Hyponatremia: An abnormally low concentration of sodium in the blood. Too little sodium can cause cells to malfunction, and extremely low sodium can be fatal. Hyponatremia has many causes including medications such as diuretics and antidepressants, hypothyroidism, cortisone deficiency (such as in Addison's disease), dehydration, vomiting or diarrhea, severe burns, kidney or heart failure, and cirrhosis.

The symptoms of hyponatremia include bloating and puffiness in the face and fingers, nausea, vomiting, muscle weakness, headache and disorientation.

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.

Women appear more prone to hyponatremia than men. About half the women studied who finished the New Zealand Ironman triathlon developed hyponatremia, compared with 14 percent of the men, according to a report in the journal Medicine and Science in Sports and Exercise.

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. (or in metric terms, 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.

Sodium is the major positively charged ion (cation) in the fluid outside of cells of the body. The chemical notation for sodium is Na. When combined with chloride, the resulting substance is table salt. The normal blood sodium level is 135 - 145 milliEquivalents/liter (mEq/L), or in international units, 135 - 145 millimoles/liter (mmol/L).