Sports drink promoters have convinced many athletes that they need special drinks to replace potassium during exercise. A recent study of female soccer players confirms that this is a myth ( International Journal of Sports Medicine, June 2009). When body levels of potassium are low, the kidneys and sweat glands conserve potassium so effectively that potassium deficiency rarely occurs.
Tiredness in healthy athletes can have many causes, but low potassium is not one of them. Many years ago, Dave Costill of Ball State University tried to create potassium deficiency in healthy national champion runners. He couldn't do it because potassium is found in all foods except refined sugar, and his athletes would not stay on a diet that consisted only of hard candy. Even with prolonged exercise in very hot weather, potassium needs can be met by eating virtually any food.
Potassium deficiency CAN be caused by certain drugs, such as diuretics or corticosteroids, or by severe diarrhea or repeated vomiting. 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. I knew that hard exercise does not cause potassium deficiency and that 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 bulemic. To control her weight, she was sticking her finger down her throat to makie herself throw up. After she was able to accept the diagnosis, she got help, stopped vomiting and went on to win several national long distance running titles.
With vomiting, you throw up the stomach's acid (hydrogen) and the blood becomes alkaline. This causes the kidneys to retain hydrogen and consequently lose huge amounts of potassium 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.