Triathlon nutrition is more complex than single-sport fuelling, and the consequences of getting it wrong are higher. You can't eat during the swim. Eating during the run is difficult. The bike is where almost all of your fuelling happens — which means the bike leg determines your run performance.
Here's how to approach each stage.
Before the race
The night before: Carbohydrate-led dinner, familiar food, low fibre, low fat. Keep the meal moderate in size. Race morning nerves combined with a heavy evening meal is a reliable recipe for a difficult start.
Race morning: Two to three hours before your start, eat your planned pre-race meal. This should be something you've practised before long training sessions — nothing new. White rice, toast, oats, banana — whatever your gut handles well. Moderate protein, low fat, low fibre.
In transition: Many athletes consume a gel in the 20 to 30 minutes before the swim start. This primes blood glucose for the initial effort. Use caffeine here if you've trained with it.
Hydration: Drink 400–600ml of water in the two hours before the race, sipping consistently rather than drinking large volumes at once.
The swim
You cannot eat during the swim. What you can control is starting the swim in a good nutritional and hydration state, and keeping the swim as efficient as possible so you exit T1 in the best condition for the bike.
One practical note: swallowing water during open water swims — especially in salt water — can cause GI distress on the bike. Keep this in mind if you historically struggle with stomach issues in the first half of the bike leg.
T1 — First transition
Keep it simple. If you have a gel planned for T1, consume it while moving through transition. Don't add complexity here — you want to be on the bike as quickly as possible.
The bike
This is where your race nutrition happens. Everything you consume on the bike directly fuels both the bike effort and the run that follows. Under-fuelling on the bike means a difficult run. Full stop.
Carbohydrate targets:
Sprint/Olympic: 30–50g per hour. Shorter duration, gel and drink are sufficient.
Half-Ironman (70.3): 60–80g per hour. Real food can be added in the middle sections when gut tolerance is better.
Ironman: 70–90g per hour. This requires practiced gut tolerance. Mix carbohydrate sources (glucose + fructose). Start at the lower end and build to the upper end as your gut adapts over the training block.
Start fuelling immediately. Within the first 15 to 20 minutes of the bike. Don't wait until you feel like you need it.
Hydration on the bike: 500–750ml per hour in mild conditions. More in heat. Include electrolytes — particularly sodium — on any bike leg over 60 minutes.
Transition to T2: In the final 20 minutes of the bike, shift to easily digestible formats — gels and liquid rather than solid food. Your gut will thank you in the first kilometres of the run.
T2 — Second transition
Most athletes take a gel at T2 for the run. If you're running a half or full Ironman, this is a sensible strategy — it provides an immediate source of fuel as you start the run.
The run
Fuelling the run in triathlon is constrained by practicality and gut tolerance. Hard running reduces gastric emptying and increases the risk of GI distress. The fuelling you've done on the bike is what carries you through most of the run.
Sprint/Olympic run: Short duration — water at aid stations is typically sufficient. A gel at the start of the run if the bike leg was hard and long.
Half-Ironman run: 20–30g of carbohydrate per hour. Gels and sports drink at aid stations. Sip rather than gulping fluids.
Ironman run: 40–60g per hour where gut tolerance allows. At this duration and cumulative fatigue level, many athletes struggle with GI issues on the run. Cola at aid stations (available at Ironman events) is a practical late-race option — it's easy to absorb, provides simple carbohydrate, and the caffeine is useful.
Don't experiment on race day. Whatever you're eating on the run, you should have practised it in brick sessions (bike-to-run training).
The GI distress problem
GI distress is the most common performance-limiting factor in longer triathlons. The causes are multiple: running after cycling, heat stress, concentrated nutrition products, inadequate gut training.
Prevention:
Train with your race nutrition in sessions that replicate race conditions
Practise eating on the bike and transitioning immediately to running
Don't over-concentrate nutrition — dilute gels if your gut is sensitive
Don't try anything new on race day
The short version
The bike leg is where triathlon nutrition happens. Under-fuel on the bike and the run becomes a survival exercise. Start fuelling early in the bike, maintain consistent intake throughout, shift to easily digestible formats near T2, and treat race nutrition as a skill to be practised — not a plan to be improvised on the day.