While there are multiple causes of hyponatremia, the main instigators are excess fluid intake in which blood sodium is diluted, and a high rate of sodium loss from sweating. Cyclists who have a high concentration of sodium in their sweat and/or a high sweat rate are more susceptible to this condition.
In some endurance athletes, measured sweat losses have exceeded 1.8 liter per hour and range from 0.5 to 2.5 liters per hour. While cyclists can check body weight pre and post-training and make some educated calculations about sweat loss, estimating sweat sodium losses is more difficult. Research measured results have ranged from 230 to 1380 milligrams per liter or even higher. Sodium sweat losses per hour are then multiplied by your sweat rate and sodium content of sweat, and for heavy sweaters and salty sweaters result in high sodium losses per hour.
You may have identified yourself as a heavy sweater or as a salty sweater and made the switch to a higher sodium sports drink, or even added electrolyte tablets to your training mix. If you are getting good results from your current sodium replacement plan, some new research recently presented at the American College of Sports Medicine Meeting this past May presented yet another twist on the benefits of smart sodium supplementation during serious training.
Exercise physiologist, researcher, and PhD student Matt Pahnke, MA at the University of Texas at Austin studied the effects of sodium replacement on brain function. “During endurance exercise we see an increase in cognitive function. When we tested subjects with very high sodium sweat losses who had a drop in blood sodium during three hours of endurance exercise, we found no improvement in cognitive function,” said Pahnke. So when these salty sweaters lost sodium and their blood sodium dropped, they did not derive the usual benefit in brain function seen with endurance exercise.
Based on these results, Phanke chose to study some salty sweaters and designed a testing protocol to determine if replacing sodium during training and subsequently maintaining blood sodium levels preserved or resulted in the expected improvement in brain function.
Subjects were heat acclimatized endurance trained males and tested in a warm environment of 33 degrees Celsius. Subjects cycled for three hours at 60 percent of maximal oxygen uptake. Body mass was maintained by consuming water with carbohydrate. Sodium chloride or a placebo was ingested in capsule form at 15-minute intervals during the cycling trials to match individual sweat sodium losses. Individual sodium losses were determined two to seven days before the experimental trial.
As would be expected, at both 120 minutes and 180 minutes into the trial, blood sodium levels were significantly lower in the subjects receiving the placebo compared to those receiving sodium replacement. Subjects maintained body mass during the trials, reflecting appropriate fluid replacement. The salty sweater subjects required on average 2.4 grams of sodium per hour to maintain blood sodium levels while remaining hydrated via drinking.
Before and after the trials, subjects were given the Stroop Color-Word Test which tests reaction time for complex tasks. “In the sodium replacement group, response time for correct answers improved,” said Pahnke. So in athletes that have high sodium losses, blood sodium will be maintained during endurance exercise with appropriate sodium supplementation designed to match sweat sodium losses. The Stroop test response time improved after exercise in subjects that maintained blood sodium with supplementation, but did not change when blood sodium was not maintained.
Determining your own sodium losses without the benefit of laboratory testing can be difficult. In some of Pahnke’s previous work, he investigated the variation in sweat sodium losses in over 130 endurance athletes, half of which were competing in the Ironman World Championship in Kailu-Kona. When exercising in a warm environment at 70 to 75 percent of heart rate maximum, Phanke found a very large range of sodium losses in sweat. While the average losses of sodium in sweat were 1.5 grams of sodium per hour, Pahnke measured losses among these endurance athletes ranged from 0.3 to 5 grams of sodium per hour, in males and females.
Pahnke followed a smaller group of subjects and found that in males, a drop in blood sodium was related to their changes in body mass, which represents both hydration levels and sweat sodium losses. “A first good step to understanding sodium losses is to measure sweat rate by measuring changes in body weight,” said Pahnke.
Check weight before and after exercise. Changes in body weight will be due primarily to sweat loss. Pahnke has a sweat calculator at www.triharder.com.
Replacing sweat losses with a higher sodium sports drink may be sufficient for many athletes. Check labels of various sports drinks to find a high sodium product that you like and tolerate. If you are on the higher end of sodium losses, as observed by white salt crystals on your skin and clothing after a training session, consuming an appropriate volume of a high sodium sports drink may be sufficient to maintain blood sodium levels. But athletes at the higher end of sodium sweat losses and with a high sweat rate can also add in additional sodium supplementation, such as electrolyte tablets to their training mix.
Monique Ryan, MS, RD, LDN is a nationally recognized nutritionist with over twenty-four years of experience and is owner of Personal Nutrition Designs, a Chicago based nutrition consulting company that provides nutrition programs for endurance athletes across North America (www.moniqueryan.com). Monique has consulted with the Chicago Fire Soccer Team for seven season, and was the nutritionist for Saturn Cycling from 1994 to 2000. She has also consulted with the Volvo-Cannondale Mountain Bike Team, the Gary Fisher Mountain Bike Team, and the Rollerblade Racing Team. Monique has consulted with USA Cycling, and was a member of the Performance Enhancement Team for the Women’s Road Team leading to the 2004 Athens Olympics. She has also provided nutrition consultation services to USA Triathlon for coaching clinics, athlete clinics, and for the resident athlete team and was a member of the USAT Performance Enhancement Team for the 2004 Athens Olympics. Monique is the author of “Sports Nutrition for Endurance Athletes,” 2nd edition (March 2007), from VeloPress, which provides sports specific nutrition for road cycling, mountain biking, running, triathlon, swimming, rowing, and adventure racing. She is also author of “Performance Nutrition for Winter Sports“(PeakSports Press), “Performance Nutrition for Team Sports” (PeakSports Press), and “Complete Guide to Sports Nutrition.” Monique is a regular contributor to VeloNews, Inside Triathlon, and Outside. She is a Certified Specialist in Sports Dietetics. As part of the FeedZone column, Monique will answer selected questions online. Please send your questions to RyanWebQA@aol.com.