Triathlon Nutrition: How to Fuel the Swim, Bike, and Run

Most triathlon nutrition guides tell you the same thing: aim for 60–90 grams of carbohydrate per hour. Take gels every 30–45 minutes. Drink when thirsty.

And then you get off your bike at kilometre 90, start the run, and your stomach locks up.

What those guides miss is that triathlon nutrition is not a single fueling strategy applied across three disciplines. It is three entirely different fueling challenges that happen to occur in sequence. Getting this right changes your race. Getting it wrong turns a good bike split into a walk-run death march.

Before the start: your pre-race fueling window

Eat 2–3 hours before your start time. You need something substantial enough to top up liver glycogen — which depletes overnight — but easy enough to digest.

• 1–4 g of carbohydrate per kg of bodyweight, depending on race duration
• Low in fat and fibre
• Foods you've eaten before long training sessions — nothing new on race day

Practical options: white rice with banana, white toast with honey, oatmeal, a bagel. Avoid: eggs, full-fat dairy, anything high in fibre.

The gap problem: In a 70.3 or Ironman, your start may be 90 minutes to 3 hours after breakfast. Take a small top-up 30–45 minutes before the gun: half a banana, a small gel, or 200 ml of sports drink.

Hydration: Drink 400–600 ml of water or electrolyte drink in the 2 hours before start. Stop drinking 20–30 minutes before the swim so you're not carrying excess fluid into the water.

The swim: the discipline you cannot fuel

You are not going to eat or drink during the swim. Full stop.

Your pre-race nutrition carries you through. A sprint swim is 15–20 minutes; an Ironman swim is 50–70 minutes for most age groupers. If you start glycogen-depleted, you will arrive at T1 already in deficit.

One thing you can control: a small gel 15–20 minutes before the swim start bridges the gap between breakfast and T1 nutrition. Many athletes skip this because of anxiety or logistics. It is worth the planning.

T1: start fueling immediately

Get something in within 5 minutes of T1 if your race is 70.3 or longer. A gel or the first sip of your bike nutrition in the first kilometre is not a nice-to-have — it is a deficit-correction.

The bike: your most important fueling window

The bike leg is where you do most of your nutritional work. You're in a stable position (no foot strike impact), your gut can tolerate higher carbohydrate volumes, and you have the most opportunity to carry and access nutrition. This is the window to load.

Carbohydrate targets by race distance:

| Race distance | Approx. bike time | Target carbs/hour | |---|---|---| | Sprint triathlon | 30–60 min | 30–45 g/hr | | Olympic distance | 60–80 min | 45–60 g/hr | | 70.3 (Half-Ironman) | 2:30–3:30 | 70–90 g/hr | | Full Ironman | 5:00–7:00+ | 80–100 g/hr (if gut-trained) |

The glucose-fructose rule: Above 60 g/hr, you need a glucose-fructose blend (typically 2:1 ratio) to access multiple gut transporters simultaneously. If you're only using glucose-based gels at 90 g/hr, you will hit an absorption ceiling. Check the label — most modern endurance products (Maurten, SiS Beta Fuel, Precision Fuel) are formulated with this ratio.

Hydration on the bike:

• 500–750 ml/hr in cool conditions (below 18°C)
• 750–1,000 ml/hr in warm to hot conditions (above 22°C)
• Include sodium: 500–700 mg per litre is standard; athletes with visible white salt rings may need more

Caffeine timing: Save caffeine gels for the second half of the bike or early run. Taking a caffeine gel at the start of a 3-hour bike splits the effect poorly; taking it at 90 minutes hits the performance window correctly.

The critical rule: Don't overcook the bike. Athletes who push to 100 g+ carbs per hour without a trained gut will arrive at T2 with an overfull, unhappy GI system — and the run's impact stress will trigger it immediately. If you haven't trained your gut to process 90 g/hr, don't race at 90 g/hr. 70–75 g/hr with a well-trained gut outperforms 90 g/hr with a naive gut every time.

T2: the gut reset

Take a small non-caffeinated gel in the last 10–15 minutes of the bike so it digests during T2 rather than the first kilometre of the run. Avoid arriving at T2 having just taken a large gel — impact from running on a freshly loaded gut is a reliable way to trigger nausea.

The run: where most nutrition plans fall apart

The run is the hardest discipline to fuel for three reasons:

1. Foot strike creates GI stress that doesn't exist on the bike. Every landing transmits vibration through the GI tract. Athletes who tolerate 90 g/hr on the bike may only tolerate 50–60 g/hr on the run.
2. You're already depleted. Unlike a standalone marathon, you arrive at the run having swum and cycled. Your glycogen is already reduced.
3. Blood flow is redistributed. Running demands more cardiovascular effort per unit of perceived effort than cycling. Less blood is available for digestion.

Carbohydrate targets on the run:

| Race distance | Approx. run time | Target carbs/hour | |---|---|---| | Sprint triathlon | 15–25 min | None needed | | Olympic distance | 35–55 min | 20–30 g/hr optional | | 70.3 (Half-Ironman) | 1:30–2:30 | 40–60 g/hr — small doses, more frequently | | Full Ironman | 3:30–6:00+ | 50–70 g/hr — mix gels, cola, broth in later stages |

Small and frequent beats large and infrequent. Half a gel every 20 minutes is better tolerated than a full gel every 40 minutes. Smaller doses keep blood sugar steadier and reduce the sugar volume hitting the GI system at once.

Late-race real food: In a full Ironman or long 70.3, gel fatigue is real. After 5+ hours of racing, sweet gels become difficult to tolerate. Flat cola (available at most Ironman aid stations) provides carbohydrate, caffeine, and sodium in a form the gut typically tolerates well. Many experienced Ironman athletes switch almost entirely to cola in the final third of the run.

How your sweat data changes the plan

The targets above are starting points. The right numbers for you depend on your sweat rate, your sweat sodium concentration (as many as 30% of endurance athletes are salty sweaters needing 1,000 mg/L or more), your training load, and race conditions.

Sweatr pulls your data automatically and builds your triathlon-specific nutrition plan: fluid and carbohydrate targets for each leg, aid station timing, and product recommendations mapped to what you've actually trained with. Get your free triathlon plan.

Race-day checklist: 70.3

Race morning (3 hours before): 100–150 g carbohydrate, low fat, low fibre; 400–600 ml water or electrolyte drink; small gel 30 minutes before swim if needed.

Swim: Controlled effort; arrive at T1 with energy in reserve.

T1: First bike nutrition within 5 minutes — don't skip this to save time.

Bike: 70–80 g carbs/hr; 600–800 ml fluid/hr; small bridge gel in final 10–15 minutes of bike.

T2: Let the bridge gel digest during transition; first run gel at kilometre 1–2.

Run: 40–60 g carbs/hr in smaller, more frequent doses; 400–600 ml fluid/hr at aid stations.

Post-race (within 30 minutes): 50–75 g carbohydrate + 20–25 g protein; continue hydrating with sodium for 2–4 hours.

The biggest mistake you can make

It's not going out too fast. It's not choosing the wrong gel brand. The biggest triathlon nutrition mistake is testing your race nutrition for the first time on race day.

Your gut is trainable. Athletes who practise 70–80 g/hr on their long brick sessions, who drink at the same rate they plan to race, who test gel tolerance at race intensity — these athletes have a fundamentally different gastrointestinal system on race day than athletes who just follow the guidelines.

Start now. Your next long training session is a nutrition test. Treat it like one.

If you want the numbers specific to your body — your sweat rate, your pace, your training data — Sweatr builds your complete triathlon fueling plan from your actual wearable data. Try Sweatr free.