ACSM Marathon Hyponatremia and Dehydration Guidelines

Getting fluids right during endurance training and events is critical - drink too much and risk hyponatremia, drink too little and risk dehydration. Dehydration is the more common problem for all marathoners, while marathon walkers and slow runners are the ones most at risk for hyponatremia. The American College of Sports Medicine published guidelines based on many past studies in the June, 2005 issue of "Current Sports Medicine Reports." The following advice is from their press release October 20, 2005.

Minimize Risk of Both Hyponatremia and Dehydration

Hyponatremia: This dangerous condition that occurs when an athlete consumes too much fluid (either water or other fluids), diluting the body's sodium levels. Exertional hyponatremia is relatively rare - probably less than 1 in 1000 marathon or triathlon finishers. But it appears to occur most often in slow-paced athletes - walking or running for over 4 hours, triathlons lasting longer than 9-13 hours).

Dehydration: Especially during hot-weather training, dehydration occurs more frequently and has severe consequences, increasing the risk of heat exhaustion and heat stroke during and immediately after activity. Dehydration can affect physical and mental performance, increase cardiovascular strain and decrease heat tolerance.

Avoid Fluid Extremes: "While hyponatremia has gotten more attention lately, far more athletes are affected by dehydration," said W. Larry Kenney, Ph.D., FACSM. "However, there are dangers associated with both extremes of behavior -- severe under-drinking and severe over-drinking. Not drinking at all is not a safe option for preventing hyponatremia. The key is 'drinking intelligently, not drinking maximally'," he added.

Drink to Match Fluid Loss and On a Schedule

The experts concluded that appropriate fluid intake (before, during and after exercise) is important to help regulate body temperature and replace fluids lost in sweat. Since fluid and electrolyte (salt) needs are widely variable based on the athlete's genetics and environmental conditions, athletes should know their bodys' hourly sweat rate (weight lost during exercise per hour + fluid consumed during exercise per hour = hourly sweat rate) and aim to replace the total amount lost during that time.

ACSM Exercise and Fluid Replacement Stand

Athletes are encouraged to drink early and at regular intervals rather than rapid fluid replacement.

Thirst May Not Be Enough: Perception of thirst is an imperfect index of the magnitude of fluid deficit and cannot be used to provide complete restoration of water lost by sweating.

Monitor Body Weight Loss: Individuals participating in prolonged intense exercise must rely on strategies such as monitoring body weight loss and ingesting volumes of fluid during exercise at a rate equal to that lost from sweating, to ensure complete fluid replacement.

Drink Consistently Rather Than Taking a Big Drink Break: Drinking over a set period of time is more effective for complete rehydration, vs. rapid replacement of fluid which instead stimulates increased urine production, reducing body water retention.

Sweat Fluid Loss: If athletes are not sweating heavily (such as slow runners) and are not thirsty then their fluid replacement needs are probably modest.

Consume Salty Foods and Beverages

Research shows foods and beverages with sodium help promote fluid retention and stimulate fluid intake. Athletes performing prolonged exercise should ingest snacks or fluids containing sodium to help offset the loss of salt in sweat, in an effort to prevent hyponatremia.

Source: ACSM Press Release, October 20, 2005.