ACSM Marathon Hyponatremia and Dehydration Guidelines

Marathoner with Water Bottle and Energy Gel
Marathoner with Water Bottle and Energy Gel. Alan Crowhurst/Getty Images Sport

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: Drinking too much water or other fluids can dilute sodium to the danger point. Slower runners and walkers on long distance events appear to have the greatest risk.

Dehydration: This is a common risk during hot-weather training. It increases the risk of heat sickness including life-threatening heat stroke. In addition to harming performance, it can strain the heart.

W. Larry Kenney, Ph.D., FACSM warns of going to either extreme of drinking too much or drinking too little. "The key is 'drinking intelligently, not drinking maximally'," he says in the press release.

Drink to Match Fluid Loss and On a Schedule

The experts concluded that drinking before, during and after exercise is needed so the body can stay correctly hydrated and able to maintain body temperature. But individuals vary considerably in exactly how much they need, especially in different conditions of temperature and humidity. The best solution is to find their individual hourly sweat rate by noting how much fluid they have taken in compared with their change in weight during an hour of exercise. Knowing that number, they can then set a schedule to drink the right amount of fluids during exercise.

Take-Away Tips from the ACSM Exercise and Fluid Replacement Stand

  • Start drinking early during exercise sessions, and replace fluids at a steady pace rather than attempting to catch up with rapid fluid replacement.
  • Thirst May Not Be Enough: An athlete may have lost more fluid than their thirst indicates. It's not a foolproof indicator, especially when sweating more than usual.
  • Monitor Body Weight Loss: Weigh before and during a long exercise session to find out how much fluid to replace and how often to drink to replace fluids lost in sweat.
  • Drink Consistently Rather Than Taking a Big Drink Break: This strategy is more effective at maintaining fluid balance. The ACSM notes that drinking a large volume at one time may result in accelerated urine production and elimination rather than fluid replacement.
  • Sweat Fluid Loss: If an athlete isn't sweating and thirsty, they may not need as many fluids.
  • Consume Salty Foods and Beverages During Prolonged Exercise: Salty snacks and sports drinks with electrolytes can replace sodim lost in sweat during prolonged exercise. Research supports taking in salt for maintaining fluid balance, and salty foods can prompt an athlete to drink. This may help prevent hyponatremia.

    More from ACSM: This update in 2007 covers in-depth the research evidence and recommendations for hydration during exercise.
    SPECIAL COMMUNICATIONS: Position Stand Exercise and Fluid Replacement. Medicine & Science in Sports & Exercise: February 2007 - Volume 39 - Issue 2 - pp 377-390 doi: 10.1249/mss.0b013e31802ca597

    Drinking Guidelines for Distance Walkers: The International Marathon Medical Director's Association issued these guidelines in 2006. They include details on how to weigh yourself to determine how much and how often you should drink during extended walking and running workouts and races.

    Source: ACSM Press Release, October 20, 2005.

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