A recent study found that women who ate half of a grapefruit every other day had a 30 percent increase of breast cancer risk when compared to women who didn’t eat the fruit. Grapefruit contains the chemical compound furanocoumarins that inhibits the way that estrogen is metabolized. This leads to a higher concentration of estrogen, which increases the risk of breast cancer.
It is recommended that post menopausal women that consume grapefruit regularly and women that have had estrogen-receptor-positive cancer should not eat this fruit. The American Cancer Society recommends eating five servings of a variety of fruits and vegetables each day to limit the exposure to possibly harmful substances from a single food.