Preparing for a marathon conjures images of grueling long-distance runs, piles of pre-race pasta and discarded paper cups along the 26.2-mile course. While replacing fluids and electrolytes during a long race is essential, it's just as important to not exceed fluid targets in order to prevent a potentially fatal condition known as "exertional hyponatremia," or low blood sodium.
"Overhydration, especially with water but even with sports drinks, is the main cause of hyponatremia," says Christine Gerbstadt, MD, MPH, RD, LDN, CSSD, CDE, a spokesperson for the Academy of Nutrition and Dietetics and a seasoned marathoner. "Runners should be aware of symptoms of early hyponatremia, like dizziness and confusion." Other signs include a bloated stomach, fingers, toes, wrists and ankles.
Hyponatremia is more commonly seen in younger female runners, runners who complete a marathon in more than four hours and those competing in their first marathon—although the condition may affect anyone. "There is a small subset of runners who may be more susceptible to this condition due to their unique kidney function, but we can't predict who these individuals are," says Gerbstadt.
It is also possible for endurance athletes to be both dehydrated and hyponatremic because the sodium lost in sweat contributes to a drop in blood sodium concentration.
While most marathoners are likely to finish the race somewhat dehydrated, the risk of exertional hyponatremia is higher than many recreational marathoners may realize. Most cases occur without noticeable symptoms, and may be corrected by urine loss and sodium consumption after the race.
So what's a runner to do? Start by developing a personalized hydration plan that includes hourly sweat rate: Step on a digital scale immediately before and after hour-long runs in various weather training conditions, particularly through a range of temperatures. Then record how much fluid is consumed during the workout.
Weight loss after a run indicates dehydration, while weight gain indicates over-drinking and the need to reduce fluid intake during training. Each pound of weight loss is 16 ounces of sweat that you did not replace. For example, if you are down one pound after a one-hour run during which you consumed 20 ounces of fluids, your total sweat loss during that run was 36 ounces. You can use this information to consume the optimal amounts of fluid per hour without overhydrating.
In addition to monitoring fluid loss, balancing electrolytes is key. Sodium sweat losses can be highly variable, ranging from 460 mg to 1,840 mg per liter of sweat, particularly among "salty sweaters" who may have white streaks of salt residue on their clothes or skin after a tough workout. Replenish electrolytes by drinking higher sodium sports drinks during the race or training and add electrolyte powder packets for enhanced sodium content. Gels, chews or bars offer other carbohydrate replacement options as body fuel stores run low, but they generally do not supply as much sodium as the enhanced sports drinks and electrolyte mixes.
"New runners and veteran marathoners can consult a sports dietitian to learn about proper hydration and electrolyte replacement," says Gerbstadt. "And if there is any question, signs or symptoms of hyponatremia, detouring from the race to the medical tent can save a life. There will always be another marathon."