Hydration During the Triathlon: 2009 Update

author : AMSSM

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How best to hydrate during an endurance event like a triathlon is currently the topic of considerable discussion and research. Here is what some of the latest research shows.

The importance of water to the function of the human body cannot be overstated. Water provides an environment for nearly all of the chemical reactions in our bodies and serves other vital purposes, such as supporting circulation and heat transfer. An understanding of the body’s response to exercise is important when discussing how to properly hydrate for a triathlon.

When we participate in endurance exercise such as the triathlon, we sweat to dissipate excess heat that comes from the work our bodies are doing. Sweating and other physiologic processes cause weight loss. Many people equate this weight loss with dehydration that is harmful to performance – this is a myth. A 2006 study from the Hawaii Ironman showed that athletes lost on average 3% of their body weight and still exercised at 80% of their maximum heart rate for ten hours without a significant increase in their core temperature.¹ This shows real world athletes losing weight during competition and not having their performance suffer. Furthermore, weight loss during exercise has been shown to protect against the development² and the dangerous effects³ of low blood sodium.

How best to hydrate during an endurance event like a triathlon is currently the topic of considerable discussion and research. Previous guidelines that recommended to drink a certain volume of fluid each hour, or to ‘drink as much as tolerable’ were based on the results of laboratory experiments⁴. These have proven, during actual events, to produce uncomfortable gastrointestinal symptoms or worse, dangerously low levels of blood sodium (hyponatremia). As shown above, studies of endurance athletes done during actual events have produced a far simpler recommendation – drink when you are thirsty. The body’s thirst mechanism is finely tuned to maintain an appropriate level of hydration.
 

Fluid replacement recommendations:

1. Drink when you are thirsty. This strategy works for every other mammal in the world that must stay active to stay alive. It also works for athletes when they exercise.
   
2. Do not attempt to replace 100% of fluid losses while exercising. Only a percentage of weight lost during endurance exercise is due to sweating, the rest is lost due to the burning of fuel used for activity. Replacing this much fluid overestimates your losses.
   
3. Weight loss is expected during endurance exercise. This does not increase your core temperature nor hampers your performance. Mild weight loss protects against hyponatremia and its dangerous side effects.
   
4. Hyponatremia is a result of overdrinking. Water and sports drinks both contribute to lowering blood sodium if enough are consumed. Sports drinks may be a good source of carbohydrates during endurance exercise greater than one hour. They DO NOT protect against hyponatremia.

Bibliography

1. Laursen, PB, et al. Core temperature and hydration status during an Ironman triathlon. Br J Sports Med, 2006;40: 320-325.

2. American College of Sports Medicine. Position Stand: Exercise and fluid replacement. Med Sci Sports Exerc. 2007;39: 377-390.

3. Almond CS, Shin AY, Fortescue EB et al. Hyponatremia among runners in the Boston Marathon. N Engl J Med, 2005;352: 1550-1556.

4. Noakes T, Sharwood K, Speedy D, Hew-Butler T, Reid S, Dugas J. Dehydration prevents the development of hyponatremic encephalopathy during exercise: Evidence from 1423 weighed competitive athletic performances. Med Sci Sports Exerc. 2005;37: s348.

 

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date: September 12, 2009