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

The exact amount of carbohydrate, fluid and sodium you as an individual needs 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 much carb?


If you’re racing for less than 1 hour then you don’t really benefit from taking in any carbs at all as 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.

However, as all long-course races will last well in excess of 3 hours they are in the territory where 60-90g of carb per hour is generally accepted to be the zone that most athletes will benefit from consuming, as shown in this diagram:

At the upper end, 90g of carb per hour can sound like a lot and 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.

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

While undoubtedly some athletes will manage to get by on less than 90g carb per hour, and some will require slightly more, the level of carbohydrate needed to race a long-course event successfully is reasonably high for everyone, reasonably consistent and 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?

Fluid and sodium intake can be grouped together, to a degree, as 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 can be applied 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. They’re further influenced by heat and humidity and the intensity you’re able to bike and run at, so a high degree of testing and trial and error is required to dial your own numbers in.

To give you an idea of the ranges involved; 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 on 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 and most people will fall somewhere in between. But they do give you a good idea of the range involved and why simply copying someone else’s hydration plan is definitely not a good idea.

Are you a heavy or light sweater?

To start to figure out if you are a heavier sweater requiring fluid intake levels closer to the top end of the range, or a light sweater needing relatively little, a useful exercise is to measure your sweat rate in training during sessions that best replicate the intensity and conditions you’ll race in.

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 get a good estimate of total sweat losses (if you take into account the volume of anything 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 and you can start to see how much you might expect to lose in a race and begin experimenting with different levels of fluid replacement and feeling how that affects your performance.

It’s important to point out that 1:1 replacement of sweat losses with fluid and sodium consumed is not the goal in training or races as it can risk over-drinking and a dangerous condition called hyponatremia where blood sodium levels are excessively diluted.

Limiting bodyweight loss to somewhere in the region of 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 done (usually at a lab or with an experienced practitioner who has sweat testing equipment).

Additionally, there are a number of wearable sweat sensors just coming onto the market that show a degree of promise at being able to measure your sweat sodium loss in real-time.

As these become more accurate and widely available, it will allow more athletes to gain a better understanding of this aspect of their physiology and how it impacts hydration requirements in races.

If you can’t access a proper sweat test or sensor then there are some clues to look out for that can indicate whether you are likely to lose more or less sodium in your sweat:

• 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.

Top image credit: Andy Lyons/Getty Images