The power of grapefruit
Winston Zhu explores how grapefruit can impact the medication we take.
Grapefruit is delicious. But did you know that the fruit and its juice contain active ingredients that alter how medications affect our body? Not only is this important for those prescribed medication, it also provides an insight into how our body processes drugs.
What happens when we take medicine orally? Just like food, the drug moves through the gut and does a ‘first pass’ of our detoxification systems. In doing so, a significant amount of the drug is deactivated before it even reaches our circulation. This phenomenon is aptly named ‘first pass’ or ‘presystemic’ metabolism, and is a major reason why certain orally administered medicines have a bioavailability - the proportion of the dose that is actually absorbed into circulation - averaging as low as 15%.
Enzymes in the liver and gut wall belonging to the cytochrome P450 family are vital to drug metabolism. These are also where grapefruit juice has its effect. Active ingredients in the fruit irreversibly inhibit a particular enzyme in this class (CYP3A4 if you’re curious). This significantly reduces our body’s ability to degrade drugs before they reach our circulation, causing an increase in how much gets into our body. For example, a glass of grapefruit juice can increase systemic levels of the cholesterol lowering drug lovastatin by 90-260%.
Generally speaking, the chemicals in grapefruit juice don’t react directly with the medicines we take and increase their potency. Rather, they achieve an amplifying effect by decreasing the body’s ability to metabolise the drugs. One exception to this rule (there’s always one!) is when drugs are administered in an inactivated form, and are actually activated by their interaction with CYP3A4. These drugs are called ‘prodrugs’. In this specific instance, grapefruit juice can negate rather than amplify their therapeutic effects.
Since the reaction between grapefruit juice and CYP3A4 is irreversible, it has long-lasting effects on our body. New enzymes must be synthesised to replace those inhibited in order for normal function to be recovered. As such, waiting a few hours after consumption may not be enough, and effects could last 1-3 days after you’ve eaten grapefruit .
The effect of grapefruit juice varies massively from drug to drug. Based on the process outlined above, the drugs which rely most heavily on CYP3A4 metabolism are those most affected by consumption of grapefruit juice. Think of this phenomenon as a force (CYP3A4) pushing down on a spring (the medicine). The greater the force, the greater the spring back when the force is removed (by grapefruit). If there is no force to begin with, then there will be no spring back! i.e. You don’t need to worry about eating grapefruit alongside medicines that are not metabolised by CYP3A4.
So what actually happens if we mix grapefruit with the wrong drug? As the interaction results in higher levels of the drug in the body, we can expect the same side effects as we would for an overdose. For the antiarrhythmic drug amiodarone, this can result in a dangerous quickening of heart rate (tachycardia). Lovastatin, mentioned above, mixed with grapefruit juice may increase risk of muscle deterioration, releasing the contents of muscle cells into the blood which, if left untreated, can lead to kidney failure (rhabdomyolysis).
Whilst the interaction between our body and grapefruit juice may lead to harmful amplification of certain medications, it might also be possible to harness its power for the benefit of patients. Supplementation with grapefruit juice might allow us easier access to drugs which are normally completely metabolised by CYP3A4. Usually, these drugs have to be administered intravenously to bypass first pass metabolism, but grapefruit juice could theoretically be used in the future to save us a painful injection!
Drug doses are carefully measured to ensure maximal benefits and minimal side effects. Physicians and pharmacists should therefore educate patients about sensible grapefruit consumption alongside medication. The NHS provides a non-exhaustive list of some medications known to show this phenomenon, and how to seek further advice from healthcare professionals if ever in doubt.