How To Prevent Hyponatremia During Long Distance Events by Monique Ryan, MS, RD

Posted: July 21, 2010 in Cycling, Information, Nutrition, Product Reviews, Race Information, Running, Swimming, Training Information

The past decade of triathlon long distance course competition has born witness to the hyponatremic meltdown of many age-groupers and professionals competitors. While this lowering of blood sodium levels can be dramatic, dangerous, and even fatal, many half and full Ironman triathletes can experience mild to severe degrees of this condition. The lower sodium blood levels drop, the more serious and life-threatening the symptoms. At the very least, hyponatremia can slow your competitive efforts and is best prevented. Anywhere from 3 to 27 percent of ultra-endurance athletes seeking medical care may suffer from hyponatremia. Researchers at one Hawaiian Ironman found that up to 30 percent of competitors were hyponatremic.

Cause and effect
While the causes of hyponatremia are many and varied, there are two main culprits. The first is excess fluid intake, which occurs when you drink too much salt-free fluid leading up to competition and drink in excess of your sweat losses during competition. High sodium sweat concentration is the second culprit. Extensive and repeated sweating as seen during Ironman can clearly result in large sodium losses. Triathletes with longer finishing times are also at great risk for fluid overload, as there is simply more time to sweat and drink during a race.

Simply put, excessive fluid intake is a big risk factor for developing hyponatremia, and how you pre-hydrate and your drinking strategies during competition can have a significant effect upon this precarious fluid balance. Being a salty sweater also exacerbates this condition.

Fluid – Just the right amount
Of course, you should consume fluid during a race as the risk of becoming dehydrated is still much greater than that of developing hyponatremia. The trick is to not over drink, and to be familiar with your individual hourly sweat loss rate. Some well acclimated triathletes may be very efficient sweaters and lose only one-half to one liter of sweat per hour, while others may reach higher levels of two or three liters per hour in hot and humid weather, despite being acclimated.

Checking your weight before and after longer training sessions can indicate how well you are keeping up with or possibly exceeding your sweat losses. A weight loss of 2-5 pounds is clear indication that fluid intake needs to increase. Conversely, if you actually gain weight during training, your drinking may be excessive. A rough estimate of your sweat rate (see table below) and checking weights before and after training can help prevent both overdrinking and dehydration.

Salty Sweat
High sodium sweat losses can further increase your risk of developing hyponatremia. Fit and acclimatized triathletes usually have less than 900 mg of sodium per liter of sweat. While higher sodium sweat losses are usually seen in unfit and unacclimatized individuals, some highly trained triathletes may be salty sweaters with losses exceeding 1400 mg per liter. If one of these salty sweater triathletes also has a high sweat rate, their losses multiply and they can lose significant amounts of sodium during a half or full Ironman triathlon. While lab testing indicates that sodium losses range from 460 to 1800 mg sodium per liter sweat, chances are your only indicator of high sodium losses is white salt rings and streaks on your clothing and skin.

Clearly salty sweaters need to replace sodium losses to maintain safe blood sodium levels. Even dehydrated triathletes can develop hyponatremia if they have high enough sodium sweat losses over a long race. Look at the sodium content of the sports drinks, gels, and other products that you consume during racing. Some triathletes will need to veer closer to higher sodium sports drinks, and others may even want to carefully supplement with sodium tablets. Salt tablets should be stored in a waterproof case or bag to prevent disintegration and should be taken with eight ounces of fluid. Generally one or two salt tablets per hour is sufficient (amounts per tablet vary). Excess salt intake without adequate fluid intake could irritate your stomach, and result in bloating and nausea. Experiment with these products in training. High sodium sports drinks are probably the most convenient strategy and also provide fluid and carbohydrates when racing.

Before and After
Pre-hydration strategies can also contribute to development of hyponatremia if not done appropriately. Keep in mind that your fluid losses may decrease during a race taper. Of course you need to hydrate leading up to the race, but there is also a risk of overhydrating at this time, just as during the race. Monitor your weight to keep tabs on hydration levels. Healthy triathletes with no specific medical concerns should consume salty foods and add salt to food in the days leading up to the race. If you are a very salty sweater, you can also increase your salt intake significantly in the days leading to the race.

Hyponatremia can also develop after a race as you attempt to rehydrate. For every pound of weight lost, aim to consume 20 to 24 ounces of sodium containing fluid. Drinks and foods that contain sodium, as well as a salty meal or snack several hours after race will help rehydration efforts. These higher sodium intakes will result in less urine production and improve rehydration efforts.  If you have gained weight during the race, rehydrate carefully.

Table 1: Blood Sodium 101

Blood Sodium Levels Symptoms
136-142 millimoles per liter Normal levels
125-135 mmol/L No obvious symptoms or mild gastrointestinal upset such as bloating and nausea
Below 125 mmol/L Symptoms are more severe and include headache, vomiting, wheezing, swollen hands and feet, unusual fatigue, confusion, and disorientation
Below 120 mmol/L Seizure, coma, brain swelling and damage, and death are more likely

Table 2: Estimating sweat losses

This method provides an estimated of sweat losses in hot weather and is fairly simple formula. Of course urine losses affect this equation, as would weight gain from any solid food consumed.

  1. Check your weight before and after training and calculate your weight loss.
    165 lb. to 162 lb. (75 kg to 74 kg)
    3 lbs. weight loss during training (1.4 kg)
  2. Know the amount of fluid that you consumed during the training session. 30 ounces of fluid weighs 2 pounds (an easier estimate is 1000-ml fluid equals1.0 kg body weight).
    Consume 60 ounces (1800 ml) of fluid during a three-hour bike ride. This fluid weighs 4 lb. (1.8 kg)
  3. Add the answers in 1 and 2.
    3 lb. + 4 lb. = 7 lb.
    (1.4 + 1.8 kg = 3.2 kg)
  4. 7 lb. equals 105 ounces fluid divided by 3 hours = 35 ounces per hour for sweat losses.
    (3.2 kg equals 3200 ml fluid divided by 3 hours = 1060 ml/hr).

Table 3: In Summary

Hyponatremia occurs during longer exercise bouts and overconsumption of fluid in relation to your individual sweat losses. High sodium sweat losses increases risk of developing hyponatremia even in the presence of dehydration. Total sodium losses during exercise are a combination of your sweat sodium concentration, sweat rate, and number of hours of sweating. Don’t overhydrate, keep up with sodium losses during the race. Prehydrate appropriately, and consume sodium before and after a race.

Nutrition for Endurance  Athletes Monique Ryan, MS, RD is the author of Sports Nutrition for Endurance Athletes, 2nd edition (VeloPress 2007). Click here to view more about the book or purchase. She was a member of the Athens 2004 Performance Enhancement Teams for USA Triathlon and USA Cycling Women’s Road Team. She is owner of Personal Nutrition Designs and offers her sports nutrition “E Program” at Monique Ryan, MS, RD is owner of Personal Nutrition Designs with programs at She is the author of Sports Nutrition for Endurance Athletes.

  1. […] How To Prevent Hyponatremia In Long Distance Events by Monique Ryan […]

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