Why You're Cramping On The Run

Q: In a recent long-course race, I felt super tired and my muscles cramped up two-thirds of the way into the run, despite drinking tons of fluid. Where did I go wrong?

A: I’m pretty sure you failed to take in enough sodium during your long-course race. Although I’m sure you’ve heard of the dangers of both under-hydrating and hyponatremia (over-hydrating, or diluting your electrolytes), the entire topic of hydrating can be very confusing to even the smartest of triathletes. You can still become dehydrated despite adequate fluid intake if you’re not keeping your sodium balance in check. It is likely that you took in too little sodium, and I recommend you increase your intake of sodium before and during your next long training or race. Increase your sports drink consumption to at least 50–75 percent of your total fluid intake during your training and race. Use an endurance sports drink (with 200 milligrams of sodium per 8 ounces versus the usual 110). I would also recommend you conduct a few sweat tests to more accurately determine your sweat rate and help ensure you are drinking the correct amount. Finally, add electrolyte tabs as needed until you are drinking enough fluid to nearly match your sweat rate. The combination of adequate total fluid and much-needed sodium will help propel you to a strong and safe finish in your next race.

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Lauren Antonucci, R.D., is a board-certified specialist in sports dietetics, three-time Ironman finisher and the founding director of Nutrition Energy in New York City.