Women who used supplements containing isolflavones, including soy, red clover, garlic, and evening primrose, for a period of at least five years were at a 25% lower risk of breast cancer, according to findings published in the International Journal of Cancer.
Researchers from the University of Toronto report that the benefits were only observed for post-menopausal women.
“Our findings suggest isoflavone supplements are associated with reduced postmenopausal breast cancer risk,” wrote the researchers.
“Furthermore, it appears that isoflavone supplements do not increase breast cancer risk and may represent a possible modifiable approach to reduce risk.”
Over one million women worldwide are diagnosed with breast cancer every year, with the highest incidences in the US and the Netherlands. China has the lowest incidence and mortality rate of the disease.
Hormone-sensitive estrogen-receptor (ER) positive and progesterone-receptor (PR) positive tumors are said to be the most common type diagnosed among breast cancer patients in the US. These tumors are stimulated to grow by the female hormones estrogen and progesterone.
Conflicting reports have clouded the picture about the beneficial effects of soy isoflavones, because isoflavones are well known phytoestrogens - active substances derived from plants that have a weak estrogen-like action.
Indeed, some studies have indicated that breast cancer cells in mice can be stimulated by isoflavones. On the other hand, population studies have shown that women with a high-soy diet generally have lower rates of breast cancer.
For the new study, the researchers analyzed data from 3,101 women with breast cancer and 3,471 healthy controls about their use of 28 isoflavone supplements in Ontario, Canada. Thirteen supplements were single ingredient supplements, and the other 15 were multiple ingredient (brand name) supplements.
In terms of daily isoflavone doses, the researchers state: “10 supplements with daily dose isoflavone values less than 0.025 mg were defined as low content, and six with daily dose values greater than 0.676 mg were high content.”
Results suggested that women who took high isoflavone content supplements, a consistent and significant reduction in breast cancer risk was observed (28% risk reduction), and the risk decreased further for longer term use (43% risk reduction when used for one to five years).
On the other hand, there were no associations between low isoflavone content supplement use and breast cancer risk. In addition, there was no difference between estrogen-progesterone tumor receptor statuses.
“Our findings of reduced postmenopausal breast cancer risk among women who ever used isoflavone supplements are compelling, however study limitations must be considered,” wrote the researchers.
“In addition to the general limitations of case control study designs, our results were based on small numbers of supplement users, especially in subgroup analyses, and are to be viewed with caution.”
“Future research, especially in large prospective studies of pre- and postmenopausal women, should ascertain supplements taken concurrently, sequentially and consistently, as well as gaps in use. These data will support the derivation of average daily isoflavone intakes from supplements and allow comparison with those from foods.”
Source: International Journal of Cancer
March 15, 2013, Volume 132, Number 6, Pages 1439-1450, doi: 10.1002/ijc.27769
“Use of isoflavone supplements is associated with reduced postmenopausal breast cancer risk”
Authors: Boucher BA, Cotterchio M, Anderson LN, Kreiger N, Kirsh VA, Thompson LU.