Hydration For Runners: Everything Triathletes Need To Know

Proper hydration plays a crucial role in our daily lives and in our sports performance. Improper hydration can lead to both dehydration or hyponatremia resulting in things like dizziness, fatigue, and poor running performance.

In this article, we shall learn more about the nitty gritties of proper hydration, especially for runners and regular exercisers.

How Much Should You Drink During Exercise?

The goal of hydrating during workouts or races is to prevent dehydration which would negatively impact your abilities or state. Having said that, some small level of dehydration is normal during prolonged workouts; you can rehydrate well after your run.

With this in mind, the current research for performing activities where you sweat a lot is to just drink when you begin to feel thirsty.   

This has also been described as ‘drinking a sufficient amount to prevent thirst’ in the study that concerned it.

Running and Hydration – Here’s The Rule of Thumb

Increase Your Daily Water Intake - The Compounding Pharmacy of America

If you’re looking for an actual quantity of water so you can plan your hydration needs in advance, here’s the average consumption:

In general, it’s recommended that athletes aim to drink 0.4-0.8 litres per hour (L/h) or 8-16 ounces per hour (oz/h).

How To Calculate Your Sweat Rate

But since your sweat rate is highly individual, you may wish to check your own sweat rate and fluid loss during running workouts or races to customize to your own needs.

One way to check how much fluid you’re losing during workouts is to weigh yourself before your workout, complete your workout (without urinating at all during the length of your workout), and then weigh again at the end of your workout to see if you have lost or maintained your weight.

If you have maintained your weight during the workout and your urine colour is not dark or concentrated, then, as a rule of thumb, you are drinking enough fluids.

If you have lost weight during your workout and your urine colour is darker, you need to increase your fluid intake during your running workout or race.

Here is a handy formula:

  • Sweat rate (ml/hr) = weight before exercise (g) – weight after exercise (g) + amount of fluid consumed (ml) – the amount of fluid urinated (if applicable)/minutes of activity x 60
  • This formula will give you an ml/h number that will show you how many ml of liquid per hour of activity you’ll need to plan for during your training runs and races and recovery.
  • If you are calculating this for a race, try to replicate conditions so you can estimate your fluid needs for the temperature and humidity you’ll likely experience.

It’s important to remember that some dehydration is normal during prolonged exercise, and attempting to replace your exact fluid quantity during exercise can be counterproductive and lead to stomach slosh: follow the sip rule outlined above and rehydrate the rest when you cross the finish line.

Another way is to calculate that 1 millilitre of sweat loss roughly represents a gram loss in body weight. So if you lost 0.25 pounds (113 grams) of weight during your workout, then you’d need to increase your fluid intake by 113 ml (about 4 fluid ounces) next time you work out under similar conditions.

The goal is to weigh the same after your workouts since any rapid weight loss is attributed to water and to prevent dehydration, you want a 1:1 replacement of fluid lost (including fluids you’ve consumed after your workout).

Once you’re done with your running workout, your goal is to replace any lost fluids from your workout (as well as electrolytes if needed), and then move into your regular daily hydration maintenance.

Risks Of Poor Hydration

Making sure that you pay attention to your hydration is not only important for optimal athletic performance, but it’s crucial to your health and well-being!


Signs and Symptoms of Dehydration - Severe Dehydration Treatment

Most people aren’t going to get dehydrated to the point of serious health consequences or death, but many people do experience dehydration symptoms like lethargy, muscle cramping, dizziness, confusion, etc.

Mild dehydration (1%-2% loss of body weight during exercise) is normal during exercise, especially during endurance events like marathons. But your goal should be to manage mild dehydration and keep it from becoming severe dehydration that can result in serious complications like seizures.

Other symptoms of dehydration include:

  • lightheadedness
  • thirst
  • fatigue and lack of energy/enthusiasm
  • lack of need to use the bathroom
  • dry mouth
  • constipation


One aspect of hydration that people don’t often address is hyponatremia, which is over-hydrating (the opposite of dehydration).

Hyponatremia is defined as an abnormally low concentration of blood sodium and it can occur when someone drinks too many low-sodium fluids over long periods compared to their sweat rate.

The greatest predictor of hyponatremia is an increase in weight during the duration of a workout, meaning you have consumed an excess of fluids.

Runners who are exercising at low to moderate intensity for long periods and who have a low body mass index are more susceptible to hyponatremia, while high-intensity exercisers are more likely to suffer from dehydration.

To prevent this, make sure that you are consuming moderate amounts of liquids for your needs during exercise, and you are also including sodium and electrolytes in your hydration routine when you’re running for longer periods (over 1-2 hours).

Making Sure You Start Your Race Hydrated

Importance of pre-hydration / Science in Sport Blog | Science in Sport

When people talk about hydration, most of the time it’s about what and how much athletes should drink during exercise.

This is clearly important, but your performance is also massively influenced by how hydrated you are when you start exercising in the first place. Drinking a strong electrolyte drink to optimize your hydration status before long, hot or really hard training sessions and events can significantly improve your performance.

This practice of “preloading”  has been widely studied in the last 20 years or so, both with astronauts and athletes.

While there’s not a completely bulletproof consensus on the subject there’s strong evidence that taking in additional sodium with fluids before you start sweating is effective in promoting increased acute fluid retention and in improving endurance performance—especially in the heat.

This article aims to give you a more solid understanding of what you can do to arrive at the start of your next event optimally hydrated.

Once you start sweating, you’re fighting a losing battle

Once you begin sweating you’re generally going to be fighting a losing battle against fluid and electrolyte loss, so starting off properly hydrated can be extremely beneficial. When you’re properly hydrated you have a larger reservoir of fluid to draw from over time than if you’re dehydrated.

Starting well hydrated has other benefits too. Optimal hydration maximizes your blood volume and this helps general cardiovascular function and your ability to dissipate the heat produced by your working muscles. This reduces fatigue and enables you to maintain your performance for longer.

Despite the relatively obvious benefits of starting exercise well hydrated, a recent study of more than 400 amateur athletes showed that around 31 per cent of them were turning up to training sessions (and, in some cases, competitions) dehydrated!

Among the data, there were strong indications that this was very likely to be compromising their performance. It’s certainly not uncommon to see people only really thinking about hydration once they turn up to a session rather than preparing in advance.

But it can be a problem for full-time athletes when training two or more times a day, or at times when they’re just under a very high total training load. That’s because uncorrected dehydration from a prior training session can make its presence felt when the next session gets underway.

We tend to overcompensate before the big day which severely impacts performance

Although athletes turning up to training a bit low on fluids is relatively common, it’s generally less of an issue before major competitions. In big events, there’s a tendency to increase fluid intake before the big day because extra priority is placed on all aspects of last-minute preparation.

The irony of this extra emphasis on pre-event hydration is that quite a lot of athletes can go from slightly under-drinking before training to significantly over-drinking pre-competition and this can lead to a different set of problems including hyponatremia (low blood sodium levels caused by inadequately replacing the sodium lost when sweating and further dilution by drinking plain water or weak sports drinks), something that can be pretty catastrophic for health and performance if it goes unchecked.

A recent study found that 10 per cent of athletes tested at the IRONMAN European Championships had hyponatremia, which shows you the extent to which hydration issues might be impacting performance.

What can athletes learn from astronauts?

NASA’s astronauts were commonly found to be suffering from low blood pressure because they were losing bodily fluids (and therefore blood volume) during their time in microgravity.

One NASA paper suggests that astronauts live with as much as a 3 to 4 per cent deficit in total body fluid levels during a typical mission. It was causing them to feel weak, lightheaded and even to black out on re-entry or once they landed back on terra firma. 

NASA then tested lots of drinks containing different carbohydrates and electrolyte mixtures and found that the more sodium you put in a drink, the more effective the drink would be at being retained in the body and bloodstream and correcting dehydration.

So, how do you “preload” effectively?

It’s about striking a balance between being aggressive enough to drive some extra fluid retention in your bloodstream without this leading to gastrointestinal issues or excessive fluid build-up making you feel bloated and sluggish.

Typical sports drinks are way too diluted to make a meaningful difference in blood volume. The reality is it’s not vastly different from drinking water.

At the other extreme, most of the scientific studies that have been conducted in this area have looked at using extremely strong electrolyte drinks containing ~3,600mg of sodium per litre.

A strength of 1,500mg of sodium per litre (32oz) seemed to be the “sweet spot” in that it’s very palatable and easy on the gut, while still being effective at boosting your blood plasma volumes and getting you optimally hydrated before your start sweating.

If you want to test whether preloading improves your performance, follow these recommendations before your next long/intense training session or “B” race:

What to do

  • Drink a strong electrolyte drink (like PH 1500) with 500ml/16oz of water the evening before your activity.
  • Drink a strong electrolyte drink (like PH 1500) with 500ml/16oz of water about 90 minutes before you start. Finish your drink at least 45 minutes before you start to give your body time to fully absorb what it needs and pee out any excess.
  • Drink the strong electrolyte drink in the water you’d have drunk anyway to ensure you don’t overdo it.
  • Don’t just drink lots of water in the build-up to a race. You can end up diluting your body’s sodium levels before you start, increasing the risk of hyponatremia.


  • Boosting your blood plasma volume before intense exercise is a proven way to enhance your performance, especially in hot conditions.
  • Having more blood makes it easier for your cardiovascular system to meet the competing demands of cooling you down and delivering oxygen to your muscles.
  • Stronger electrolyte drinks are very effective at increasing your plasma volume as they contain more sodium than a typical sports drink. That extra sodium helps to pull water into your bloodstream and keep it there.
  • Preloading may allow you to get away with drinking considerably less in shorter/harder events where previously they would have had to try to consume more on the move. It can also help reduce the number of times you need to pee before you start.
  • You can’t preload anywhere near as effectively with weaker sports drinks as you’ll lose a large proportion of the fluid as urine. Or it’ll slosh around in your stomach without being properly absorbed.
  • Drinking a stronger electrolyte drink before you start can also help you avoid/alleviate muscle cramps, especially if you’re prone to suffering from them late on in events and especially when it’s hot.

How to Prehydrate Between Intense Workouts and Races

You should drink fluids four hours before working out or participating in a race if you have been doing intense physical activity recently.

For pre-hydration, consume 5-7 ml of fluid per 1 kg of body weight slowly.

This is approximately the same as a large glass of water for an average person who weighs 75kg.

If you are concerned about hydration during your workout or event, you can add some sodium-containing snacks or fluids to your pre-hydration routine to help with water retention.

If you have had more than 12 hours since your last workout, or you are already hydrated, you do not always need to prehydrate. It’s always a good idea to check your hydration levels before going for a run or race!


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