Many sports drinks advertise that they contain potassium, but potassium deficiency is not common in athletes unless they are trying to control weight by vomiting.

A few years ago, one of the best female long-distance runners in the country came to me to find a cause for her sudden drop in performance. All tests I ordered were normal except for a low blood level of potassium. The most common cause of potassium deficiency is vomiting, but she repeatedly denied doing this. I then requested that she collect her urine for one day, and the laboratory reported that it contained three times as much potassium as normal. This proved that she was bulimic. To control her weight, she was sticking her finger down her throat and making herself throw up. After she was able to accept the diagnosis, she got help, stopped vomiting and went on to win several long distance running titles.

The kidneys and sweat glands conserve potassium so effectively in response to low body levels that potassium deficiency rarely occurs in healthy athletes. Even with prolonged exercise in very hot weather, potassium needs can be met by eating a normal diet because potassium is found in virtually all foods except refined sugar. On the other hand, potassium deficiency can be caused by drugs, such as diuretics and corticosteroids. It can also be caused by diarrhea or repeated vomiting. With diarrhea, potassium is lost in the stool. With vomiting, potassium is lost in the urine. In both athletes and non-athletes, the most common cause of low potassium blood levels and high potassium urine levels is vomiting.