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How Does Dehydration Affect Triathlon Performance?


What difference does dehydration make to your triathlon performance? We checked in with Precision Hydration founder Andy Blow to find out.

Dehydration is a common issue for triathletes and one that’s important to consider when putting together a hydration and nutrition plan for longer training sessions or races.

We asked the experts at Precision Hydration to find out more about why performance drops when you get dehydrated and how to make sure you don’t end up there in the first place.

What Is Dehydration?

The path to dehydration begins when our bodies sweat in an attempt to regulate core temperature, the liquid evaporating on the surface of the skin to cool us down.

Sweating is therefore an essential function that allows us to exercise for prolonged periods without overheating. But the longer we train and race, the more fluid we lose.

When too much is lost, we enter a state of dehydration where there’s insufficient fluid in the body to perform at our potential, limiting performance.

Why Does Dehydration Impact Triathlon Performance?

Andy explains what goes on in the body as we get dehydrated: “Dehydration can, beyond a certain point, cause massive performance issues primarily because it manifests itself in reduced blood volume and increased blood viscosity (thickness), both of which impair cardiovascular function and heat dissipation.”

This means the heart can’t pump as much blood to working muscles or to the skin to cool you down, making it easy to see why performance would drop as a consequence.

 “The negative effects of dehydration are especially relevant in events that are very long and hot (such as Ironman races and ultra marathons) and for athletes who are training hard and sweating a lot on back-to-back days. That’s because, in these scenarios, the volume of sweat and sodium losses can be quite dramatic.”

As well as impaired cardiovascular function and compromised cooling, there several other dehydration symptoms that can put a serious blocker on your triathlon performance.

Running In The Sun And Heat
Long runs or races in the heat can cause serious dehydration issues unless you stay on top of your fluid and electrolyte intake. (Photo: Isaac Wendland – Unsplash)

These include dizziness, light headedness, weakness, blurred vision and headaches. If you become overheated due to dehydration, you can even get chills even in the heat, causing uncomfortable goose bumps and increased skin sensitivity.

On top of this, losing too much sodium through sweating can cause its own issues to overcome.

“As well as maintaining fluid balance, sodium plays an important role in the absorption of nutrients in the gut, maintaining cognitive function, nerve impulse transmission and in muscle-contraction.”

This means that without enough sodium, your calorie intake is likely to be compromised, it could be hard to think straight and cramping could come into play, so you certainly wouldn’t be racing to your potential.

At What Point Does Dehydration Affect Performance?

With all these possible issues, it’s clear that any athlete wanting to race strongly should avoid getting dehydrated, but the point at which performance takes a hit is difficult to define.

“The generic advice suggests that if you get more than 2% dehydrated, then your performance will start to deteriorate. That advice is based on older, lab-based studies, and more recent research would suggest some athletes can tolerate a higher amount of dehydration, with Ironman finishers reporting a bodyweight change of around 1-6%.”

Ironman finish line exhaustion
By the end of an Ironman, hard effort and dehydration can mean significant body weight decreases. (Photo: Charlie Crowhurst/Getty Images for Ironman)

“It’s worth noting that not all bodyweight loss can be assumed to be sweat/fluid as you’ll also be burning carbs, fats and some protein during exercise.”

All this is backed up by a 2013 study on Ironman athletes, which found an average weight decrease of 1.9kg due to several indicators for dehydration as well as glycogen loss.

“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% body weight loss for most athletes.

“Most athletes should be able to tolerate a measurable degree of dehydration when doing longer endurance events, assuming they start properly hydrated in the first place.”

Beware Over-Replacement Of Fluids

While it’s good to have that 2-4% of bodyweight guidance, it’s hard to measure when you’re out on a long ride or pounding your way through an Ironman marathon. But stressing about replacing lost fluid can lead to over-drinking, which could be even more detrimental to your race.

“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.”

Handing out water in Ironman event
Drinking cup after cup of water to try and stay hydrated could do more harm than good. (Photo: Tom Pennington/Getting Images for Ironman)

Indeed hyponatremia – dilution of sodium levels due to excessive drinking – is a much more serious condition than dehydration and one that’s caused several deaths in endurance events such as marathons over the years, where advice in the past has been to drink as much as you possibly can.

The Importance Of Sodium

Not drinking enough can lead to performance-limiting dehydration, however drinking too much water alone can cause hyponatremia, so how do you hit the sweet spot in between?

The answer is to stay on top of your sodium. This major electrolyte increases fluid retention, helping you stay hydrated while taking on board more avoids over-dilution or losing too much from sweat.

“Sodium is the electrolyte we all lose in the greatest proportion in our sweat and, due to this and its importance in fluid balance in the body, should always be the focus of electrolyte replacement during prolonged periods of sweating.”

So put simply, more sodium means more fluid in the body, less sodium mean less fluid.

“It will be important to use electrolytes during particularly intense training sessions or those where your sweat losses are likely to be high and you need to replace the sodium you’re losing in that sweat.

“Ultimately, as with anything you’re planning to eat or drink during a race, it’s important to ‘practice what you race’ in training.”

About Precision Hydration

Precision Hydration provides triathletes with personalised hydration plans using different strength electrolytes to match how you sweat.

Triathletes all over the world trust PH products to stay hydrated with pro athletes including Sarah Crowley, Carrie Lester and Emma Pallant using PH to perform at their best.

Find which strength of electrolytes is right for you and receive a personalised hydration plan by taking PH’s online Sweat Test.

Triathlon Vibe
Triathlon Vibe
Triathlon Vibe is the home of triathlon training advice for beginner to expert triathletes. From sprint to Ironman, we share how to swim, bike and run stronger and faster.


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