When considering what substances we put into our bodies, we tend to put food and drugs into separate categories. Yet what we eat and drink on a daily basis are every bit as capable of altering our biology as pharmaceutical drugs. Just as different drugs can interact with one another, so too can natural chemicals in our food interact with our medication to produce harmful effects. Among the most unusual examples of this is grapefruit.
Grapefruit by itself is perfectly healthy, containing a decent amount of fibre, vitamins C and A, and many more beneficial vitamins and minerals. It also contains some antioxidant compounds that may protect against various diseases including diabetes and heart disease. Unfortunately, there’s also a lengthy list of drugs with which grapefruit can cause harmful interactions, including:
So what makes grapefruit special to give it so many interactions? The answer lies in the way our digestive system handles drugs. Many drugs are broken down in the small intestine by an enzyme called CYP3A4. This means that less than 100% of the medication you take will actually enter your blood, and less still may reach the target organs. This is something that drug manufacturers and doctors have to account for when calculating the dosage for a given drug.
Grapefruit interferes with this system because it contains compounds called furanocoumarins that inhibit CYP3A4. This means that less drug is broken down and more enters the bloodstream than intended, resulting in an overdosing. If you consume enough grapefruit, it is even possible to reduce the liver’s ability to remove drugs from the blood, as the liver also uses CYP3A4 enzyme. It hardly needs to be stated why this is a bad thing, especially for drugs with a low therapeutic index – that is to say, where a very small rise in blood concentration can turn a drug from safe to toxic. Grapefruit can interact with drugs taken as long as 3 days after its consumption.
Different people have different amounts of CYP3A4, meaning that it is impossible to account for the effects of grapefruit, as they vary from person to person. That’s not all, though: strangely, while grapefruit can cause overdosing of some drugs, more recent studies have found that it can actually cause underdosing of others. That’s because compounds in grapefruit can also block the proteins that transport certain drugs into our blood or into our cells. This leads to less drug reaching its target, making it less effective.
At this point, you may be wondering why grapefruit would have evolved the ability to affect any of these proteins. Firstly, it should be noted that grapefruit is not the only fruit that contains furanocoumarins, simply the most significant and most studied. Other citrus fruits can also interact with drugs, and this will be indicated on the packet when relevant. As for why some plants produce these chemicals, they appear to be a defence mechanism against fungi and small animals. Furanocoumarins are toxic in sufficient quantities, and some are also phototoxic, meaning they can damage DNA in the presence of ultraviolet light.
The presence of these compounds shouldn’t deter you from including citrus fruits in your diet, provided you aren’t taking any drugs that could be affected by them. These fruits remain very beneficial overall and are not dangerous for humans outside of drug interactions.
Grapefruit Juice and Some Drugs Don't Mix: https://www.fda.gov/consumers/consumer-updates/grapefruit-juice-and-some-drugs-dont-mix#:~:text=Here%20are%20examples%20of%20some,Adalat%20CC%20(both%20nifedipine).
Time course of recovery of cytochrome p450 3A function after single doses of grapefruit juice: https://doi.org/10.1016/S0009-9236(03)00118-8