The bike leg of a triathlon is the only place you can eat. You can't chew during the swim and you can't stomach much on the run. Everything you need to get through the last 10km of the marathon has to go into your body between T1 and T2.
Most triathletes under-fuel the bike and then blame the run when it collapses. But the run didn't collapse — the nutrition plan did, three hours earlier, when you took two gels in a 5-hour Ironman bike leg and convinced yourself that was enough.
We've talked to Sebastian Kienle, Kristian Blummenfelt and the sports scientists behind modern high-carb fuelling on the podcast. What follows is the distilled version: what to eat, when, how much, and why.
Carb Targets That Actually Work
The last decade has completely rewritten triathlon fuelling. Where 60 g/hr was once considered aggressive, elite Ironman racers and Tour de France riders now routinely train their guts to tolerate 120 g/hr or more. The numbers below are what work for trained amateurs with a reasonably prepared gut.
Sprint and Olympic: 30-60 g/hr. Short enough that glycogen stores matter more than live fuelling. Top up early, don't stress.
Middle distance / 70.3: 60-90 g/hr. You need real fuel. An average 70.3 bike leg at 2:30 should take 180-220g of carbohydrate in total across the bike.
Full Ironman: 90-120 g/hr. This is where under-fuelling destroys races. A 5-hour bike at 100 g/hr is 500g of carbs just on the bike, plus whatever you took pre-race and during the run. That is the range modern Ironman bike legs demand.
Caveat: these targets require a trained gut. Do not turn up to race day and try to push 120 g/hr for the first time. Your gastrointestinal tolerance adapts the same way your FTP does — slowly, with repeated exposure. Train your gut on every long ride. Our in-ride nutrition guide covers the progression.
Why the Bike Leg Fuels the Run
Here is the bit most triathletes miss: the run is not fuelled by the run. It is fuelled by the bike.
Your stomach becomes progressively less tolerant as the race goes on. Heat, fatigue, aero position compressing your abdomen, and elevated sympathetic drive all conspire to slow gastric emptying. By kilometre 21 of an Ironman marathon, most athletes can only tolerate roughly half the carbohydrate intake they could in the first hour of the bike.
That means you cannot "catch up" on the run. You can only defend what you banked on the bike. Under-fuel the bike by 200g of carbs and the first half of your marathon will be fine, but the last 10km becomes a slow-motion car crash as glycogen taps run dry and your brain starts hoarding glucose for itself.
Lionel Sanders has talked about this on the podcast — when he raced at his peak, his bike nutrition was so aggressive that T2 felt like the start of the race. That isn't heroic. It is the correct outcome of correct fuelling.
The 1:0.8 Glucose to Fructose Ratio
The single biggest upgrade to modern sports nutrition is multiple transportable carbohydrates, a line of research led by Asker Jeukendrup. Glucose is absorbed through SGLT1 transporters, which cap oxidation at around 60 g/hr. Fructose uses GLUT5 — a completely separate transporter. Combine them and you can push total exogenous carbohydrate oxidation beyond 90 g/hr. Classic Jeukendrup trials used a 2:1 glucose-to-fructose ratio; newer work from Maurten and others has moved to roughly 1:0.8, which appears to support oxidation rates above 100 g/hr with less GI distress at the highest intakes.
Most modern endurance carb products are built on some version of this ratio — Maurten's 1:0.8 hydrogel, SiS Beta Fuel (moved from 2:1 to 1:0.8 in 2021), Neversecond C30, and similar. Precision Fuel & Hydration's glucose-to-fructose ratios vary by product, so read the label. If you are still taking straight-maltodextrin gels at 90 g/hr you are over-loading SGLT1, leaving carbs stranded in your gut, and almost certainly going to vomit.
Check your labels. A 40g carb gel with 25g maltodextrin and 15g fructose is roughly 1:0.6, which is good. A gel that says "maltodextrin" and nothing else is glucose-only.
A Practical Fuelling Schedule
Here is a concrete schedule for an Ironman bike leg targeting 100 g/hr. Adjust proportionally for 70.3 or shorter.
T1 (0-5 min): Take a gel (about 30g carb) with a mouthful of water. This kicks the gut into fuelling mode immediately and front-loads calories while tolerance is highest.
Minutes 15-20: First bottle sip. If you are using a concentrated bottle (eg. 160g carb in 500ml), 100-150 ml in the first 20 minutes. Otherwise start steady sipping at 150-200 ml / 20 min.
Every 20 minutes: Carb intake. Rotate a chew, a gel, a swig of concentrated mix — anything that adds roughly 30-35g of carbs per 20 minutes. The point is consistency. Do not let 40 minutes go by with nothing.
Every 15 minutes: Fluid intake. Small sips, often. Don't slam a full bottle in one go — gastric emptying hates volume spikes. 500-800 ml per hour total depending on conditions.
Every 30 minutes: Sodium top-up if not built into drinks. 500-1000 mg sodium per hour total for most athletes, higher in heat.
Last 30 min of bike: One final gel 15-20 minutes before T2 to prime the run with a fresh glucose spike. Keep it small to avoid overloading the gut as running starts.
Set it on your head unit as lap alerts. Do not rely on feel. By hour three you will forget.
Fluids, Sodium and Heat
Carbs get the attention but fluids and sodium decide whether your carbs get absorbed. Dehydration and sodium depletion both slow gastric emptying, meaning the gel you took at hour two is still sitting in your stomach at hour three, sloshing.
Sweat rate testing is worth doing once. Weigh yourself naked before a 60-minute hard ride in conditions similar to race day, weigh yourself naked after (pre-towel), and calculate weight lost plus fluid consumed. That is your hourly sweat rate. Replace 60-80 per cent of it on the bike.
Sodium targets scale with sweat rate and sweat concentration — 500-1000 mg/L of fluid is the safe working range for most amateurs. Heavy sweaters in hot races can need 1500 mg/L. Do not guess, especially in heat.
Our energy gels guide covers product selection and rotation specifically.
Rehearse the Plan
No nutrition strategy survives first contact with race day if you have not practised it. Every long ride in the last 8 weeks of your build should use race-day products at race-day volumes. If Maurten 320 destroys your stomach in training, it will destroy your race.
The gut is trainable. Push your hourly carb intake up by roughly 10 g/hr every 2-3 weeks on long rides. A triathlete who starts a build at 60 g/hr tolerance can be comfortably absorbing 100-110 g/hr within 12 weeks. That delta alone is worth minutes on the run.
Key Takeaways
- Target 60-90 g/hr for 70.3 and 90-120 g/hr for Ironman — most age-groupers chronically under-fuel.
- Use products with a 1:0.8 glucose-to-fructose ratio to nearly double your absorption rate without GI distress.
- Start fuelling within 15-20 minutes of T1. You cannot catch up later.
- Pair carbs with 500-800 ml/hr of fluid and 500-1000 mg/L of sodium. Heat and size shift these up.
- The run is fuelled by the bike — defend what you bank in the first 2 hours.
- Train the gut on every long ride. Tolerance is trainable; race day is not the day to test it.
- Pair your plan with a proper in-ride nutrition guide and smart energy gel selection.
- Need a coach to build your fuelling plan and rehearse it session-by-session? See our coaching options or apply to work with us.