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

Credit: Precision Hydration

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…


“During exercise, athletes should start drinking early and at regular intervals in an attempt to consume fluids at a rate sufficient to replace all the water lost through sweating (i.e. body weight loss), or consume the maximal amount that can be tolerated.”.

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.

So, this advice soon came under fire from a lot of people in the sports medicine community and significant evidence contradicting it was unearthed. As a result the ACSM revised it’s guideline in 2007 to include the more conservative (if slightly vague) statement that…

“The goal of drinking during exercise is to prevent excessive (>2% body weight loss from water deficit) dehydration and excessive changes in electrolyte balance to avert compromised performance.”

Again, although the revised guidelines were undoubtedly more conservative and accept that 100% like for like replacement of sweat losses is not necessary during exercise, it still suggests that only dehydration of up to 2% (defined by body weight loss) is ‘acceptable’.

The suggestion seems to be that dehydration beyond a 2% bodyweight loss must be detrimental to your performance (or health), meaning that in practice athletes ought to try to get pretty close to balancing ‘sweat out’ with ‘drinks in’ when training and competing.

Just to calibrate your thinking on this, 2% dehydration defined by bodyweight loss would amount to an approximate 1.4L (100oz) drop in mass for a 70kg (154lb) athlete.”

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

There are a number of older lab-based studies in the scientific literature that do seem to point to an approximately 2% level of dehydration (by body weight loss) being detrimental to exercise performance and these are presumably what the 2007 ACSM guidelines are based on.

The issue is that quite a few of these studies have been criticised as being unrepresentative of what happens in the ‘real world’.

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.

Few athletes would deliberately try to do a hard training session or race having deliberately dehydrated themselves immediately before it! In the real world, fluid losses usually tend to build up over time once athletes have started exercising in a well-hydrated state.

Although these criticisms of the early research are valid, this doesn’t mean there’s nothing to be learned from it. What they do show pretty conclusively is that uncorrected dehydration can massively impact physical performance in certain circumstances.

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.

One of the most famous studies managed to obtain pre and post race bodyweight data from legendary distance runner Haile Gebrselassie. It demonstrated that he dropped a staggering 9.8% of his mass during the 2009 Dubai marathon – a race that he won in 2:05:29. This example in particular has often been used to attack the ‘2% dehydration impairs performance’ theory head on.

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?

Changes in bodyweight have long been used as a measure for estimating sweat losses in athletes. That’s because fluid loss through sweating accounts for the majority of any change in body mass during exercise. Body weight is also really easy to measure with a set of scales, certainly easier than running behind athletes collecting all the sweat that drips off them anyway!

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

But some scientists have cast doubt over this assumption, when it comes to longer duration exercise at least.

The problem with assuming that essentially all weight loss during exercise is fluid lost via sweating is that it fails to take into account the fact that you’re also burning through a lot of stored fuel when you’re working hard.

You burn carbs, fats and some protein during exercise. The by-products of these biological processes are CO2 (that you breath out and so you become lighter) and water which is released and becomes available to the total pool of fluid in your body, so it can be assumed to offset some of what you lose in your sweat.

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?

In my own experience of preparing for an Ironman (or other long events) I would tend to taper my training aggressively in the final few days, eat more food (especially carbs to top up my glycogen stores) and take on more sodium to help retain fluid. This would result in a significant weight gain in the final few days before my event. I’m also pretty sure I’m not alone in doing this either.

The result of this resting and feeding would typically leave me standing on the start line for a long distance event 1.5-2kg heavier than my ‘normal’ day to day weight. I always took this as a positive sign that I was sufficiently fortified for the long battle ahead and in numerical terms it would have meant that I was 2.2-2.8% heavier than usual.

Exactly how much of that additional weight was water (versus extra glycogen and body fat) would be hard to speculate, but even it if was 50% then this would presumably have a sizeable impact on the amount of sweat I could lose before becoming noticeably dehydrated. And what about training, where I’m unlikely to have followed the same pre-session ritual?

I actually think that this might be a factor in case studies like the 9.8% bodyweight drop seen in Gebrselassie in his Dubai marathon win. Most marathon runners do a considerable amount of carb loading pre-race (as do triathletes) and so him losing so much mass on the day could well have been partly influenced by being somewhat above ‘normal’ bodyweight at the start of the event.

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.

There is also highly likely to be a great deal of difference in the level of dehydration one athlete can tolerate to the next. Can Gebrselassie tolerate 10% bodyweight loss during a marathon because he’s adapted to that over years of training and competition at the highest level? Or has he got an innate ability to cope with high levels of sweat loss and is this something that contributes to him being a world class runner?

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.

The sweet spot is likely to be highly individual and may even be slightly different for the same athlete on different days and in different climates. But, it probably falls somewhere between 2-4% bodyweight loss for most people.

How can I avoid dehydration?

It’s definitely worth thinking about the role of sodium in retaining fluid in your quest to maintain your hydration status. Sodium helps you retain fluid in your bloodstream, useful when your sweat output is high.

Studies such as this recent one by researchers in Spain looked at fluid and sodium intake during a hot half-Ironman event in 2015. Their data showed that athletes who took in more sodium tended to lose less bodyweight in percentage terms than a control group who only took a placebo.

The group that took the sodium in (and so maintained a better hydration status) outperformed those taking the placebo by an average of 26 minutes in the race. There seemed to be a clear and positive correlation between increased performance and losing less bodyweight overall by using sodium supplementation to absorb and retain more fluid.

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.

Andy Blow has a few top 10 Ironman and 70.3 finishes and an Xterra World Age Group title to his name. He founded Precision Hydration to help athletes solve their hydration issues. He has a degree in Sport and Exercise Science and was once the Team Sports Scientist for Benetton and Renault F1 teams.

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