Long-course triathlon is as much a fuelling problem as a fitness problem. The run leg exposes every under-fed hour on the bike — however strong your thresholds look.
For most age-group 70.3 athletes the race lasts 4–6 hours: long enough to deplete glycogen, short enough that many still under-fuel. Carb targets in WattX scale with weight, predicted split, and Race Pacing Targets — riding harder burns carbs faster, which leaves less margin.
Carbs
Why Carbs Matter in Long-Course Racing
At race effort, you have enough stored glycogen for roughly 90–120 minutes. A 70.3 lasts far longer. Without exogenous carbohydrate, pace collapses — often suddenly. Fuel early; waiting until you feel hungry means you are already behind.
Current science supports 60–90 g carbohydrate per hour during sustained endurance when the gut is trained. The upper band needs a 2:1 glucose:fructose blend so two intestinal transporters can work in parallel. Single-source glucose alone often caps absorption around 60 g/h and increases GI distress.
Upper band needs 2:1 glucose:fructose. Front-load on the bike; simplify on the run.
By leg
Fuelling by Leg
No mid-race fuel
70–150 g carbs 2–3 h pre-start. Familiar, low fibre. Swim is 20–35 min for most.
Main fuelling window
Train 70–90 g/h when tolerated. WattX defaults: 60 g/h (70.3) · 75 g/h (IM).
Simpler formats
Gels every 25–30 min from km 1. Defaults: 45 g/h (70.3) · 55 g/h (IM).
The Nutrition Strategy tool uses conservative per-distance defaults from your weight, predicted split, and temperature — train higher in practice, race from the playbook.
Hydration
Hydration and Sodium
Do not rely on thirst alone in competition.
Physiology
Why It Works
Once stored glycogen runs out, you “bonk” and your pace collapses. That's why you fuel steadily from early on.
As you sweat, your blood volume drops. If you lose >2% of your body weight in fluid, your heart has to pump significantly faster to maintain the same power output (cardiac drift).
Sodium dictates how well your gut actually absorbs the water you drink. Without adequate sodium, water just sloshes in your stomach. It is also critical for maintaining muscle contraction and preventing cramps.
Wisdom
Race-Day Rules
Check the race nutrition partner. Try their products during your longer training days and race simulations. If your stomach or palate rejects it, you will need to carry 100% of your own fuel.
Don't try a new gel, hydration mix, or carb ratio on race day.
Knowing exactly where the aid stations are dictates how much you actually need to carry.
On race day, if you feel you are no longer absorbing your nutrition, stop eating/drinking. Your gut may have shut down. Slow your pace, and give it a bit of time before trying again.
It is tempting to blow past an aid station when you feel great and don't want to break your rhythm. Don't do it. Missing a scheduled bottle or gel creates a deficit that will derail your race later on. Slow down, grab what you need, and stay on plan.
Pacing link
Linking Pacing and Fuel
Harder bike pacing increases carb oxidation. Under-fuel on the bike and the run pays regardless of fitness.
Set Race Pacing Targets in the Race Simulator — the predicted split feeds straight into Nutrition Strategy.
Your numbers
Calculating Your Numbers
Carbohydrate needs scale with weight, pace relative to your CSS, FTP, and FT, and conditions. WattX totals swim, bike, and run carbs, fluid, and sodium from your profile — and refreshes when simulator time or distance changes.