How to fuel a triathlon: swim, bike and run

How do you fuel a triathlon? You eat almost nothing in the swim, you do most of your eating on the bike, and you top up with smaller amounts on the run. How much depends on how long you'll be racing: a sprint needs very little, and an Ironman needs as much carbohydrate as your gut can take, for many hours.

That's the short answer. The long answer is where most age-groupers get it wrong, because triathlon fuelling isn't three separate plans. It's one plan for the whole race, and the bike leg has to carry most of it.

Start with the clock, not the distance

Carbs per hour depend mostly on how long you're going. Asker Jeukendrup's 2014 review is still the clearest guide: for efforts of about an hour, small amounts of carbohydrate, or even just rinsing a sports drink around your mouth, can help. For two to three hours, aim for about 60g an hour. For longer, ultra-endurance work, up to about 90g an hour, using a mix of glucose and fructose so your gut can absorb it.

So work out roughly how long your race will take you, then fuel for that time. A fast 70.3 athlete and a first-timer at the same race are doing two very different events.

Treat those as ranges to train with, not targets to hit on the day for the first time.

What real Ironman athletes actually eat

The best real-world data comes from a 2012 study by Beate Pfeiffer and colleagues. They asked 221 endurance athletes what they ate and drank during races, including two Ironmans and a 70.3.

Three things stand out to me:

• Ironman finishers averaged 62 to 71g of carbs an hour. The 70.3 group averaged 65g. Marathon runners in the same study averaged just 35g.

• The range was wild. Individual athletes took anywhere from 6g to 136g an hour. There is no single number everyone hits.

• More carbs went with faster Ironman times. Athletes who ate more carbohydrate finished faster in both Ironman races. They also reported more nausea and wind, which tells you the gut is the limit, not the benefit.

That last point is the whole game. Serious gut problems hit about 31% of the Ironman athletes and 14% of the 70.3 athletes, and the people most likely to suffer were the ones with a history of gut trouble. The answer isn't to eat less. It's to train your gut so you can eat more without it rebelling. Our gut training guide covers how.

The swim: fuel before you get wet

You can't eat in the water, so the swim is fuelled by breakfast. Eat a familiar, high-carb, low-fibre breakfast a few hours before the start, the same one you've eaten before long training days. If there's a long gap between breakfast and the gun, a gel or a few sips of sports drink shortly before the swim is a common way to top up.

Honestly, for a sprint or Olympic, the swim is short enough that most people don't need to overthink this. For an Ironman, you'll be in the water for an hour or more before you can eat again, so starting with full tanks matters.

The bike: this is your kitchen

The bike is where you eat. You're sitting down, your body is fairly still, and you can carry bottles and food. If you're doing a 70.3 or an Ironman, the carbs you take on the bike largely decide how your run goes.

A few rules I'd stick to:

• Start early. Give yourself a few minutes to settle after T1, then start. Don't wait until you feel hungry. By then you're behind.

• Eat little and often. A small, regular drip sits far better than big doses whenever you remember.

• Use drinks and gels for most of it. At race intensity, liquid and gel carbs are easier on the gut than solid food. Some long-course athletes like a bar or real food early on the bike, when the effort is steady. Use whatever you've practised.

• Stop eating a bit before T2. Ease off solid food towards the end of the bike so your stomach isn't full when you start running.

If you're aiming for 90g an hour, you need a mix of glucose and fructose. Glucose alone tops out at about 60g an hour, however well trained your gut is.

The run: smaller amounts, still steady

The run is where under-fuelled triathletes come apart, usually somewhere in the second half of the run. Your gut takes more of a beating on foot, so most people can't keep up their bike intake. Our marathon fuelling guide covers why.

The fix isn't to stop eating. It's to go smaller and steadier. Gels with water at aid stations, sports drink, cola later on if you've practised with it. Keep taking something regularly, not in big catch-up doses. If you've fuelled the bike properly, you're topping up on the run, not trying to catch up.

Drinking: don't overdo it

Fluid needs vary hugely with heat, your size and how much you sweat, so there isn't one right number. Two things are worth knowing.

First, more water isn't always better. Drinking well beyond your needs over many hours can dilute the sodium in your blood, a condition called hyponatremia. It's one of the real medical risks in long-course triathlon. The 2015 international consensus on the condition says to drink when you're thirsty, rather than to a fixed amount or "as much as you can".

Second, salt matters more as races get longer and hotter, especially if you're a heavy or salty sweater. Our sodium guide covers how much, and a sweat rate test tells you your own fluid loss per hour instead of guessing.

Carb loading: long course only

Carb loading earns its place before a 70.3 or an Ironman. For a sprint or Olympic, a normal high-carb day before and a good breakfast is enough. Most people don't need to do more. If you're racing long, our carb loading guide walks through it.

Practise the whole race, not just the bike

The athletes who blow up on fuelling are almost never the ones who got the science wrong. They're the ones who never rehearsed it. Practise your race products, your race amounts and your race timing on long bike rides, and especially in brick sessions, where you run straight off the bike. That's the only way to find out what your gut will put up with when you start running on tired legs.

And nothing new on race day. If the race hands out a drink you've never tried, either train with it or carry your own.

FAQ

Do I need to eat during a sprint triathlon? If your sprint takes about an hour, most people don't. A good breakfast covers that. A few sips of sports drink on the bike won't hurt if you like it.

How many gels do I need for a 70.3? Work it out from your time and your hourly target. At 60 to 90g an hour over five or six hours, you're looking at 300g or more of carbs in total, most of it on the bike. Check the carbs on the label of the gels you've trained with, and count your drink as well.

Can I fuel during the swim in an Ironman? No. That's why breakfast, and a small top-up before the start, matter so much for long course.

Why do I get stomach problems on the run? Usually it's a mix of too much at once, not enough water with your gels, and a gut that hasn't been trained to handle fuel at race effort. Build it up in training, especially in brick sessions.

Sources

• Jeukendrup A. A step towards personalized sports nutrition: carbohydrate intake during exercise. Sports Medicine, 2014.

• Pfeiffer B, et al. Nutritional intake and gastrointestinal problems during competitive endurance events. Medicine and Science in Sports and Exercise, 2012.

• Hew-Butler T, et al. Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference, Carlsbad, California, 2015. British Journal of Sports Medicine, 2015.

In MOVE, every planned session on your Plan tab gets its own carbs per hour, and the sweat test gives you a fluid target for each hour, so you can rehearse your race numbers on your long rides and bricks before race day.