Phytoestrogens, which are abundant in flaxseed and soy, have
chemical structures resembling those of estrogens in your body
and have been shown to exert hormonal effects, thereby affecting
chronic diseases.

Estrogen is metabolized in two ways. Along one pathway, it is
converted into a powerful metabolite, 16alpha-hydroxyestrone
(16alpha-OHEthat acts to stimulate target tissues. Levels of
16alpha-OHE1 can rise in response to obesity, alcohol
consumption, and toxic exposure. High levels of this potent
metabolite are linked with increased risk and poorer prognosis in
conditions associated with estrogen excess, including breast
cancer and lupus.

Alternately, the body can break down estrogen into a much weaker
metabolite, called 2-hydroxyestrone (2-OHE1). This metabolite
binds weakly to cell receptors and may slow cell proliferation.
However, excessive levels of 2-OHE1 may increase the risk of
developing conditions associated with estrogen deficiency, such
as heart disease, depression, and osteoporosis. A proper balance
between 2-OHE1 and 16alpha-OHE1 is a key to optimal health. A
substantial proportion of women have too much 16alpha-OHE1, which
leads to problems such as breast cancer. Whether you eat soy or
flaxseed may influence the balance of these two estrogen
metabolites.

A study conducted at the Department of Nutritional Sciences,
University of Toronto compared the effects of consuming flaxseed
versus soy on estrogen metabolism in 46 postmenopausal women.

Urinary concentrations of 2-OHE1, but not 16alpha-OHE1, increased
significantly in the flaxseed group, suggesting that 2-OHE1
levels increased relative to 16alpha-OHE1. In the group who ate
soy, there was no change.

So, should you eat flaxseed instead of soy? It depends on whether
your estrogen metabolites are in balance, or not. It’s easy to
find out with a simple urine or blood test. Just ask one of our
physicians about an estrogen assessment. If the test shows that
your levels of 16alpha-OHE1 are too high, we suggest that you eat
flaxseed instead of soy.

Source: Brooks, JD et al, Am J Clin Nutr. 2004;79(2):318-25