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Bicycle Training and Fitness

Fluids and Endurance Exercise

by ExperiencePlus! - Tuesday, July 27, 2010

It wasn’t too long ago that we were told that as endurance athletes we could not drink enough water during exercise. Drink all you can get in was the common hydration guide for long distance events. The reasoning was that dehydration is the greatest challenge facing the athlete, especially in hot weather. That certainly seems logical.

But recently, sports scientists have come to realize that athletes can indeed drink too much water. Excessive water intake may well detract from exercise performance. For example, the most common cause of failure to finish in the Ironman Hawaii Triathlon is related to overdrinking of water. In extreme circumstances overconsumption of plain water can even lead to death. In fact, there have been three deaths attributed to excessive water intake in marathons in recent years.

The condition that is believed to have caused these race abandonments and deaths is called “hyponatremia” – low sodium levels. For optimal physical function the body needs to maintain a fairly constant level of sodium in body fluids. If the body’s sodium concentration is diluted by excessive water intake the body experiences a “shutting down” of basic functions. Among the earliest signs of hyponatremia are weakness, nausea, dizziness, disorientation, and headache. All of these may not appear in a given athlete in a given situation. One that is more common is swelling of the extremities as evidenced by watchbands, rings and elastic sock bands leaving deep impressions in the skin.

What makes this condition especially dangerous is that it may be diagnosed by the suffering athlete, companions, and even medical staff as dehydration. If this mistake is made and the athlete is given even more plain water the condition can be made much more serious. In long distance events such as hiking and bicycle tours professionals are now much better informed on this condition. As a further example of the changes taking place in this area of fluid replacement, many marathons are even reducing the number of aid stations to help prevent hyponatremia.

Dehydration is still a concern. But it is the problem experienced most commonly by the fastest athletes in a race who are unable to get in adequate fluids during activity due to their torrid pace. Hyponatremia is most likely to occur in the slowest participants, especially in events lasting longer than four hours that are conducted in hot conditions.

So how can you prevent hyponatremia? The key is to avoid drinking water only in exercise sessions lasting longer than about 60 to 90 minutes. Sports drinks containing not only sodium but also carbohydrate will keep you well hydrated, maintain sodium levels, and help prevent low glycogen levels which can result in bonking. The less sodium you take in the less fluids you should take in. If using a sports drink only limit your fluids to about 24 to 32 ounces per hour since the sodium concentration is likely to be low relative to your need if it is hot. If taking in supplemental sodium you can probably drink more than this.

How much sodium is necessary? This is a difficult question to answer as the amount needed varies with body size and sweat rate. The science is also not well-established on this but my experience in coaching many athletes in extreme conditions has led me to believe that 600 to 1000 mg per hour in hot events lasting longer than four hours is safe and effective. Again, the range depends on your size, weather conditions, and your sweat rate. Where you may be in this range is open to some conjecture tempered by experience, but the downside of taking in too much sodium is far less risky than that of taking in too little. Total sodium intake during exercise should be calculated based on all sources including sports drinks, gels, sodium capsules, and food. Sodium capsules may be purchased through sports nutrition catalogs and online at sites such as www.pbn.com. It’s probably also wise to take in more sodium the day before long exercise sessions by putting extra salt on your food.