Sweet and flavorful grapefruits are naturally low-calorie foods, with a whole grapefruit providing only approximately 100 Calories. Grapefruits are low in protein and fats, so most of the calories come from sugars. They are also low in sodium and cholesterol. However, grapefruits are a great source of Vitamin C and Vitamin A.
The beta carotene and lycopene in grapefruits are thought to act as antioxidants, or substances that prevent cell damage, by removing free radicals (oxidants) in our bodies. Beta-carotene is also a precursor for Vitamin A.
Grapefruits and their close relatives, pomelos, also contain a class of compounds known as furanocoumarins. Plants produce furanocoumarins as defense mechanisms against predators. For humans, however, the furanocoumarins can interfere with the action of various prescription medications, so some medicines contain warnings not to eat grapefruit.