Half-distance triathlon fuelling: carbs, fluid and sodium per hour

How many carbs per hour does a half-distance triathlon need?

For exercise beyond about 2.5 hours the sourced ceiling is up to 90 g of carbohydrate an hour, and 30 to 60 g an hour is the standard range below that. This guide plans for a 5 to 6 hour finish, which puts you in the top band, as long as your gut is ready for it.

The advice does not depend on body weight, and it is based mostly on trained athletes, so newer athletes may need to go lower (Jeukendrup, 2014). In the observational study, a higher carbohydrate intake went with better performance in IRONMAN® races and also with more nausea and flatulence, and intakes ranged from 6 to 136 g an hour. That spread is why your own testing matters more than any average.

How much carbohydrate on the bike, and how much on the run?

Take most of your carbohydrate on the bike, 60 to 80 g an hour, up to 90 g if you have trained for it. On the run drop to 40 to 60 g an hour. The bike is steadier, and you can bank fuel there that you could not comfortably take on the run. These splits are our recommendation.

For the full race that adds up to about 280 to 355 g of carbohydrate, our arithmetic from the plan below. Your plan also depends on your pace and on the aid stations, so read the athlete guide for your race and carry what you cannot count on finding. For how the race fits a wider plan, see the half-distance triathlon plan.

Which carbohydrates should you use: glucose, fructose, gels or food?

Use a mix of glucose (or maltodextrin) and fructose once you aim above about 60 g an hour. A single carbohydrate source is absorbed at up to about 60 g an hour, and a blend lets you use more. Drinks, gels and low-fibre solid food all work, so pick what you tolerate and can carry.

• A single source can be used at up to about 60 g an hour. Above that, the carbohydrate must be a multiple transportable blend (Jeukendrup, 2014).
• Studies used maltodextrin or glucose with fructose in a 2 to 1 ratio. Read the label of your drink mix and gels for both sugars.
• The performance benefit of a blend has mostly been seen in studies of 2.5 hours or more, and it starts to show in the third hour of exercise.
• Form is flexible: liquid, semi-solid or solid, as long as solids are low in fat, protein and fibre. Our recommendation: drink mix in the bottles, gels as the main top-up, and a little solid food in the first bike hour only.

How much should you drink in a half-distance triathlon?

Drink to thirst, which for most athletes comes to about 500 to 900 ml an hour. Sweat rates differ a lot, so use a long training session to find yours. The aim is to limit losses without drinking beyond thirst, which is the bigger medical risk in long events.

• In a study of 94 IRONMAN® finishers told to drink only to thirst, median intake was 900 ml an hour, and only 1 of 94 finished with low blood sodium (Frontiers in Nutrition, 2021).
• Aim to keep body-mass loss from sweating to 2 to 3 percent (review for triathlon, 2011). Weigh yourself before and after a long session to estimate your own rate.
• Overdrinking beyond thirst is the main cause of exercise-associated low blood sodium (review, 2017). Do not drink on a timer when you are not thirsty.
• Typical sweat rates range from about 0.5 to 2 litres an hour, with rare cases above 3 (review, 2017). Hot days push it up.

Our recommendation: 500 to 900 ml an hour on the bike, 400 to 700 ml on the run, and more only if you are thirsty and it is hot.

How much sodium do you need?

There is no proven universal number. As a starting point we use 300 to 600 mg of sodium an hour on the bike and run, from your drink mix and food. Go higher only if you know you sweat heavily or the day is hot, and treat anything higher as something to test in training.

• Whole-body sweat sodium ranges from about 10 to 70 mmol/L (Sports Medicine, 2017). At 23 mg per mmol, that is about 230 to 1,600 mg per litre of sweat (our conversion).
• Measured sodium loss ranges from 0.2 to 7.3 g an hour. Replacement should be individual, and no evidence shows that athletes gain from replacing more than they lose (NATA position statement).
• A systematic review of five studies tested 280 to 900 mg an hour. Only one found a benefit, at 504 mg an hour, so the evidence for performance is thin (review).
• Our recommendation sits inside that tested range. Take the low end if you sweat lightly, the high end if you sweat heavily, and always with fluid you drink to thirst.

What does an hour-by-hour plan look like for a 5 to 6 hour race?

Below is a plan for a 5 hour 30 minute finish: a 45 minute swim and transition, a 3 hour bike and a run of about 1 hour 45 minutes. It is our recommendation, built from the ranges above. Scale the hours to your own splits and the grams to your own tolerance.

Over the race that is about 280 to 355 g of carbohydrate, our arithmetic from the rows. Late in the run, one study of a low caffeine dose taken toward the end of a race, for example from flat cola, found effects comparable with higher doses taken an hour earlier (ISSN caffeine position stand). The studied range is 3 to 6 mg/kg, so test any caffeine in training first.

How do you train your gut for race-day numbers?

Raise your intake gradually in long sessions, using the exact products you will race with. Gut complaints are common, and nutritional training and sensible choices can reduce the risk. We keep this short here, because a dedicated guide on gut training is a topic for later.

1. Start from what you already take in your long rides and runs. Add about 10 g an hour at a time over several weeks, our rule of thumb, until you reach your race target.
2. Practise on the bike first, then in a brick workout so the run-off-the-bike intake is rehearsed too.
3. Use the same drink mix, gels and bottles you will race with, at race intensity, and keep a short note of what you took and how you felt.
4. If a product causes trouble, change one thing at a time and retest. History matters: athletes with past stomach problems reported more of them in races (2012 study).

What are the most common fuelling mistakes in a half distance?

Most mistakes come from starting too late, trying something new on race day or drinking by the clock. They are easy to avoid with a plan, a rehearsal and one checklist. These are our observations, so use them as a prompt rather than as findings.

• Starting fuel late. Waiting until you feel hungry leaves you behind on carbohydrate before the run even begins.
• Saving fuel for the run. Taking little on the bike and a lot on the run is the wrong way round for most stomachs.
• Drinking more than thirst asks for. This is the main route to low blood sodium.
• Using a new product or a higher dose on race day. Test everything in training first.
• Forgetting the logistics: where gels are carried, how bottles are refilled and what your race's aid stations actually serve.