydrate per hour, your body hits an absorption ceiling with a single sugar type — with a mix of two sugar types (usually glucose and fructose) it can process significantly more, because different transport pathways in the gut are used simultaneously. That's almost irrelevant on the short course — on long-distance, where you need energy for hours, it's the difference between strength and crisis.
The uncomfortable truth: in a study of long-distance triathletes, 59% of athletes reported gastrointestinal symptoms during the race — and race duration itself correlated more closely with this than the sheer amount of carbohydrate consumed. Nutrition here isn't an afterthought, it's a trainable skill.
For most age groupers the realistic target is 60–90 g of carbohydrate per hour on the bike, with a mix of several sugar types for better tolerance. That's significantly more than many beginners intuitively consume — and often exactly why the marathon breaks down.
Official recommendations for ultra-endurance exertion cite a framework of roughly 450–750 ml of fluid per hour and over 575 mg of sodium per liter in heat. That's not an arbitrary number: too much plain water without sodium is one of the main drivers of hyponatremia — a dangerous drop in blood sodium.
In an analysis of 1,089 long-distance triathletes, one in ten (10.6%) had documented hyponatremia at the finish — mostly mild, but severe cases occurred too. Especially at risk: women and athletes with longer finish times — the first cases appeared from hour 9 onward, critical cases in the clusters finishing in hours 12–14. Drink to a plan, not by thirst alone, and replenish sodium consistently.
Pure gels become unappealing in taste and digestion for most athletes after several hours. Bars, banana or salty snacks in the bento box add variety and cover the enormous calorie demand over 5–7 hours of cycling more realistically than gel alone.
Practice your exact race-day plan in at least three to four long training sessions. Your gut responds to repeated exposure with better tolerance — you can't make that up on race day, only train it beforehand.
Drink to a plan, not by thirst — both too little and too much fluid without sodium are risky. If you experience confusion, severe nausea or swelling, alert medical staff on course immediately — these can be signs of hyponatremia, not normal fatigue.
With persistent GI distress, reduce pace and switch to simple, well-tolerated calories rather than stopping eating entirely — an empty tank is the bigger risk on long-distance.
Keep a simple fueling log for your long training sessions: what, how much, when, how your stomach reacted. After three to four sessions you'll have a solid race-day plan, not a guess.
A trick experienced age groupers use: they prefer eating solid food on flat or slightly uphill sections, not right before technical descents — the stomach tolerates digestion better when the rest of the body isn't maximally demanded at the same time.
Realistically 60–90 g per hour for most age groupers, ideally from a mix of several sugar types for better absorption. That's significantly more than feels intuitively right — practice the amount beforehand in training.
Typical signs are confusion, severe nausea, dizziness or swelling — different from normal exhaustion. In a study of over 1,000 long-distance finishers, one in ten was affected, mostly mild but with severe cases too. When in doubt, alert medical staff immediately.
Because race duration itself correlates more closely with GI distress than the sheer amount of carbohydrate — in one study, 59% of long-distance athletes reported symptoms. The longer the exertion, the more time digestive stress has to build.
A rough framework for ultra-endurance exertion is 450–750 ml per hour plus adequate sodium, especially in heat. Drink to this framework and your individual sweat loss, not by pure thirst.
Long-Distance Triathlon: The Marathon After 180 km on the Bike