High in fibre, Grapefruit has significant amounts of nutrients, namely Vitamin C, Vitamin B1 Thiamine, Vitamin B9 – Folate as well as potassium and pectin.
The active constituents of Grapefruit are naringin, which are flavonoids as well as furanocoumarins like bergamottin.
Increased bioavailability and plasma concentrations of numerous drugs potentially lead to toxicity. Grapefruit with its high flavonoid content – limonite, quercetin, kaempferol, naringenin and obacunone inhibit the hepatic microsomes containing the phase 1 enzymes which increases the bioavailability of various drugs. Check with the GP for best practice. If drinking Grapefruit juice separate from the drug by a few hours.
Grapefruit may interfere with quinine found in tonic water, as it has an inhibitory effect on cytochrome P450. Avoid in those with cardiac rhythm disorders such as long QT syndrome that may worsen with quinine. This is a red flag.
Also, red wine with grapefruit has an additive inhibitory effect on phase I enzyme. In theory, increasing the risk for interactions with other drugs. Research has not found this to be the case with white wine.
Hermans et al 2003 and Chan et al 1998