Just a note to add that if you are taking certain medications (for example, statins to control your cholesterol levels), grapefruit juice can cause serious interactions. So please check with your pharmacist and/or MD about your meds if you are adding grapefruit or other citrus to your diet.

Here are simple explanations:
http://www.mayoclinic.com/health/food-and-nutrition/AN00413/
http://www.fda.gov/ForConsumers/ConsumerUpdates/ucm292276.htm