According to wikipedia betacyannin is what causes the beet to be the color it is, and for individuals who can't break it down, it causes they're waste products to turn a disconcerting shade of pink or red. Only about 14%of the population has difficulty breaking this down.
One source I read suggested that it meant the individual was iron deficient, however after talking to a doctor, they said they did not believe that to be true.
