Phytoestrogens (plant estrogen preparations) are sometimes used in the attempt to improve perimenopausal symptoms, such as hot flashes, night sweats, vaginal dryness or mood swings. They are plant-derived estrogens, chemically similar (even though distincly different) to human and animal estrogens. A number of different phytoestrogen compounds exist. The more commonly known ones are isoflavones. Soybeans are a particularly rich source of phytoestrogens, with their major phytoestrogens being the isoflavones called genistein and daidzein. Another type of phytoestrogen compounds, called coumestans, is found in high concentrations in clover sprouts.
Phytoestrogens can bind to human estrogen receptors and may influence human hormone levels in a variety of different ways. They can exert very weak estrogen-like activity when consumed in large amounts. However, a systematic review concluded that “there is no evidence of effectiveness in the alleviation of menopausal symptoms with the use of phytoestrogen treatments” [ref 1]. Regarding their safety, they have not been studied regarding their potential risk for blood clots. A few case reports have been published of blood clots occurring in relationship to phytoestrogen intake [ref 2,3]. However, these publications do not provide any useful information, as it is impossible to tell whether the association of phytoestrogens and blood clots was causative or incidental.
Black cohosh does not contain phytoestrogens and does not have estrogen-like effects. From a safety point of view it appears fine for the woman with a history of blood clots or clotting disorder to take it (no matter whether she is on or off blood thinners at the time). However, it also appears that black cohosh is not effective in relieving hot flashes [ref 6,7].
I am not aware of any existing guideline regarding the use of phytoestrogens in women with a clotting tendency (thrombophilia) or a history of blood clots. Some noteworthy guidelines/consensus statements:
- A publication by a “Practice Committee of the American Society of Reproductive Medicine” does not take any reference to thrombosis risk [ref 4].
- An American Heart Association Advisory made a general statement that “use of isoflavone supplements in food or pills is not recommended” because efficacy and safety of soy isoflavones are not established [ref 5].
Due to the structural and some functional similarity of phytoestrogens and human estrogens I would advise the woman with a history of blood clots against the use of high doses of phytoestrogens. However, I would also highlight in our discussion that (a) there is an absence of study data and (b) we just don’t know whether phytoestrogens are safe or not. If the woman was on an anticoagulant and wanted to take phytoestrogens, I would not have a major problem with that.
- Lethaby AE et al. Phytoestrogens for vasomotor menopausal symptoms. Cochrane Database Syst Rev. 2007 Oct 17;(4):CD001395. Review.
- Guimarães J et al. Phytoestrogens as a risk factor for cerebral sinus thrombosis. Cerebrovasc Dis. 2005;20(2):137-8.
- Lai Cheong JE et al. Retinal vein thrombosis associated with a herbal phytoestrogen preparation in a susceptible patient.Postgrad Med J. 2005 Apr;81(954):266-7.
- Practice Committee of the American Society for Reproductive Medicine. Status of environmental and dietary estrogens–are they significant estrogens? Fertil Steril. 2006 Nov;86(5 Suppl 1):S218-20.
- Sacks FM et al. Soy protein, isoflavones, and cardiovascular health: an American Heart Association Science Advisory for professionals from the Nutrition Committee. Circulation. 2006 Feb 21;113(7):1034-44. Epub 2006 Jan 17.
- Newton KM et al. Treatment of vasomotor symptoms of menopause with black cohosh, multibotanicals, soy, hormone therapy, or placebo: a randomized trial. Ann Intern Med 2006;145:869-879.
- Geller SE et al. Safety and efficacy of black cohosh and red clover for the management of vasomotor symptoms: a randomized controlled trial. Menopause 2009;16:1156-1166.
Disclosure: I have no conflict of interest relevant to this post.
Last updated: Oct 17th, 2011
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