Hyponatremia

Hyponatremia is an abnormally low concentration of sodium in the blood. Sodium is found mostly in body fluids outside the cells. It is very important for maintaining blood pressure. Sodium is also needed for nerves and muscles to work properly. When the amount of sodium in fluids outside cells drops, water moves into the cells to balance the levels. This causes the cells to swell with too much water. Although most cells can handle this swelling, brain cells cannot, because the skull bones confine them. Brain swelling causes most of the symptoms of hyponatremia.

Severe or rapidly progressing hyponatremia can result in swelling of the brain (cerebral edema), and the symptoms of hyponatremia include bloating and puffiness in the face and fingers, nausea, vomiting, muscle weakness, headache and disorientation. Hyponatremia is most often a complication of other medical illnesses in which either fluids rich in sodium are lost (for example because of diarrhea or vomiting), or excess water accumulates in the body at a higher rate than it can be excreted.

Hyponatremia has increasingly become a problem in high-endurance events such as marathons, ultramarathons, hiking and long military marches. Athletes need to drink regularly, for example during a long race, to prevent dehydration, but excess water can lead to hyponatremia.

Hyponatremia can be prevented in high-endurance activities. A racer can calculate their "sweat rate" ahead of a race by weighing before and after exercising for half an hour at the pace and under the conditions anticipated in the race. For every pound lost through sweating, the racer should drink a pint per hour during the race (for every kilo lost, the racer should drink about a liter per hour during the race). A sports beverage (e.g. Gatorade) that contains carbohydrates and electrolytes, may help prolong an athlete's peak performance and replace sodium lost in sweat. Athletes should also salt their food for several days before a high-endurance event.