Hydration Mistake Tied to Postrun Fatigue
A sweat test pointed to what was in the bottle, not just how much was drunk.
A runner used the Nix Hydration Biosensor during several runs to better understand lingering fatigue, headaches, and afternoon slumps. The device showed that even a relaxed, sub-50-minute five-mile run on a roughly 70-degree day led to an estimated 17 ounces of fluid loss. Since the runner usually carried no water for runs of 10 miles or less, the result changed how those ordinary workouts looked.
Sports dietitian Kylee Van Horn recommended replacing about 75 percent of fluid lost each hour during training. Losing more than 2 percent of body weight through fluids is when dehydration begins to affect performance; for this runner, that meant avoiding a loss above 2.5 pounds. The testing also measured electrolytes: the easy run involved 915 milligrams lost, including 430 milligrams of sodium.
After four runs, the runner’s average sodium loss was about 430 milligrams per hour, varying by roughly 30 milligrams. Matching sodium to the amount of fluid replaced became the key adjustment: 13 ounces of water called for about 308 milligrams of sodium, while 17 ounces called for 430 milligrams. The biggest issue appeared during regular eight- to 10-mile runs and 90-minute workouts, where the runner had not been taking in the same support used during longer efforts.
- Four runs showed average sodium loss near 430 milligrams per hour.
- An easy five-mile run produced an estimated 17 ounces of fluid loss.
- The suggested replacement target was about 75 percent of hourly fluid loss.
- Dehydration can affect performance after fluid loss exceeds 2 percent of body weight.
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