Tiredness and cramps in athletes can have many causes, but lack of potassium in their diets is not one of them. Many years ago, Dave Costill of Ball State University tried to create potassium deficiency in 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.
The kidneys and sweat glands conserve potassium so well that you don’t lose much. If an athlete develops potassium deficiency, it is usually caused by drugs, such as diuretics or corticosteroids, or by diarrhea or repeated vomiting. Some athletes try to control their weight by making themselves vomit. This is called bulimia, and the person usually denies vomiting. Their physicians can demonstrate that they are vomiting by ordering blood and urine tests. If blood levels of potassium are low and urine levels are high, vomiting is a likely cause. Ask your doctor to do a work-up for other causes of chronic tiredness. If none can be found, you may overtraining and should talk to your coach or a personal trainer about changing your workouts.
Also see The Potassium Deficiency Myth