More studies are needed to understand potential favorable or conflicting actions between these two chemicals. The effects of phytoestrogens on women taking birth control pills or being treated with postmenopausal hormonal therapy have not been examined. Both of these treatments use estrogen, and since phytoestrogens can act like the hormone estrogen, phytoestrogens might disrupt or amplify the effect of the estrogen in individuals with a diet very high in phytoestrogens. However, such effects have not been reported in groups of women who have diets high in phytoestrogens. The regulatory bodies of several countries, including Great Britain, Switzerland, Australia, and New Zealand, have suggested that soy infant formulas be used only in children who are not breast fed and are definitely intolerant to cow's milk.
Soy formulas contain much higher amounts of phytoestrogens than is seen in human breast milk.
In addition, infants fed soy formula have blood levels of phytoestrogens that are far greater than normal levels of estrogen in infants. No studies have examined the health effects of children eating phytoestrogen-rich foods. Pregnant or breast-feeding women should not use phytoestrogen supplements or consume substantial amounts of flaxseeds on a regular basis. In animal studies, the phytoestrogens found in high amounts in flaxseeds have been shown to cause developmental abnormalities and some studies of soy phytoestrogens have shown a possible increase in susceptibility to cancer in offspring. Eating moderate amounts of soy or flax products should present no problem.