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.
Drug interactions 
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.
Grapefruit juice has also been reported to increase both the bioavailability of some benzodiazepines, (such as diazepam) and greatly slow the rate of metabolisation. 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.
See also 
- The World's Healthiest Foods; Grapefruit. The George Mateljan Foundation. Article
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- He K, Iyer KR, Hayes RN, Sinz MW, Woolf TF, Hollenberg PF (April 1998). "Inactivation of cytochrome P450 3A4 by bergamottin, a component of grapefruit juice". Chemical Research in Toxicology 11 (4): 252–9. doi:10.1021/tx970192k. PMID 9548795.
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- Bailey DG, Dresser GK (2004). "Interactions between grapefruit juice and cardiovascular drugs". American Journal of Cardiovascular Drugs 4 (5): 281–97. doi:10.2165/00129784-200404050-00002. PMID 15449971.
- Bressler R (November 2006). "Grapefruit juice and drug interactions. Exploring mechanisms of this interaction and potential toxicity for certain drugs". Geriatrics 61 (11): 12–8. PMID 17112309.
- Bakalar, Nicholas. Experts Reveal the Secret Powers of Grapefruit Juice. New York Times. Published: March 21, 2006. Article
- http://www.hon.ch/News/HSN/532584.html hon.ch
- Bakalar, Nicholas (2006-03-21). "Experts Reveal the Secret Powers of Grapefruit Juice". The New York Times. p. F6. Retrieved 2006-11-21.
- Ozdemir M, Aktan Y, Boydag BS, Cingi MI, Musmul A (1998). "Interaction between grapefruit juice and diazepam in humans". European Journal of Drug Metabolism and Pharmacokinetics 23 (1): 55–9. doi:10.1007/BF03189827. PMID 9625273.
- "Health | Grapefruit link to breast cancer". BBC News. 2007-07-16. Retrieved 2010-03-15.
- Kim EH, Hankinson SE, Eliassen AH, Willett WC (January 2008). "A prospective study of grapefruit and grapefruit juice intake and breast cancer risk" (PDF). Br. J. Cancer 98 (1): 240–1. doi:10.1038/sj.bjc.6604105. PMC 2359690. PMID 18026192. Retrieved 26 June 2009.