Grapefruit’s medicinal caution
Margaret Beyer, of Dunedin, asked:
Anyone taking cholesterolreducing drugs is advised not to also take grapefruit juice. Why is this so? What is the mechanism of this interaction? New Zealand grapefruit are a slightly different fruit from grapefruit grown in the United States, so does the advice apply to all grapefruit?
Belinda Cridge, a toxicologist at the University of Otago, responded:
It’s commonly accepted that the compounds that cause the interaction between grapefruit and medicines are the compounds naringin, naringenin and bergapten. Of these, naringin is thought to be the most important. It acts as an inhibitor of the enzyme CYP3A, which means it slows it down or stops it working completely.
CYP3A is one of the major enzymes that our bodies use to remove substances such as drugs from the body, thus limiting their action. If you inhibit the enzyme, the drug can build up to higher, possibly toxic, levels. This effect can happen with many different drugs and is why doctors and pharmacists recommend you stay away from grapefruit and grapefruit juices if you are on medication.
Is there a difference with New Zealand grapefruit?
It doesn’t appear so. A group of researchers from the Pharmacy School at Otago measured naringin levels in commercial grapefruit juices in New Zealand and also from handsqueezed fresh fruit (Ho et al, Pharm Acta Helv. 2000 Apr;74(4):37985). They found that the levels of naringin varied between 115mg/L and 656mg/L. This is comparable to fruit and juice from California, which had naringin levels between 104mg/L and 628mg/L (Wanwimolruk and Marquez, Drug Metabol Drug Interact. 2006;21(34):23343).
So, no it does not appear as if New Zealand grapefruit are lacking in this compound and I would expect levels to vary based on season, growing conditions, variety and preparation. And why only grapefruit? The levels of naringin in orange juice and other citrus is fairly low unless it is a sour orange variety. It does depend on the citrus, but, generally, the more sour/bitter flavours are associated with higher naringin content.
To complicate things, naringin itself may lower cholesterol levels, but this is outweighed by the possible toxicity when it is combined with the prescribed cholesterollowering drugs. However, for those that want to stay healthy and are not on medications, grapefruit has some excellent properties.