Nutrition: grapefruit juice

PUBLISHED : Tuesday, 14 August, 2012, 12:00am
UPDATED : Tuesday, 14 August, 2012, 1:24pm

It's widely known that taking grapefruit juice with certain medications can make the drugs more potent, causing dangerous side effects. But what if it could be used in a controlled fashion to increase the efficacy of a drug?

A clinical trial by researchers at the University of Chicago Medicine has shown that a glass of grapefruit juice daily lets patients get the same benefits from an anti-cancer drug with a much lower dose. This could not only help patients avoid the side effects associated with high drug doses, but also lower medication costs.

In the study, published this month in Clinical Cancer Research, patients who drank a glass (235 millilitres) of grapefruit juice a day slowed the body's metabolism of the drug called sirolimus, increasing the drug levels in the patients by 350 per cent. A drug called ketoconazole that also slows drug metabolism increased sirolimus levels by 500 per cent.

The study was supported by the US National Institutes of Health and not by a pharmaceutical company. Sirolimus was the first of a series of drugs developed to prevent rejection of transplanted organs but that also have anti-cancer effects.

Grapefruit juice has the ability to inhibit enzymes in the intestines that break down sirolimus and several other drugs. The effect begins within a few hours of consuming the juice and wears off over a few days.

Previous studies have shown that the active ingredients in grapefruit juice that are responsible for the drug interactions are chemicals called furanocoumarins, which attach themselves to enzymes and destroy their ability to interfere with drug absorption.

The 138 patients enrolled in the clinical trial, all with incurable cancer and no known effective therapy, were divided into three groups. One received only sirolimus; one, sirolimus plus ketoconazole; and the third, sirolimus plus grapefruit juice.

The optimal cancer-fighting dose for those taking only sirolimus was about 90 milligrams per week. At doses above 45mg, however, the drug caused serious gastrointestinal problems, such as nausea and diarrhoea, so patients switched to 45mg twice a week.

The optimal doses for the other groups were much lower. Patients taking the drug plus ketoconazole needed only 16mg per week to maintain the same levels of drug in the blood, and the sirolimus plus grapefruit juice group needed between 25 and 35mg per week.

Although ketoconazole proved slightly stronger in drug retention, grapefruit juice is non-toxic, with no risk of overdose.

The effect of grapefruit juice can vary among individuals, but tests of enzyme levels in patients may be able to predict one's response.