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 — and what this one covers — 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. The rules that apply on the bike do not apply on the run. What you do or don't do in the swim sets up everything that follows.

Getting this right changes your race. Getting it wrong turns a good bike split into a walk-run death march.

Here is the discipline-by-discipline breakdown.

Before the start: your pre-race fueling window

Everything that happens during the race depends on what you do in the 3 hours before the gun goes.

Race morning breakfast

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 that it is not sitting in your stomach when the swim starts.

The formula is straightforward:

1–4g of carbohydrate per kg of bodyweight, depending on race duration and how your gut handles food under stress
Low in fat and fibre (slows gastric emptying, increases GI risk)
Foods you have eaten before long training sessions — nothing new on race day

Practical options: white rice with banana, white toast with honey, oatmeal with no added fat, a bagel. Avoid: eggs, full-fat dairy, anything high in fibre.

The gap problem

In a 70.3 or full Ironman, your start time may be 90 minutes to 3 hours after you have finished breakfast. You will need a small top-up 30–45 minutes before the gun: half a banana, a small gel (without caffeine at this stage), or 200ml of a sports drink. This prevents blood sugar from drifting low before you even hit the water.

Hydration

Drink 400–600ml of water or electrolyte drink in the 2 hours before start. Urine should be pale yellow — not clear, not dark. Stop drinking 20–30 minutes before the swim so you are not carrying excess fluid into the water.

The swim: the discipline you cannot fuel

This is non-negotiable: you are not going to eat or drink during the swim. Full stop. You are in the water. The swim leg is the exception that defines everything around it.

What this means in practice:

Your pre-race nutrition has to carry you through the swim. For most athletes, a sprint swim is 15–20 minutes. An Olympic is 20–30 minutes. A 70.3 swim is 30–40 minutes. A full Ironman swim is 50–70 minutes for the majority of age groupers. These are long enough that if you start the swim glycogen-depleted, you will arrive at T1 already in deficit.

The swim increases your fueling urgency at T1. You have been working at an elevated heart rate without any carbohydrate intake. Your glycogen stores have been ticking down. The bike leg is your first — and most important — fueling window, and it starts the moment you rack your wetsuit.

One thing you can control: if there is a significant wait between transition arrival and the swim start, use it. A small gel (20–22g carbohydrate) 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: the transition that sets up your whole race

Transition one is where many triathletes make their first nutrition mistake: they arrive at T1 depleted and don't start fueling immediately.

Get something in within 5 minutes of T1 if your race is 70.3 or longer. A gel or a hit of your bike nutrition in the first kilometre on the bike 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. Here is why: your body is in the most stable position of the three disciplines (no foot strike impact), your gut can tolerate higher volumes of carbohydrate, and you have the most opportunity to carry and access nutrition. This is the window to load.

Carbohydrate targets by race distance and intensity:

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

The glucose-fructose rule

Above 60g of carbohydrate per hour, you need to use a glucose-fructose blend (typically 2:1 ratio) to access multiple gut transporters simultaneously. If you are only using glucose-based gels or maltodextrin drinks at 90g/hr, you will hit an absorption ceiling and GI distress follows. Most modern endurance-specific products — Maurten, SiS Beta Fuel, Precision Fuel — are formulated with this ratio. Check the label.

What to drink on the bike

Your sweat rate on the bike is meaningfully different from your sweat rate on the run. Cycling typically produces 60–80% of the sweat rate of running at similar perceived effort, because you generate your own airflow while moving. In hot conditions, this advantage narrows significantly.

As a baseline:

Drink 500–750ml per hour in cool conditions (< 18°C)
Drink 750–1,000ml per hour in warm to hot conditions (> 22°C)
Include sodium in your bike hydration — 500–700mg sodium per litre is the standard range; athletes with high sweat sodium concentration (visible white salt rings on kit) may need more

Caffeine timing

If you are using caffeine gels, save them for the second half of the bike or the early run. Caffeine's ergogenic effect peaks 45–90 minutes after ingestion. Taking a caffeine gel at the start of a 3-hour bike splits the effect poorly; taking it at the 90-minute mark of the bike — or in the first kilometres of the run — hits the performance window correctly.

The critical rule: don't overcook the bike

This is the most common nutrition mistake in 70.3 triathlon, and it is not about eating the wrong thing. It is about eating too much of the right things, too fast.

The bike is forgiving. The run is not. Athletes who push to 100g+ carbohydrate per hour on the bike without a trained gut will arrive at T2 with an overfull, unhappy GI system — and the run's impact stress will trigger it immediately. The number of athletes who had a great bike split and then walked miles 10–20 because of GI distress is enormous.

Conservative approach: If you have not trained your gut to process 90g/hr, don't race at 90g/hr. 70–75g/hr with a well-trained gut will outperform 90g/hr with a naive gut every time.

T2: the gut reset

If you have been fueling well on the bike, you should arrive at T2 with solid energy and manageable GI status.

Transition two is a brief opportunity to:

Take a small gel (non-caffeinated) in the last 10–15 minutes of the bike so it digests during T2 rather than during the first kilometre of the run
Or carry the first gel in your run kit and take it within the first kilometre of the run

Avoid arriving at T2 having just taken a large gel — the impact of running on a freshly-loaded gut is one of the more reliable ways to trigger nausea.

The run: where most nutrition plans fall apart

The run leg of a triathlon is the hardest discipline to fuel, for three reasons:

Foot strike creates GI stress that does not exist on the bike. Every landing transmits a vibration through the GI tract. Athletes who tolerate 90g/hr on the bike may only tolerate 50–60g/hr on the run.
You are already depleted. Unlike a standalone marathon, you arrive at the run having swum and cycled. Your glycogen stores are already reduced. The run leg cannot be treated as a standalone fueling event — it is a continuation of a multi-hour deficit-management process.
Blood flow is redistributed. Running demands more from the cardiovascular system per unit of perceived effort than cycling. Less blood is available for digestion. The same gut that functioned fine on the bike will under-perform on the run.

Carbohydrate targets on the run:

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

Small and frequent beats large and infrequent

On the run leg, half a gel every 20 minutes is better tolerated than a full gel every 40 minutes. The smaller dose keeps blood sugar steadier, reduces the volume of sugar hitting the GI system at once, and is easier to chase with water at aid stations.

What to drink on the run

The run introduces a new hydration challenge: unlike the bike, you cannot carry a 750ml bottle and sip continuously. You are fueling at aid stations.

Target 400–600ml per hour depending on conditions
If both sports drink and water are available at aid stations, take sports drink in the first half, water in the second half (to manage total carbohydrate and sugar load)
Don't skip aid stations just because you feel fine — dehydration compounds across the run leg, and by the time you feel thirsty in a race, you are already behind

Late-race real food

In a full Ironman or long 70.3, the run can extend to 3 hours or more. Gel fatigue is real — after 5+ hours of racing, sweet gels become difficult to tolerate. This is where flat cola (available at most Ironman aid stations), pretzels, and broth become genuinely useful. Flat cola provides carbohydrate, caffeine, and sodium in a form the gut typically tolerates well after hours of racing. 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 variables that are specific to your physiology:

Your sweat rate — some athletes lose 1 litre per hour; others lose 2.4 litres. The same ambient temperature will affect these athletes completely differently. A 1-litre-per-hour sweater may not need to drink at every aid station; a 2.4-litre sweater who misses two stations is in serious trouble.
Your sweat sodium concentration — athletes with high sodium loss (visible salt on skin, cramping history) need significantly more sodium than the 500mg/litre baseline. As many as 30% of endurance athletes are "salty sweaters" who need 1,000mg/litre or more to maintain performance.
Your training load — a 70.3 in the middle of high training volume depletes glycogen stores faster coming into race week. Your carbohydrate needs per hour may be slightly higher than an athlete who has tapered fully.
Race conditions — a 70.3 in 28°C humid conditions requires a completely different hydration plan than the same race in 15°C overcast conditions. Your static plan needs a weather adjustment protocol.

This is where personalised wearable data moves from nice-to-have to essential. Your Apple Watch or Garmin captures heart rate, pace, training load, and environmental data across every session. Combined with your sweat rate profile, that data can be used to build a per-discipline hydration and fueling plan that accounts for your body — not a generic 75kg male average.

Sweatr pulls that 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 have actually trained with. Get your free triathlon plan.

A practical framework: race-day execution

Here is the discipline-by-discipline checklist for a 70.3:

Race morning (3 hours before start):

Breakfast: 100–150g carbohydrate, low fat, low fibre
400–600ml water or electrolyte drink over 2 hours
30 minutes before swim: small gel or half banana if needed

Swim (30–40 min):

No fueling possible
Focus: controlled effort, arrive at T1 with energy in reserve

T1:

First bike nutrition within 5 minutes — gel or first sip of nutrition bottle
Don't skip this to save time

Bike (2:30–3:30):

Target: 70–80g carbohydrate/hour
Target: 600–800ml fluid/hour (weather-dependent)
Final 10–15 minutes of bike: small gel to bridge T2 transition
Avoid pushing over your gut-trained maximum

T2:

Let the bike gel digest during transition
Don't take another gel immediately — wait until kilometre 1–2 of the run

Run (1:30–2:30):

Target: 40–60g carbohydrate/hour in smaller, more frequent doses
Target: 400–600ml fluid/hour at aid stations
Accept that run nutrition will feel different to bike nutrition — smaller amounts, lower volume, steady and patient

Post-race:

Within 30 minutes: 50–75g carbohydrate, 20–25g protein
Continue hydrating with sodium for the following 2–4 hours

The biggest mistake you can make

It is not going out too fast. It is 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 practice 70–80g/hr on their long brick sessions, who drink at the same rate they plan to race, who test their 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. No spreadsheets. No generic averages. Your plan, for your race. Try Sweatr free.