According to new research, it is not the flavonoids in grapefruit that interact dangerously with some drugs, it is the furanocoumarins. Some medications, such as those for cholesterol and hypertension, are affected by furanocoumarins so that they enter the bloodstream too quickly.

You can read about this study in the American Journal of Clinical Nutrition.

There is an enzyme in our intestine which destroys a quantity of some drugs, thus slowing down the amount entering the bloodstream. Furanocoumarins inhibit the enzyme, thus more of the drug enters the bloodstream.

Patients on some medications have to avoid grapefruit juice. Others take grapefruit juice in order to get more of the drug into their system. It depends which drug the patient is on.

Researchers at the University of North Carolina at Chapel Hill, USA, proved that furanocoumarins were the culprits, not flavonoids. They did this by giving volunteers two types of grapefruit juice. One had furanocoumarins in it, while the other didn't - it had been taken out.

The volunteers were divided into three groups. They were all given 10 milligrams of Felodipine, a hypertension drug. One group consumed fresh orange juice, another had normal fresh grapefruit juice, and the third group had grapefruit juice with the furanocoumarins removed.

After a series of blood tests the researchers found that the absorption rate of the drug into the bloodstream had been altered by the regular grapefruit juice group only - the group that was given grapefruit juice with the furanocoumarins left in.

Dr. Paul Watkins, one of the researchers, said that this study gives clear evidence that furanocoumarins interacted with the drug in a way that accelerated the rate at which it entered the bloodstream. When they got rid of the furanocoumarins from the grapefruit juice, the interaction was not present.

Perhaps, for patients on such medications who wish to continue drinking grapefruit juice, makers should consider making this juice with the furanocoumarins removed, said the researchers. They added that it would be plausible to screen foods for the presence of furanocoumarins.

Furanocoumarins could also be useful in helping some drugs to enter the bloodstream more easily - some orally administered drugs.

See the abstract in the American Journal of Clinical Nutrition A furanocoumarin-free grapefruit juice establishes furanocoumarins as the mediators of the grapefruit juice-felodipine interaction

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