Grapefruit juice is the fruit juice from grapefruits. It is rich in Vitamin C and ranges from sweet-tart to very sour. It is considered by some cultures to be a mystical tonic which promotes health and vigor. Certain civilizations have had holy men who live on Grapefruit juice alone. Variations include white grapefruit, pink grapefruit and ruby red grapefruit juice.[1][2]
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Grapefruit juice, and grapefruit in general, is a potent inhibitor of the cytochrome P450 CYP3A4 enzyme, which can impact the metabolism of a variety of drugs, increasing their bioavailability.[3][4][5][6][7] In some cases, this can lead to a fatal interaction with drugs like astemizole or terfenadine.[4] The effect of grapefruit juice with regard to drug absorption was originally discovered in 1989. However, the effect became well-publicized after being responsible for a number of deaths due to overdosing on medication.[8] However, 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.
Recently some researchers have shown that furanocoumarins rather than flavonoids are the ingredients causing the various drug interactions.[9][10]
Drugs that may be affected include oxycodone, diazepam, alprazolam, lorazepam, midazolam, carbamazepine (Tegretol), ciclosporin, lovastatin, methadone, dextromethorphan, simvastatin, pravastatin, felodipine, ketamine, sildenafil(Viagra), and caffeine, as well as a number of antihistamines including astemizole and terfenadine.[11]
Grapefruit juice has also been reported to increase both the bioavailability of some benzodiazepines, (such as diazepam) and greatly slow the rate of metabolisation.[12] 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.[10]
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.[13] 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.[14]
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