More half Ironman races are lost to the gut than to the legs. Athletes who train for months still blow up at mile 8 of the run — not because they lacked fitness, but because they under-fueled the bike or asked their stomach to absorb more than it was trained to handle. This guide gives you the actual numbers: how many grams of carbohydrate per hour you need for a 70.3, how to hit them, and how to train your gut so race day fuel stays down.
The short answer
| Athlete | Carbs per hour (bike) | Carbs per hour (run) |
|---|---|---|
| First-timer, 6+ hour finish | 50–60 g | 30–40 g |
| Mid-pack, 5–6 hour finish | 60–80 g | 40–50 g |
| Experienced / front of pack, under 5 hours | 80–100 g | 50–60 g |
If you remember one number, make it this: most age-group athletes perform best around 60–90 g of carbohydrate per hour on the bike, scaled to experience and gut training. Almost nobody fails a 70.3 from eating slightly too much on the bike — thousands fail from eating too little.
Why these numbers
Your body can store roughly 400–500 g of glycogen — around 1,600–2,000 calories. A 70.3 costs most athletes 3,500–5,000 calories. The gap has to come from body fat (which fuels easy efforts well but not race effort) and from the carbohydrate you swallow during the race. Stored glycogen alone runs out somewhere in the first half of the run, which is exactly where most bonks happen.
Absorption is the bottleneck. Glucose crosses the gut through a transporter that saturates at about 60 g per hour. To go beyond that, you add fructose, which uses a separate transporter — this is why nearly every modern gel and drink mix uses a glucose-fructose blend, and why intakes of 90 g per hour and more are possible with training. The practical ceiling is not your stomach size; it is which transporters you have trained.
Build your race plan backwards
Work in grams, not calories or “a gel every so often”. An example for a 70-kg athlete targeting a 5:30 finish, 75 g/hour on the bike:
- Bike (2:50): about 210 g total. For example: 2 bottles of mix at 45 g each (90 g) + 4 gels at 25 g (100 g) + a few chews (20 g). Set a timer: something every 15–20 minutes from minute 15 onward.
- Run (1:55): about 80–90 g total. A gel every 25–30 minutes plus cola or sports drink at aid stations in the final 5 km.
- Swim: nothing during, obviously — but a gel 10 minutes before the start tops off liver glycogen.
Front-load the bike. The gut absorbs best early, when effort is controlled and the body is fresh — the final 30 minutes of the bike and the whole run live off what you banked in the first two hours. Pair the plan with your pacing targets from our long-course pacing guide: going 20 watts too hard shuts the gut down faster than any nutrition mistake.
Hydration and sodium belong in the same plan
Carbohydrate absorption needs fluid. As a baseline, aim for 500–750 ml per hour on the bike, more in heat, and 400–600 mg of sodium per litre of sweat replaced. Dehydration past about 3% of body weight slows gastric emptying — which means the fuel you swallow just sits there. Our full triathlon hydration guide covers sweat testing and heat adjustments in detail.
Train your gut like you train your FTP
The single most skipped workout in triathlon: eating at race rate. The gut adapts in 4–6 weeks, just like muscles.
- Pick one long ride per week and fuel it at your full race target (not less).
- Practice eating in aero position, at race cadence, with race products.
- Do at least three brick sessions where you take a gel in the first 10 minutes of the run.
- Increase by 10 g per hour every couple of weeks if the current rate sits well.
Race morning matters too: 2–3 g of carbohydrate per kg of body weight, 3 hours before the start, from familiar low-fiber foods. The full protocol for the days before is in our carb loading guide, and the race-morning menu in what to eat before a triathlon.
Troubleshooting on race day
Gut feels bloated and sloshy
Back off pace for 10 minutes, sip plain water, skip one feeding, then resume at a slightly lower hourly rate. Sloshing means absorption stalled — usually from pace, heat or a too-concentrated mix, not from the total amount.
Nausea on the run
Switch from gels to liquid carbs (sports drink or flat cola). They empty from the stomach faster at run intensity.
Cramping late in the run
More often a pacing and conditioning signal than pure sodium loss — but if you are a salty sweater (white crust on kit), add 300–500 mg sodium per hour next time and test it in training.
FAQ
How many gels is that in total?
For a typical 5:30–6:00 finisher at 70 g/hour on the bike and 45 g/hour on the run: the equivalent of 11–14 standard 25 g gels, with drink mix replacing several of them. Mix formats — all-gel fueling is a common nausea trigger.
Can I just use the on-course nutrition?
Only if you trained with it. Check your race’s official product list months out, or carry your own on the bike and switch to course nutrition as backup late in the run.
Do carbs matter for a 6.5–7 hour finisher?
More, not less — you are out there longer. Lower intensity means fat covers a bigger share, so the hourly target drops to 50–60 g, but the total is larger and skipping it still ends in a walk.
Fueling is one leg of the stool. The training that gets you to race day is in our free 20-week 70.3 plan and complete 70.3 guide.
