Compounds such as genistein, naringenin, kaempferol, and lignans such as enterodiol and enterolactone, were not linked to increased incidence of adverse events, according to an analysis of 92 randomised controlled trials with almost 10,000 participants published in The American Journal of Medicine.
“Our findings have implications for women seeking safe […] alternatives to hormone replacement therapy,” wrote the researchers, led by Clemens Tempfer from the Medical University Vienna.
“Based on our findings, they can be assured that there is no indication of serious unwanted side effects of phytoestrogen treatment such as those found in women on hormone replacement therapy, for example, thrombosis, myocardial infarction, stroke, and breast cancer,” they added.
“It has to be acknowledged, however, that our study does not make any statement about the efficacy of phytoestrogen treatment, which has been discussed controversially.”
Soy isoflavones are naturally occurring oestrogen-like compounds, and supplements are currently marketed as a way of reducing symptoms of the menopause and offer an alternative to hormone replacement therapy.
Conflicting reports however have clouded the picture about the beneficial effects of soy isoflavones, with some studies indicating that breast cancer cells in mice were stimulated by the isoflavones. Population studies have shown that women with a high-soy diet generally have lower rates of breast cancer.
The new meta-analysis looked at all isoflavones, and identified 92 randomised controlled trials with 9,629 participants.
Comparing the phytoestrogen groups to the placebo or control groups, Tempfer and his co-workers report that the incidence of side effects was approximately the same, with 36.7 and 38 per cent, respectively.
While higher rates of gastrointestinal side effects were recorded in the users of phytoestrogen, no significant differences were observed regarding gynaecological, musculoskeletal, or neurological side effects, added the researchers.
Furthermore, hormone-related side effect rates, including breast and endometrial cancer, were not significantly different between groups.
EFSA’s opinion
Earlier this year data was sent to a European Food Safety Authority (EFSA) working group investigating isoflavones following a symposium in Italy. Twenty isoflavone experts met in Italy to examine studies collected over the last 20 years, and as well as newer trials, and concluded that emerging human studies in isoflavones demonstrate the “modest but valuable benefit for menopause relief”.
The scientists concluded the soy and red clover-derived isoflavones do not increase the risk of breast cancer and can offer very real relief to post-menopausal women.
Source: The American Journal of Medicine
October 2009, Volume 122, Issue 10, Pages 939-946.e9
“Side Effects of Phytoestrogens: A Meta-analysis of Randomized Trials”
Authors: C.B. Tempfer, G. Froese, G. Heinze, E.-K. Bentz, L.A. Hefler, J.C. Huber