Hydration for Endurance Athletes: A Practical Guide

When we exercise, we sweat — even when sea swimming. Sweating is essentially a form of dehydration, as your body loses both water and electrolytes.

The amount you sweat is highly individual. Heavier athletes, those moving at higher intensities, and those exercising in hot conditions generally sweat more. In contrast, lighter athletes, those exercising at lower intensities, or those training in cooler conditions, tend to sweat less. Of course, many athletes sit somewhere in the middle. For example, a light but fast runner on a mild day may still have significant fluid losses.

There are also differences in sweat composition. Some athletes are naturally "salty sweaters", while others lose relatively little sodium in their sweat. The aim of this guide is to help you make an informed estimate of your hydration needs for training and racing. Better still, you can do it for less than a penny per session — although there are certainly more expensive options available!

Why Hydration Matters

We lose mainly water and salt (sodium) when we sweat, so effective hydration should replace both.

Ironman University suggests that the most common fluid loss during Ironman-intensity exercise is 1-1.5 litres per hour. However, environmental conditions play a huge role.

Athletes sweating less than 1 litre per hour are considered low sweaters, while those losing more than 2 litres per hour are considered high sweaters. Losses above 2.5 litres per hour are exceptionally high.

Importantly, you do not need to replace every drop of fluid lost during exercise. Research suggests that performance can generally be maintained provided body weight losses remain within approximately 2–4% of starting body weight.

However, replacing fluid without replacing sodium can be problematic and, in extreme cases, dangerous. Spending time refining your hydration strategy before race day can make a significant difference to both performance and comfort.

How Much Sodium Do You Need?

Approximately 700–1000mg of sodium per litre of water is suitable for most athletes. As a rough guide, 1000mg of sodium is provided by just under half a level teaspoon of sea salt. If you choose sea salt rather than highly refined table salt, it also contains trace amounts of other minerals such as magnesium, potassium and zinc. While these amounts are small, some athletes prefer sea salt as a more natural option (although slightly higher levels of these minerals are better for maintaining neural signalling and muscle contraction).

At the lower end of the spectrum, some athletes may need only 500mg of sodium per litre. At the upper end, particularly salty sweaters may require as much as 2000mg per litre.

One of the easiest ways to estimate your sodium losses is to look at your clothing and skin after hard training sessions or races lasting longer than an hour.

Which Type of Sweater Are You? (reference Precision Hydration)

Sweating buckets

Drenched in sweat and covered in visible white salt marks after hard exercise.

Recommendation: 2000mg sodium per litre (approximately 1 level teaspoon of sea salt).

Pretty darn sweaty

Very sweaty but with fewer visible salt deposits.

Recommendation: 1500mg sodium per litre (approximately ¾ level teaspoon of sea salt).

Fairly sweaty

Wet all over but not completely drenched.

Recommendation: 1000mg sodium per litre (approximately ½ level teaspoon of sea salt).

A bit sweaty

A light but noticeable sheen of sweat.

Recommendation: 750mg sodium per litre (approximately ⅜ level teaspoon of sea salt).

Not very sweaty

Little visible sweat; others may barely notice you have been exercising.

Recommendation: 500mg sodium per litre (approximately ¼ level teaspoon of sea salt).

It is worth noting that while your sweat rate changes considerably with temperature and fitness, your sweat sodium concentration tends to remain relatively stable. During colder weather, sweat losses are often lower, but the concentration of sodium in sweat may be slightly higher.

Testing Your Sweat Rate

Some products, such as the Gatorade GX Sweat Patch, can provide a more detailed assessment of sweat composition. However, for most athletes, the guidelines above will get you remarkably close.

Once you have an idea of your sodium concentration, the next step is determining how much fluid you need. Because fluid requirements depend heavily on exercise intensity, duration and environmental conditions, the best approach is to perform a simple sweat test.

1. Weigh yourself immediately before training.

2. Complete your session.

3. Record everything you eat and drink during the session.

4. Weigh yourself again immediately afterwards.

As a rule of thumb, 1kg of body weight lost equates to approximately 1 litre of fluid. Don't forget to account for any fluid or food consumed during the session when calculating your true sweat loss.

Use the calculator below to estimate your percentage body weight loss and determine an appropriate rehydration strategy.

The key is consistency. Perform these tests regularly across different conditions and training intensities to build a detailed picture of your personal hydration requirements.

Putting It Into Practice

Drink regularly throughout training and racing, using thirst as your primary guide. Add the appropriate amount of sea salt or an electrolyte product to your fluids, based on your estimated sodium requirements.

As a simple starting point for almost every athlete — male or female, heavy or light, fast or slow — I recommend adding a generous pinch of sea salt to your water bottle and drinking to thirst.

Most importantly, don't allow yourself to become excessively thirsty. By the time you are very thirsty, dehydration is already affecting how you feel and how you perform.

Hydration doesn't need to be complicated. A little experimentation in training can help you arrive at a strategy that is simple, inexpensive and highly effective on race day.