Breast cancer is the unregulated growth of cells in breast tissue. Many of these cells are naturally stimulated to grow by estrogens, steroidal hormones present in both men and women. Some breast cancer may be stimulated to grow faster when exposed to estrogens. Phytoestrogens are chemicals in plants such as soy and flax seed that have weak estrogenic activity. No conclusive research has demonstrated that soy or flax seed increases the risk of breast cancer, or whether it is helpful or harmful if consumed after a breast cancer diagnosis.
Hormone-Receptive Breast Cancer
The division of cells in your body is regulated by dozens of genes and cellular checkpoints. If damage and mutations to the DNA of cells in the breast accumulate in a way that allows the cell to replicate unregulated, breast cancer develops. Estrogen is a hormone present in both men and women that regulates a large number of genes. Many cells in the breast are stimulated to grow when estrogen is present. In many cases, estrogen will increase the rate of growth of breast cancer.
Phytoestrogens in Soy and Flax
Isoflavones in soy and lignans in flax seed may be converted into chemicals that stimulate signaling through the estrogen receptor. In most cases, phytoestrogens have much lower affinity for the estrogen receptor than estrogens your body produces. Phytoestrogens actually lower estrogen signaling when estrogen is present, since they compete with estrogen to activate the estrogen receptor. If estrogen is not present, phytoestrogen increase estrogen signaling by a small amount.
The biological activity of phytoestrogens depends strongly on how they are metabolized, both by bacteria in the intestines and enzymes in your body. Although phytoestrogens can be metabolized to be highly estrogenic, this only occurs in a third of Western populations, according to research cited by the Linus Pauling Institute.
Research
Many epidemiological studies have been performed to determine whether phytoestrogens raise or lower the risk of developing breast cancer. Although some studies found a decrease in risk with higher phytoestrogen intake, overall results have been inconclusive. This may be due to the inter-individual variability in the metabolism of phytoestrogens. A Chinese study found that soy isoflavones lowered the risk of breast cancer regrowth in survivors. Some studies have found that high intake of soy isoflavones can increase the rate of cancer growth or interfere with breast cancer drugs such as tamoxifen, although these studies were not performed in humans.
Recommendations
Research suggests that there is no reason to avoid soy or flax seed in the amounts consumed in food during breast cancer treatment. More concentrated sources such as supplements should only be taken under the supervision of the doctor supervising your cancer treatment. If you choose to try isoflavone or lignans in supplements, monitor your progress closely with your doctor.
