Grapefruit juice is the fruit juice from grapefruits. It is rich in Vitamin C and ranges from sweet-tart to very sour. Variations include white grapefruit, pink grapefruit and ruby red grapefruit juice.
Grapefruit juice, and grapefruit in general, is a potent inhibitor of the cytochrome P450 CYP3A4 enzyme, which can affect the metabolism of a variety of drugs, increasing their bioavailability. In some cases, this can lead to a fatal interaction with drugs like astemizole or terfenadine. The effect of grapefruit juice with regard to drug absorption was originally discovered in 1989. The first published report on grapefruit drug interactions was in 1991 in the Lancet entitled "Interactions of Citrus Juices with Felodipine and Nifedipine," and was the first reported food-drug interaction clinically. However, the effect only became well-publicized after being responsible for a number of deaths due to overdosing on medication.[NOTE: the article referenced in  says nothing about deaths, only aching muscles]
Grapefruit juice has also been reported to increase both the bioavailability of some benzodiazepines, (such as diazepam) and greatly slow the rate of metabolization. An easy way to tell if a medication may be affected by grapefruit juice is by researching whether another known CYP3A4 inhibitor drug is already contraindicated with the active drug of the medication in question. Examples of such known CYP3A4 inhibitors include cisapride (Propulsid), erythromycin, itraconazole (Sporanox), ketoconazole (Nizoral), and mibefradil (Posicor) .
The flavonoid existing in highest concentration in grapefruit juice is naringin, which in humans is metabolised to naringenin. Other flavonoids exist in grapefruit juice in lower concentrations as well. Orange juice does not contain naringin in as high a concentration, instead containing hesperetin. It is sometimes recommended as a substitute. Juice of limes and Seville oranges can also inhibit drug metabolism, however, as can apple juice with some drugs.
A study published in the July 2007 edition of the British Journal of Cancer reported that eating grapefruit every day could raise the risk of developing breast cancer by almost a third. The study found that in the test subjects, 50,000 post-menopausal women, eating just a quarter of a grapefruit daily raised the risk by up to 30%. It is believed that the fruit boosts the levels of estrogen, which in turn increases the risk of developing the disease. However, a 2008 study has shown that grapefruit consumption does not increase breast cancer risk and has found a significant decrease in breast cancer risk with greater intake of grapefruit in women who never used hormone therapy.
In 2009, a third study conducted by European Prospective Investigation into Cancer and Nutrition (EPIC) observed 114,504 women and their dietary intake of grapefruit. This study concluded that there was no association between grapefruit intake and estradiol or estrone among postmenopausal women. Researchers found no evidence of an association between grapefruit intake and risk of breast cancer among this group.
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