Training > Long distance

How much dehydration can you tolerate during an Ironman?

Sports scientist Andy Blow of Precision Hydration discusses how much dehydration athletes can cope with during a long-distance triathlon, without it having a detrimental effect on their tri performance

Not all that long ago, the prevailing opinion in sports science was that you needed to replace 100% of your sweat losses to maintain your performance when exercising. In 1996, the American College of Sports Medicine stated that…


These guidelines were not written specifically with long distance triathletes in mind, but if they’re taken at face value the statement seems to imply that, during an Ironman, some competitors should be aiming to drink as much as 2-3 litres an hour in order to replace 100% of their sweat output. That kind of sweat loss is not uncommon in hotter events.

 That’s a hell of a lot of drinking! In fact, it’s beyond what’s physically possible for most people.

That statement - along with the general “drink, drink, drink” marketing messages coming from the sports drink industry -  has been blamed for driving the worrying increase in cases of hyponatremia (ill health - or even death in extreme cases - caused by the over-consumption of fluid) seen in amateur sports. In fact, a study conducted at Ironman Frankfurt a few years ago found that as many as 10% of finishers had hyponatremia to some extent, which will have (at best) impacted their performance.

Dehydration and sodium: why replacing salt is crucial







Why 2% dehydration and not 3%, 4%, 5%?



For example, some of the original lab studies into dehydration either used periods of induced sweating (saunas or hot baths) or diuretics (drugs that make you pee a lot) to artificially shed body water before exercise tests started, rather than letting dehydration accrue more gradually as it usually does during the course of normal training or racing. This has procedural advantages in terms of how fast and accurately dehydration could be achieved in the subjects taking part, but it’s also artificial.



In the 2000s, a number of sports science researchers cast aside their lab coats and ventured outdoors to collect hydration related data from athletes competing and training in the field.

Several studies looked at bodyweight changes in athletes taking part in endurance races (including Ironman triathlons) to estimate their sweat losses and rates of fluid replenishment and compare that with performance data.


After all, if someone can win a marathon in a world class time having basically dropped 10% bodyweight, how can dehydration credibly be said to be negatively impacting their performance?

Numbers were also crunched during the 2000 and 2001 IM South Africa and the 2004 IM New Zealand races. The majority of finishers are clustered in the 1-6% bodyweight loss area during an Ironman. Whilst that tells us little about their performance compared to their potential, it’s not unreasonable to say that something in that area would be considered an ‘average’ level of loss, for Ironman events (in climates similar to South Africa and New Zealand at the very least).

This research has lead to the consensus in sports science shifting towards the idea that some level of uncorrected fluid loss is acceptable rather than athletes having to aim for a 100% replacement of sweat whilst exercising. That’s especially the case if they’re allowed to respond to the dictates of thirst and to start exercises well hydrated in the first place.

But, defining an exact point at which dehydration starts to negatively effect performance (if one even exists) is still proving elusive.

What’s the relationship between bodyweight and sweat loss?


So, bodyweight loss has almost become synonymous with sweat loss, with 1kg of weight loss being said to equal 1 litre of sweat output.




In an Ironman - where a lot of calories are burned (~7,000 - 11,000 Kcal) - the amount of fuel used up can be a significant confounding factor. So, assuming weight loss equals sweat loss is more problematic.

It’s also worth noting that your body stores around 500g of glycogen in your muscles and liver and that with every gram of glycogen something like 1-3g of water is also stored alongside it. As this glycogen is burned up (and you can burn through all of it in 90 minutes if you’re going at it hard) this process releases something like 0.5 to 1.5L of additional ‘free water’ for your body to re-use internally.

If you do the maths on what all the above could mean during a long event, it could theoretically mean that an athlete could lose 4.5-6% of their bodyweight without seeing a significant change in total body water. In other words, they would maintain a good hydration status despite losing a hell of a lot of weight.

What about pre-exercise bodyweight fluctuations?





So, what level of dehydration is acceptable in an Ironman?

It seems clear that aiming to replace all of your sweat losses during an Ironman is unnecessary.

There are very few examples of athletes losing more than 10% bodyweight after finishing events under their own steam. It’s probably fair to say that something in the ~1-6% range seems to be ‘normal’ for Ironman events, in hot climates at least.


I think it’s clear that all athletes ought to be able to tolerate a measurable degree of dehydration when doing an Ironman, assuming they start properly hydrated in the first place. No-one needs to aim for 100% fluid replacement on the move. In fact, trying to do so could lead to an increased risk of hyponatremia which is bad for your performance and health.


How can I avoid dehydration?




It’s well worth doing some testing in training and racing to get an idea of how much you’re sweating and how salty your sweat is, then you can take this free online Sweat Test (developed by my company, Precision Hydration) to help you understand what kind of fluid and electrolyte intake regime might work for you. This can then be refined through some good ol’ fashioned trial and error in training.


Electrolyte sports drinks: can you overdose?

Hyponatremia: causes and symptoms of low sodium levels

How to plan your hydration


Daily deals from top retailers


Back to the top