Endurance Running and Hydration: How much should I drink?

Introduction

There are many factors involved in determining how much fluid an athlete should drink during an endurance event such as a marathon. These include level of exercise intensity, body size and sweating rate. Although the rate of which an athlete sweats is largely genetically determined, environmental factors such as acclimatisation to heat can also play a part. The athlete also needs to be wary of excessive over hydration to avoid the complication of Exercise Associated Hyponatraemia (see below for details). Replacing 100% of sweat losses is unnecessary and not drinking any fluids is not advocated.

Endurance exercise performance becomes impaired when
dehydration exceeds 2% of body mass.

Examples

The following examples have been created as a guide to help athletes create their own individual fluid intake regime:

1. Elite runner at 2:10 pace in a marathon in warm weather drinking 0.5L/h would be dehydrated >2% with a significant portion of the race remaining. The same athlete ingesting 1L/h would maintain loss at 1.7%
   
2. Recreational runner greater than 50kg at 4:00 pace in a marathon in warm weather drinking 0.5L/h would be dehydrated >2%
   
3. Recreational 50-90kg runner in warm weather drinking 1L/h would produce weight gain
   
4. Recreational 70-90kg runner in cool weather drinking 0.5L/h would maintain hydration
   
5. Recreational 50kg runner in cool weather drinking 0.5L/h would be fluid overloaded
   
6. All runners in cool weather drinking 1L/h would be fluid overloaded
   

Weight

To preserve performance, athletes are advised to drink enough to prevent 2% loss of body mass. Drinking rates should never result in weight gain. Sweating rates can be estimated by weighing nude before and after exercise at race pace under anticipated environmental conditions. Optimal drinking lies in between 2% body mass loss and weight gain.

Exercise Associated Hyponatraemia

A serious consequence of overconsumption of fluid and insufficient weight loss is Exercise Associated Hyponatraemia (EAH). This occurs when an athlete’s excessive fluid intake leads to a low blood sodium level. At its most severe, EAH can lead to brain damage and respiratory problems.

Risk factors for developing this condition include low or high body mass index, lighter pre-race weight, less marathon experience, female gender, NSAID usage and higher heat stress weather conditions.

Most patients with EAH do not have a dramatic clinical presentation. Signs and symptoms can occur at the finish line or up to 6 hours after the race and can include confusion, nausea, vomiting, excessive fatigue, dizziness, lightheadedness, headache, bloating and cramps.

The diagnosis of EAH should be confirmed with a blood test to detect a low blood sodium level. If testing facilities are not available at the race medical station and an athlete’s condition is deteriorating, then they should be transferred immediately to a hospital.

Mild cases of EAH can be treated with water and hypotonic fluid restriction (such as sports drinks) and consumption of salty food. Production of dilute urine is a good indicator that the body is getting rid of excess fluids. More severe cases should be transferred immediately to hospital for intravenous fluid and monitoring.

Summary of recommendations on fluid intake during an endurance event

Athletes should record their pre and post exercise weight with the knowledge that dehydration of 2% body weight loss will impair race performance.

Using the examples above as a guide, athletes should individually tailor their fluid depending on the level of competition (elite vs. recreational), the weight of the athlete and environmental factors (cold vs. warm weather) of the endurance event.

It is neither necessary nor appropriate to drink to match 100% of fluid losses. Not drinking any fluids is also not advocated.

Athletes should be aware that over consumption of fluids can lead to EAH and its serious complications of brain damage and respiratory problems

Ultimately, there is a fine balance between over consumption of fluids causing EAH and under consumption leading to impaired performance. It is the recommendation of this author that the athlete drinks to thirst but also keeps in mind their individually tailored fluid intake based on their weight and race conditions.