In fear of becoming dehydrated, some runners actually drink too much water on race day. This can lead to a hyponatremia, a condition caused by dangerously low sodium levels in the blood.
While hydration is crucial for runners, forcing yourself to drink excessive amounts of water can dilute sodium in the body and cause cells to swell, experts say. Symptoms include nausea, vomiting, confusion, loss of energy, muscle weakness and body swelling.
Slower marathon runners can be especially vulnerable to hyponatremia, says David Watt, executive director of the American Running Association and the American Medical Athletic Association.
"The longer you are out there, the more you drink," he says. "(But) if you go out there and overdo it with a sports drink, you can still get in trouble."
According to Watts, such large numbers of competitors suffer hyponatremia that aid stations at many races are now equipped with intravenous saline bags.
How to Avoid Hyponatremia: To keep your sodium in check, you should consume fluids based on your thirst and urine color. Drink at a comfortable and natural rate when you are thirsty, instead of at a pre-defined schedule. If your urine is pale yellow (when you hit the Porta Potty), you are likely well hydrated.