Warning on fruit juice and drug interactions

September 6, 2008

Sorry to be still on the fruit juice trail, but news this week had a real deja-vu quality about it for me. In the 1980′s, I was involved with a naturopath in the writing of a book called ‘The Medicine Chest’ which looked at the relationship and interaction between foods, vitamins and medicines. One of the things we flagged up then was how food can affect your medication. One example of this is the drug warfarin which interacts with vitamin K, which we get from food, and from the bacteria in our gut. Vitamin K is involved in the formation of special liver proteins, known as coagulation factors, which reduce the risk of haemorrhage or bleeding. Conversely, if you are susceptible to blood clotting, warfarin (because of how it interferes with the formation of these vitamin-K-dependent factors) may be prescribed for you. So you can see that if you increase the amount of vitamin K-rich foods then you can alter the effect that the warfarin will have in your body. Such foods include everyday items like spinach, lettuce, beef, broccoli and soy beans – good foods in themselves but to be discussed with your doctor if you are on warfarin.

Now the scientific world seems to have caught up with the research done by naturopaths over the years, which has always treated food as ‘medicine’ and been much more aware of its effects. Recent research presented at a US conference has now suggested that a chemical in grapefruit, orange, and possibly also apple juice, could stop anti-allergy drugs being absorbed properly. Grapefruit juice is already known to interfere with blood pressure drugs and indeed some medicines carry a warning that taking them alongside grapefruit juice could cause an overdose and normally your pharmacist will point this out to you. However, the latest finding shows that grapefruit juice had the reverse effect on fexofenadine, an antihistamine drug, making it less rather than more potent. Volunteers took the drug with either a single glass of grapefruit juice, or just water.

When it was taken with juice, only half the drug was absorbed, potentially reducing its effectiveness. The active ingredient of the juice, naringin, appears to block a mechanism which moves drug molecules out of the small intestine into the bloodstream and this substantially decreases the absorption of certain drugs.

The three juices mentioned have also been found to affect etoposide, a chemotherapy drug, some beta-blocker drugs used to treat high blood pressure, and cyclosporine, taken by transplant patients to prevent rejection of their new organs. Dr David Bailey of the University of Western Ontario, the study’s author, said: “This is just the tip of the iceberg – I’m sure we’ll find more and more drugs that are affected this way.”


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