The kidneys pass riboflavin out in the urine, so when there is too much riboflavin in the body, the part that one does not want will not stay in the blood for long. If there is much riboflavin in the urine, the urine becomes bright, fluorescent yellow. Vitamin pills, or a meal with a lot of liver or egg white turns the urine yellow because such foods put so much riboflavin into the blood.