Quercetin - Studies have shown that grapefruit juice significantly increases estradiol levels in the blood. 1, 2 One of the flavonoids found in grapefruit juice is quercetin. In a test tube study, quercetin was found to change estrogen metabolism in human liver cells in a way that increases estradiol levels and reduces other forms of estrogen. This effect is likely to increase estrogen activity in the body. However, the levels of quercetin used to alter estrogen metabolism in the test tube were much higher than levels found in the body after supplementing with quercetin.
There is evidence from test tube stuudies that another flavonoid in grapefruit juice, naringenin, also has estrogenic activity. It has yet to be shown that dietary or supplemental levels of quercetin (or naringenin) could create a significant problem.
Grapefruit - In a small, controlled study of women with surgically removed ovaries, estradiol levels in the blood were significantly higher after estradiol was taken with grapefruit juice than when estradiol was taken alone. These results have been independently confirmed, suggesting that women taking oral estradiol should probably avoid grapefruit altogether.
Source: Interactions with Supplements
How the Drug/Grapefruit Interaction Works
Cytochrome P-450 is a group of enzymes located throughout the body, with the largest concentration found in the liver and the intestinal walls. This family of enzymes is responsible for triggering the chemical reactions required to breakdown (metabolize) many different compounds, from food to drugs. CYP3A4 is the most abundant member of the cytochrome P-450 enzyme family and is responsible for breaking down approximately 60% of the drugs we take.
Grapefruit — more specifically a compound in grapefruit that has yet to be identified — inhibits the activity of CYP3A4. This means that when grapefruit or grapefruit juice is consumed, a compound within the grapefruit disrupts the CYP3A4 enzyme’s ability to metabolize a drug. If a drug is not adequately metabolized, higher levels of the drug than intended may enter the bloodstream, which can lead to a potentially dangerous situation.
Grapefruit/drug interactions have been observed within a few hours after consuming grapefruit and may last for up to 24 hours. As little as eight ounces or 250 milliliters can have an effect on the metabolism of some drugs.
Drugs that Interact with Grapefruit Juice:
Hormone replacement: cortisol, estradiol, methylprednisolone, progesterone, testosterone
Source: Is Grapefruit Affecting Your Medications?
