This is a question that no-one has conclusively answered yet. Some clinical trials say isoflavone supplements reduce menopausal symptoms, whilst others disagree. One area of agreement is that Japanese women (whose diet contains high levels of soy, a rich source of isoflavones) seem to suffer less from menopausal symptoms (1).
Isoflavones are phyto-oestrogens (plant oestrogens) and their role is hormonal in plants just as in humans. They are remarkably similar compounds to human oestrogen.
There are many phytooestrogens in red clover, but the four main ones which we are interested in are named Daidzein, Genistein, Formononetin and Biochanin-A. For simplicity we will call them D,G,F and B (shown above).
The active forms that have oestrogen-like activity in humans are D and G. These compounds are made from F and B respectively. This bio-conversion is carried out in your intestines by very specific species of bacteria. Only 30-50% of women host the necessary bacterial species in their tums to benefit fully from isoflavone supplementation (1).
Most commercial supplements are a crude extract of a mixture of undefined isoflavones. Red clover crude isoflavones only contains about 4% D and G, the rest being F and B (2).
Supplements declaring “Red clover extract 400mg” simply says there is 400mg/tablet content of red clover extract, and nothing about its isoflavone make up whatsoever. Typically, red clover extract might contain 0.02% isoflavones. At this level, each tablet may contain less than 10mg isoflavones.
Supplements declaring “40mg isoflavones” says that there is 40mg/tablet of mixed crude isoflavones. G and D are present in very small and variable proportions in this, perhaps 2% each.
Without standardisation of individual isoflavone contents, it is very uncertain how effective any supplement is likely to be to the majority of women (3). The measuring and standardisation of component contents in red clover extracts is one area I am covering in my research. This should pave the way for more effective and predictable dietery supplements. With the huge and rapidly growing market for isoflavone supplements, such quality control is overdue!
References
(1) Patisaul, H. B., & Jefferson, W. (2010). The pros and cons of phytoestrogens. Frontiers in neuroendocrinology, 31(4), 400-419.
(2) Krenn, L., Unterrieder, I., & Ruprechter, R. (2002). Quantification of isoflavones in red clover by high-performance liquid chromatography. Journal of Chromatography B, 777(1-2), 123-128.
(3) Setchell, K. D., Brown, N. M., Desai, P., Zimmer-Nechemias, L., Wolfe, B. E., Brashear, W. T., ... & Heubi, J. E. (2001). Bioavailability of pure isoflavones in healthy humans and analysis of commercial soy isoflavone supplements. The Journal of nutrition, 131(4), 1362S-1375S