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October 14, 2011, 12:05PM PT in Fitnovatives Blog  |  1 Comments

Marathon Training: Preventing Tragedy with Attention to Hydration

marathon water stationLast weekend, William Caviness, a 35-year-old North Carolina man collapsed 500 yards from the finish line at the Chicago Marathon and was pronounced death soon after.

An experienced marathoner and firefighter described as “the picture of health,” his sudden death is now the sixth in Chicago Marathon’s history in the past decade and half. This marathon, however, isn’t the only event plagued by tragedy and all raise safety concerns of endurance challenges. 

While the autopsy on Caviness was inconclusive, some experts speculate that, in this case, cardiac arrest is to blame. Underlying heart conditions, dehydration, and over-hydration (hyponatremia) most often are the causes of life-ending races in young athletes.

As a result, serious attention to hydration on race day can prevent a tragic outcome on what is supposed to be a celebration of a commitment to physical activity and a remarkable feat of mental toughness.

Sadly, it turns out that not many runners are paying serious attention to hydration. A New York Times piece that ran last week highlighted two studies, which showed that a whopping 65 percent of the athletes studied were “not at all” concerned about keeping themselves hydrated. This nonchalance comes at a cost.

Drinking too little can lead to dehydration — a scary condition exercisers have been cautioned against in every text, handout and presentation on fluid replacement. Dehydration results from a sweat rate that is beyond fluid replenishment.

Several factors increase the likelihood of dehydration, by either increasing sweat rate or decreasing fluid intake:

  • Exercising at very high intensities: Sweat rate is increased; the time available to focus on rehydration is diminished; and the stomach and intestines are less able to process and empty fluids into the bloodstream.
  • Exercising in very hot and/or humid conditions: Sweat rate is dramatically increased in an effort to rid the body of excessive heat. Because water also functions to cool the body temperature, severe dehydration in these conditions can lead to heat stroke.
  • Decreased fluid intake: Lack of fluid consumption during exercise can lead to dehydration. Poor consumption may be due to inaccessibility, a low level of fluid tolerance, dislike of the available beverage, or failure to understand the importance of staying hydrated.

Dehydration, along with high exercise intensity, hot and humid environmental conditions, poor fitness level, incomplete heat acclimatization, and a variety of other factors can all raise body temperature and together lead to heat stroke.

While dehydration is a serious concern, athletes should also be aware that drinking too much — out of fear of not drinking enough — could lead to hyponatremia, a less well known and understood, but equally frightening condition characterized by a low blood sodium level.

Exertional hyponatremia results from excessive intake of low-sodium fluids during prolonged endurance activities -- that is, drinking a greater volume of fluid than the volume lost in sweat--and possibly, to a lesser extent, from inappropriate fluid retention.

A study of 488 Boston Marathon runners published in the New England Journal of Medicine found that 13% (22% of women and 8% of men) had hyponatremia, and 0.6% had critical hyponatremia, at the end of the race.

Runners with hyponatremia were more likely to have low body mass index, consume fluids at every mile (and more than 3 liters total throughout the race), finish the race in more than 4 hours, and gain weight during the run. The greatest predictor of hyponatremia was weight gain, which researchers attributed to excessive fluid intake.

But hyponatremia is not limited to runners. Anyone exercising at a low to moderate intensity for an extended period of time (generally 4 hours or more) while consuming too much water can be at risk.  

The goal of fluid intake during exercise is to prevent performance-diminishing or health-altering effects from dehydration or hyponatremia. Here’s how:

  • Aim for a 1:1 fluid replacement to fluid loss ratio: Ideally, people should consume the same amount of fluid as they lose in sweat. Exercisers can compare pre/post-exercise body weight. The goal is to avoid weight loss greater than 2%. If determining individual needs is not feasible, the American College of Sports Medicine recommends that athletes could consider aiming for a 0.4 to 0.8 L/h (8-16oz/h) replenishment with the higher rate for faster, heavier athletes in a hot and humid environment and the lower rate for slower, lighter athletes in a cool environment. Because people sweat at varying rates and exercise at different intensities, this range may not be appropriate for everyone. However, when individual assessment is not possible, this recommendation works for most people.
  • Drink fluids with sodium during prolonged exercise sessions: If an exercise session lasts longer than two hours or an athlete is participating in an event that stimulates heavy sodium loss (defined as more than 3 to 4 grams [g] of sodium), then the athlete should consider consuming a sports drink that contains elevated levels of sodium. In one study, researchers did not find a benefit from sports drinks that contain only the 18 mmol/L (or 100 milligrams per 8 oz) of sodium typical of most sports drinks and thus concluded that higher levels would be needed to prevent hyponatremia during prolonged exercise. The Institute of Medicine recommends that people exercising for prolonged periods in hot environments consume sports drinks that contain 20-30 mEq/L (450-700mg/L) of sodium to stimulate thirst and replace sweat losses and 2-5mEq/L (80-200mg/L) of potassium to replace sweat losses. Alternatively, exercisers can consume extra sodium with meals and snacks prior to a lengthy exercise session or a day of extensive physical activity. Additional sodium or supplementation with salt tablets seems to be unnecessary based on the limited research to date on this topic.
  • Drink carbohydrate-containing sports drinks to reduce fatigue: If an athlete exercises for longer than one hour, he or she should also get some additional carbohydrate with fluids. With prolonged exercise, muscle glycogen stores become depleted and blood glucose becomes a primary fuel source. To maintain performance levels and prevent fatigue, athletes should consume drinks and snacks that provide about 30 to 60 g of rapidly absorbed carbohydrate for every hour of training. As long as the carbohydrate concentration is less than about 6% to 8%, it will have little effect on gastric emptying.
  • Following exercise, athletes should aim to correct any fluid imbalances that occurred during the exercise session: This includes consuming water to restore hydration, carbohydrates to replenish glycogen stores, and electrolytes to speed rehydration. If the athlete will have at least 12 hours to recover before the next strenuous workout, then rehydration with the usual meals and snacks and water should be adequate. The sodium in the foods will help retain the fluid and stimulate thirst. If rehydration needs to occur quickly, then the athletes should drink about 1.5L of fluid for each kilogram (or .75L of fluid for each pound) of body weight lost. This will be enough to restore lost fluid and also compensate for increased urine output that occurs with rapid consumption of large amounts of fluid. A severely dehydrated athlete (>7% body weight loss) with symptoms (nausea, vomiting, or diarrhea) may need intravenous fluid replacement.
  • Those at greatest risk of hyponatremia should be careful not to consume too much water following exercise and instead should focus on replenishing sodium.

At the end of the day, the human body is well-equipped to withstand dramatic variations in fluid intake during exercise and at rest with little or no detrimental health effects. For this reason, most recreational exercisers will never suffer from serious hyponatremia or dehydration and should not be alarmed. It is under extreme situations of prolonged or very high intensity exercise in excessive heat and humidity that risk elevates. And even then, if athletes replenish sweat loss with equal amounts of fluid, hydration problems can be avoided and performance optimized. The key to a safe and successful finish is a few ounces of education and prevention.


References

  1. Almond CS, Shin AY, Fortescue EB, et al. Hyponatremia among runners in the Boston Marathon. N Engl J Med. Apr 14 2005;352(15):1550-1556.
  2. Institute of Medicine. Fluid Replacement and Heat Stress: Institute of Medicine;1994. Hew-Butler TD, Sharwood K, Collins M, Speedy D, Noakes T. Sodium supplementation is not required to maintain serum sodium concentrations during an Ironman triathlon. Br J Sports Med. Mar 2006;40(3):255-259.
  3. Rodriguez NR, DiMarco NM, Langley S. Position of the American Dietetic Association, Dietitians of Canada, and the American College of Sports Medicine: Nutrition and athletic performance. J Am Diet Assoc. Mar 2009;109(3):509-527.
  4. Sawka MN, Burke LM, Eichner ER, Maughan RJ, Montain SJ, Stachenfeld NS. American College of Sports Medicine position stand. Exercise and fluid replacement. Med Sci Sports Exerc. Feb 2007;39(2):377-390.
  5. Speedy DB, Thompson JM, Rodgers I, Collins M, Sharwood K, Noakes TD. Oral salt supplementation during ultradistance exercise. Clin J Sport Med. Sep 2002;12(5):279-284.

By Natalie Digate Muth
Natalie Digate Muth, MD, MPH, RD, FAAP

Natalie Digate Muth, MD, MPH, RD, FAAP is the Healthcare Solutions Director for the American Council on Exercise, a board-certified pediatrician and Fellow of the American Academy of Pediatrics, a Diplomat of the American Board of Obesity Medicine, registered dietitian and board-certified specialist in sports dietetics, and ACE-certified health coach. She is the author of "Eat Your Vegetables" and Other Mistakes Parents Make: Redefining How to Raise Healthy Eaters" and the upcoming textbook "Sports Nutrition for Health Professionals". She has been ACE certified since 1998.

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