Written for Ironman.com
As the summer approaches, nutritionist Nancy Clark offers a hydration education in sweat, sodium and cramping.
Hydration is a hot topic in the endurance and IRONMAN communities. But an issue that often goes ignored in the conversation is the problem of overhydration, which can be as dangerous to your health as underhydration.
Dr. Timothy Noakes’ recent book, Waterlogged: The Serious Problem of Overhydration in Endurance Sports, is an interesting yet controversial resource that addresses that question. His book challenges the advice, “drink before you get thirsty.” Noakes believes elite athletes who become champions can tolerate significant sweat loss without untolerable thirst. He contends that the associated weight lost via sweating enhances performance. Others question if those athletes could run better if better hydrated.
So what does a sweaty athlete need to know about staying adequately hydrated? Below are a few drops of less-controversial hydration information to help you quench your thirst, perform well and stay out of the medical tent when you find yourself competing at hot races.
Most athletes feel thirsty at about two percent dehydration. At that point, they’ll start looking for water. Ultrarunners can maintain performance at three percent dehydration. (To determine your percent dehydration, weigh yourself naked before and after your workout. A one-pound drop equates to a loss of 16-ounces of sweat; two percent dehydration equates to a three-pound sweat loss for a 150 pound person.)
Thirst is a powerful fluid regulator. Noakes disapproves of the advice to drink before you are thirsty because that can create problems with overhydration. Yet others contend drinking on a schedule can help endurance athletes maintain proper hydration as long as they do not aggressively overhydrate, but rather replace fluids according to their sweat losses (as learned during training via pre-post exercise weigh-ins).
Also known as hyponatremia, overhydrating can lead to low blood sodium and occurs when athletes drink excessively. It can also occur when dehydrated endurance athletes lose significant amounts of sodium in sweat. Data from 669 ultramarathoners indicates 15 percent experienced low blood sodium. Of those, 24 percent were overhydrated, 36 percent were dehydrated, and the rest were in fluid balance (but not sodium balance).
The amount of sodium lost in sweat varies from person to person. Some people are salty sweaters. Athletes accustomed to exercising in the heat retain more sodium than unacclimatized athletes. (Compare the saltiness of your sweat on first hot day of spring versus in the fall.)
Athletes lose relatively more water than sodium, so under standard conditions, the blood sodium level can actually increase during exercise (unless you overhydrate). But with abnormally high sodium high losses, such as during an ultramarathon, blood sodium can be low even in a dehydrated athlete. Hence, sodium replacement can be a wise idea.
Noakes says evidence is lacking to prove that athletes who cramp have low serum sodium or are more dehydrated than non-crampers. He suggests muscle cramps are related to fatigue, not sodium deficiency. If sodium deficiency was the problem, wouldn’t the entire body cramp, not just one muscle?
Exercise-induced muscle cramps occur in muscles that perform repetitive contractions. Athletes who get cramps tend to be those who do high-intensity exercise, as well as those who have a history of cramping. Note: Many exercise scientists believe there are two types of muscle cramps: some related to fatigue, others related to sodium imbalance. One thing that’s known for sure is that stopping exercise to stretch resolves muscles cramping.
What you can do
So what’s a sweaty endurance athlete supposed to do during prolonged exercise? Learn your sweat rate and drink accordingly. If fluid in your stomach starts “sloshing,” stop drinking. The body can absorb about 600 to 1,000 ml/hour (women/ men). Adding carbohydrates and sodium to the water enhances fluid absorption as well as palatability and performance. Consuming “real” foods (salty pretzels, pickles, chicken broth, ham-cheese-mustard wrap) during ultraendurance events can also do a fine job of providing needed electrolytes. Just don’t get too aggressive with water or sodium intake—and remember to have fun.
Nancy Clark, MS, RD, CSSD counsels both casual and competitive athletes. Her Sports Nutrition Guidebooks are invaluable for new runners, marathoners, and triathletes.