Estradiol Interactions with Dietary Supplements

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.³ 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.⁴ It has yet to be shown that dietary or supplemental levels of quercetin (or naringenin) could create a significant problem.

Vitamin D
In controlled studies, the addition of 300 IU per day of vitamin D3 (cholecalciferol) did not improve the bone-preserving or fracture-preventing effects of hormone replacement with estradiol plus a progestin (a synthetic form of progesterone) in postmenopausal women without osteoporosis.^{5 6} However, in a controlled study of osteoporotic women, only those receiving both hormone replacement and vitamin D had increases in bone density of the hip; no improvement occurred in the hip with hormones alone.⁷ More research is needed to determine conclusively when vitamin D is important to add to hormone replacement.