Why Should Foods Such as Grapefruit and Pomegranate Be Avoided During Cancer Treatment?

Substances in some foods and alternative medicine products may interact with your cancer medicine, causing treatment side effects or reducing the medicine’s effectiveness. Grapefruit is widely known to interact with cancer medicines. But is grapefruit the only food that interacts with chemotherapy medicines? Are there other foods or products that can affect your treatment, and how do they do so?

To work, all medicines, including cancer medicines, must be broken down and absorbed in the intestines after entering the body, distributed through the blood and metabolized. They must then be eliminated from the body. When you take medicines orally as tablets, capsules or liquids, this process begins with intestinal breakdown and absorption and continues through metabolism. The same is actually true of food: after being eaten, it is broken down in the intestines, digested and absorbed into the blood. Absorbed nutrients are first transported to the liver, where metabolism continues.

Some substances entering the body become part of our structure, some are stored (for example, as fat) and some are excreted. A medicine being metabolized means that the medicine undergoes biological transformation, and this process is also called biotransformation . This metabolism occurs in the liver, kidneys, intestines, skin and lungs through various enzyme systems. The most important are CYP3A enzymes.

What Are CYP3A Enzymes?

We know that a group of bodily substances called cytochrome P (CYP) enzymes participates in intestinal absorption and metabolism in other organs.

There are many CYP enzymes, including a group called CYP3A. This group contains enzymes called CYP3A4 and CYP3A5. They are important in determining how medicines are absorbed from the intestines.

Some foods and medicines affect CYP3A. These enzymes may affect the amount of cancer medicine absorbed, making the medicine stronger or weaker.

Foods to Avoid During Cancer Treatment

Grapefruit is one of the most common foods to avoid; it contains a substance that blocks (inhibits) CYP3A. However, this substance also occurs in varying concentrations in other fruits.

• grapefruit and its juice
• blood orange 
• pomegranate 
• star fruit (carambola)

During cancer treatment, these fruits should not be consumed regularly, particularly as freshly squeezed juices. However, a patient who ate them without knowing this should not feel they have made a terrible mistake. If you strongly crave them, eating a slice of grapefruit or 1 spoonful of pomegranate is not objectionable, but you should not increase intake to normal consumption amounts.

Medicines

Many different types of medicines may affect CYP3A enzyme activity, including:

• some antibiotics (erythromycin, clarithromycin)
• some cancer medicines (ifosfamide, tamoxifen, vinblastine, imatinib)
• antifungal medicines
• HIV treatments
• epilepsy medicines (anticonvulsants)
• medicines called calcium channel blockers
• antidepressants
• statins (cholesterol-lowering medicines)
• steroids

Herbal Supplements (Phytotherapy Products)

Many different herbal supplements also strongly affect CYP3A enzymes: 

• St. John’s wort 
• Black cohosh
• Ginseng 
• Ginkgo biloba 
• Milk thistle

For these reasons, it is very important to tell your oncologist about herbal supplements, regular medicines, concentrated nutritional products and any special diet you follow. Your doctor can tell you whether any medicine or supplement you take may affect CYP3A enzymes and therefore your cancer treatment. If you must take a medicine affecting the CYP3A enzyme for another health condition, your doctor will adjust the dose of your cancer medicine.