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Here's some bittersweet news for juice drinkers: Grapefruit juice may soon come packaged with a warning label.

There's no doubt that grapefruit juice can be part of a healthy diet. The tart and tasty beverage has no fat, cholesterol or refined sugar, and it's packed with vitamins and minerals. A single serving provides 100 percent of the recommended daily allowance for vitamin C, and it's an excellent source of folate, a B-vitamin that helps prevent certain types of birth defects.

The juice carries the American Heart Association's official stamp of approval -- it contains compounds that may reduce the risk for cardiovascular disease and cancer. A study at the University of Western Ontario showed that grapefruit juice could prevent the growth of breast cancer cells in rodents. Mice receiving grapefruit juice in place of drinking water developed 50 percent fewer breast tumors, and those tumors were less likely to spread.

The juice-drinking mice were also found to have lower levels of low-density lipoprotein (LDL) cholesterol, a substance known to contribute to heart disease. In other trials, rabbits that drank grapefruit juice showed a 32 percent reduction in their LDL cholesterol levels.

With all this good news, you may be tempted to drink up. But there's a downside to guzzling grapefruit juice. In a study at the Harvard School of Public Health, researchers found that a daily 8-ounce serving of grapefruit juice was associated with a 44 percent increase in the risk of kidney stone development. Of the 17 beverages assessed, grapefruit juice was associated with the highest increase in risk, surpassing tea, colas and other citrus juices.

Grapefruit juice may be hazardous to your health in other ways, especially if you take certain medications. Nearly a decade ago, researchers investigating drug interactions made an interesting discovery. Grapefruit juice appeared to have a significant impact on how the body absorbs and degrades certain medicines.

The observation came from a study designed to assess the interaction between alcohol and felodipine, a medication used in the treatment of hypertension and angina. To mask the taste of alcohol, volunteers were given either grapefruit juice or orange juice. Researchers were surprised to discover that blood levels of felodipine were much higher in volunteers receiving grapefruit juice. This chance finding prompted the scientists to investigate the effects of the juice on other drugs.

Additional studies have confirmed that grapefruit juice does indeed interfere with the absorption of medications. Drinking just one cup of grapefruit juice can lead to a whopping nine-fold increase in the concentration of certain drugs in the bloodstream. Among the drugs found to react to grapefruit juice are many calcium channel blockers used to treat hypertension, certain hormones, antihistamines, sedatives and several of the statin drugs used to treat high cholesterol. As research continues, additional drugs are being added to the list.

Not only does grapefruit juice increase the concentration of certain drugs in the bloodstream; it also delays their elimination from the body. Both actions can increase the frequency and intensity of medication side effects.

Scientists have pinpointed the agent responsible for the problem. Naringin, the component that gives the grapefruit its characteristic bitter taste, also appears to interfere with the absorption and elimination of some medications. Naringin blocks the action of enzymes in the liver and small intestine that are responsible for drug degradation. Other citrus fruits don't contain naringin -- the substance is unique to whole grapefruits and grapefruit juice.

Some drug researchers are conducting experiments to determine if this unusual phenomenon can be put to good use. Drinking grapefruit juice could mean reduced drug dosages for some people, which could lower treatment costs. But until more is known, the challenge is to educate consumers about the effects of the juice on the absorption of certain medications.

In spite of efforts to inform the public, there's plenty of work to be done. Only a few pharmaceutical companies have made serious attempts to warn doctors, pharmacists and patients of potential drug-juice interactions. Some consumer advocates have recommended placing warning labels on grapefruit juice containers, but juice makers aren't making any promises. The Food and Drug Administration is currently preparing guidelines that would require manufacturers to place notices on packaging, but the process could take years to complete.

For most folks, grapefruit juice is a wholesome addition to a healthy diet. But for some, it can be a very real health hazard. If you have kidney stones or take medications, ask your doctor if drinking grapefruit juice is safe for you.

Dr. Rallie McAllister is a family physician in Kingsport, Tenn. Her column will appear three times monthly on this page.

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