Nabbed: culprit of grapefruit juice--drug interaction.

Drinking grapefruit juice is a medical no-no for people who take any of several widely prescribed drugs. The drink affects how the body metabolizes the medications. Now, researchers have pinned down the class of natural juice compounds that's responsible for the unwanted chemical interaction.

Researchers discovered around 1990 that grapefruit inhibits the enzyme CYP3A4, which participates in the metabolism of about half of all prescription drugs. Inhibition of that enzyme causes drugs to stay in the body longer, potentially overdosing the patient. Doctors subsequently advised many patients not to consumer the juice while using certain medications.

Paul B. Watkins of the University of North Carolina at Chapel Hill and his colleagues tested the idea that compounds called furanocoumarins, which are abundant in grapefruit juice but scarce or absent in most other citrus juices, are the metabolism-altering culprits.

The researchers filtered and processed grapefruit juice to remove its furanocoumarins. Then they gave 18 healthy volunteers either the processed juice, normal grapefruit juice, or orange juice. The volunteers also took felodipine, a blood pressure--lowering medication that's known to interact with normal grapefruit juice.

Over the next 24 hours, the researchers monitored felodipine concentrations in each volunteer's blood. The drug lingered about twice as long in volunteers who had consumed normal grapefruit juice as it did in volunteers who'd drunk furanocoumarin-free grapefruit juice or orange juice. The findings appear in the May American Journal of Clinical Nutrition.--B.H.