How much carbohydrate, fluid and sodium do I need per hour for an Ironman?

How much carbohydrate, fluid and sodium you require per hour during an Ironman to optimise race performance depends on a number of factors.

These include: your expected pace and total race time, the weather conditions, your sweat rate and level of sodium loss, your level of practise and tolerance for eating and drinking on the move, and, to some extent, your body size.

As these factors move around so will your need for carbs, fluid and sodium, but what follows here is a guide for how to get started by getting these numbers in roughly the right ballpark.

How many carbohydrates do I need per hour during an Ironman?


If you’re racing for less than 1 hour then you don’t really benefit from taking in any carbs at all. You can rely on stored energy for that duration.

For events up to 3 hours, even if you’re pushing hard, the evidence suggests that working up to about 60g carb per hour is sufficient to maintain performance.

All long-course races will last well in excess of 3 hours. Therefore, they are in the territory where 60-90g of carb per hour is generally accepted to be beneficial as shown in this diagram:

At the upper end, 90g of carb per hour can sound like a lot. Working up to taking in that amount often requires commitment to training your gut to tolerate it. This means practising race-level intake of gels, chews and sports drinks in key training sessions.

There is an emerging body of evidence that this process is worthwhile, resulting in better performances with less stomach issues on race day.

How high can you go?

Many pro and elite athletes have even experimented with higher intakes (around 120g/h) in long races with great success in recent years. This shows how far this aspect of performance can be pushed.

Some athletes will manage to get by on less than 90g carb per hour, and some will require slightly more. But the level of carbohydrate needed to race a long-course event successfully is reasonably high for everyone. It doesn’t tend to vary much with weather conditions or other factors.

As such, whatever you find works for you in a cool race will be a good indication of what would also work in the heat and vice-versa. The same cannot be said for fluid and sodium intake, though…

How much fluid and sodium do I need per hour during an Ironman?

We can group fluid and sodium intake together, to a degree. They are intrinsically linked in the body and are lost together in sweat. The more you sweat the more fluid, and consequently more sodium, you’ll need as well.

Unlike with carb intake there is no handy diagram or universal ‘rule of thumb’ that we can apply to the question of how much to drink (and how much sodium to take with your fluids).

This is because sweat rates and sweat sodium losses vary so dramatically from person to person. Heat and humidity and the intensity you’re able to bike and run are also influential. So you need a high degree of testing and trial and error to dial your own numbers in.

The Precision Fuel & Hydration online database of case studies from hundreds of long-course triathlons contains examples of athletes with lower sweat rates doing long-course races in cool conditions. Some consumed as little as 265ml of fluid per hour, along with negligible amounts of sodium.

At the other extreme there are examples of athletes consuming 1.7 litres of fluid per hour with a relative sodium concentration of 1,377mg/L!

Clearly, both of these examples are on the extremes of the continuum. Most people will fall somewhere in between. But they do give you a good idea of the range involved. They show why simply copying someone else’s hydration plan is definitely not a good idea.

Are you a heavy or light sweater?

To figure out if you are a heavier sweater or a light sweater, measure your sweat rate in training during sessions that replicate race conditions and intensity.

You can do this by weighing yourself before and after a session and seeing how much weight you lose. One litre of sweat weighs almost exactly 1kg. So you can estimate total sweat losses (if you take into account what you drink in the session) and convert that into an approximate hourly sweat rate.

Do that a handful of times for bike and run sessions. Then you can start to see how much you might expect to lose in a race. Begin experimenting with different levels of fluid replacement and see how that affects your performance.

1:1 replacement of sweat losses with fluid and sodium consumed is not the goal in training or races. It can risk over-drinking and a dangerous condition called hyponatremia where blood sodium levels become excessively dilute.

Limiting bodyweight loss to between 2-4% tends to be about right for the majority of athletes doing longer sessions or in long races.

What is your individual sweat sodium concentration?

When it comes to sodium replacement, understanding your individual sweat sodium concentration is a very useful data point for long-course athletes, especially those racing in heat and humidity.

You can get an accurate picture of this by having a sweat test. This is usually done at a lab or with an experienced practitioner who has sweat testing equipment.

Additionally, a number of wearable sweat sensors show a degree of promise at being able to measure your sweat sodium loss in real-time.

As these become more accurate and available, they will allow more athletes to better understand how much to drink in racing.

Signs that you’re a heavy sweater

Without a proper sweat test or sensor look out for these clues that can indicate your sweat rate.

• Seeing large salt deposits on your training kit after training in the heat
• Suffering frequent muscle cramps during or post exercise
• Having sweat that really stings or burns when it gets in your eyes
• Struggling with low blood pressure post-exercise
• Craving for salt after training sessions or races

These are all signs that you’re likely to be a heavier and saltier sweater.

If you tick more than a couple of these boxes, experimenting with higher levels of sodium replacement in training with a view to increasing intake in races is definitely a good idea.

To give you an idea of the typical range of sodium intake seen in long-course races, more data from the Precision Fuel & Hydration database suggests that some athletes can complete an event on as little as 112mg/L of sodium in their drinks whereas others need as much as 2,909mg/L.

Failure to prepare for the nutritional demands of a long-course event is preparing to fail on race day.

So starting to work out what you think you’ll need in terms of carb, fluid and sodium, then testing and adjusting this in key training sessions in the build-up to your next race is an investment that will pay huge dividends when the big day arrives.