More threads by David Baxter PhD

David Baxter PhD

Late Founder
Experts Reveal the Secret Powers of Grapefruit Juice
March 21, 2006
By Nicholas Bakalar

In 1989, a group of Canadian researchers studying a blood pressure drug were astonished to discover that drinking a glass of grapefruit juice dangerously increased the drug's potency.

Refreshing but Risky
They were testing the effects of drinking alcohol on a medicine called Plendil. The scientists needed something that would hide the taste of alcohol so that subjects would know only that they were taking the drug and not know whether they were drinking alcohol with it.

"One Saturday night, my wife and I tested everything in the refrigerator," said David G. Bailey, a research scientist at the London Health Sciences Center in London, Ontario, and the lead author on the study. "The only thing that covered the taste was grapefruit juice."

So they used it in their experiment, expecting the grapefruit juice to be irrelevant to their results. But blood levels of the drug went up significantly in the control group that drank just grapefruit juice, without alcohol.

"People didn't believe us," Dr. Bailey said. "They thought it was a joke. We had trouble getting it published in a major medical journal."

Eventually the paper was accepted and published by Lancet, in February 1991.

Finding why juice had that effect was the next question.

The answer, it turned out, lay in a family of enzymes called the cytochrome P-450 system, in particular one known as CYP 3A4. This enzyme metabolizes many drugs, and toxins as well, into substances that are less potent or more easily excreted or both.

Grapefruit juice interferes with the ability of CYP 3A4 to do that, increasing the potency of a drug by letting more of it enter the bloodstream, in effect producing an excessive dose.

Grapefruit interacts with this enzyme only in the intestines, not in the liver or other places where it is found. As a result, the effect is seen only with medicines taken orally, not with injected drugs.

Numerous studies now show the interaction of grapefruit juice with many widely used medicines. Most interactions have no serious consequences, but a few do. For example, drugs used to lower cholesterol, like Lipitor, Mevacor and Zocor, have increased potency when taken with grapefruit juice. Excessive levels of those drugs can lead to a serious and sometimes fatal muscle disorder called rhabdomyolysis.

Does this mean a person could reduce the amount of medicine required simply by drinking grapefruit juice? No, according to Dr. Bailey.

"The problem is the unpredictability of the effect," he said. "You can't just lower your dose of Lipitor and increase your consumption of grapefruit juice. There's no uniformity from one individual to another or from one bottle of grapefruit juice to the next.

"There's huge variation in the amount of enzyme people have in their guts. Fooling around with grapefruit juice is not a good idea."

Grapefruit juice can also interfere with the metabolism of selective serotonin reuptake inhibitors, or S.S.R.I.'s, like Prozac, which are used to treat depression.

Dr. Marshall Forstein, an associate professor of psychiatry at Harvard, said he told patients to switch from grapefruit juice to something else because most oranges and other citrus fruits do not have the same effect.

"If they insist," Dr. Forstein said, "I try to prescribe the S.S.R.I. or other medication to be taken at a time when the grapefruit juice would have mostly been metabolized."

Among fruit juices, grapefruit has the strongest effect, but lime juice and orange juice made from Seville oranges similarly inhibit the CYP 3A4 enzyme. With some drugs, apple juice may interact in the same way.

While Dr. Bailey suggests avoiding grapefruit juice entirely when taking medicine, some experts say the effect of the juice should not be exaggerated.

"The circumstances under which an interaction will occur are relatively unusual," said Dr. David J. Greenblatt, a professor of pharmacology at Tufts. First, he said, the drug has to be metabolized significantly by intestinal CYP 3A4, and relatively few are. "When you look at the actual data for each drug, the scientific conclusions are that the interactions are unusual, sometimes quite small and not of clinical importance. But there are some cases in which it's significant."

Dr. Greenblatt and his co-investigators at Tufts have conducted research sponsored by the National Institutes of Health in this field for years, and he has been a paid consultant to the Florida Citrus Commission.

Dr. Richard B. Kim, a professor of medicine and pharmacology at Vanderbilt University, agreed that the interaction was a serious health concern in some patients.

"Grapefruit consumption is a clinically relevant issue, especially for the elderly, who are most likely to be taking the drugs affected by it," Dr. Kim said. "If you're taking multiple medications, or have recently switched to a different type of medication, you should be particularly careful. The easiest thing to do under those circumstances is to take the medicine with water and avoid the juice completely."


thanks for this article Dr. B.. gonna print it ang give it to my mum. she loves grapefruit jiuce and takes lipitor.. maybe she can show this to her doc. she hasn't been well for awhile so her doc can look at this and either rule out or in a connection.
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