A review of the clinical effects of phytoestrogens
Knight, D.C. and Eden, J.A. 1996. Obstet Gynecol. 87:897-904.
Phytoestrogens are naturally occurring compounds that are structurally or functionally similar to estradiol, and include lignans, isoflavones, coumestans and resorcyclic acid lactones. These naturally occurring estrogens in plants and food appear to play a significant role in influencing disease in humans. This article reviews the research for the years 1980 to 1995 for phytoestrogens, with emphasis on the absorption, metabolism, and effects in humans of lignans and isoflavones. Oilseeds such as flax contain the highest amount of lignans, although smaller amounts are found in cereals, legumes and other foods. Plant lignans, matairesinol and secoisolariciresinol are converted by colonic bacteria to the major mammalian lignans, enterolactone and enterodiol. Oilseeds, especially flax seed oil, produce these lignans in amounts 100 times that of other plants. The important foods containing isoflavones are soybeans and soy products such as soy drinks. Lignans and isoflavones possess both estrogenic and antiestrogenic effects, as well as other effects such as anticancer, antioxidant and antiproliferative effects. Osteoporosis is associated with estrogen deficiency and it has been found that bone loss and fracture rates decrease in countries where consumption of soy products is higher. Some symptoms of menopause such as hot flashes, also appears to be lower in populations consuming soy products. Human studies show that both isoflavone and lignan supplementation may ease menopausal symptoms. Epidemiological research has also shown that higher consumption of phytoestrogens such as soy products is related to lower incidence of cancer, such as breast cancer. High lignan levels in humans are also related to lower breast cancer risk. The antiproliferative effect of lignans has been demonstrated in human breast cancer cell lines. These data suggest that phytoestrogens such as the lignans found in flax seed may play a protective role in breast cancer. Not surprisingly, phytoestrogens may also reduce the risk of cardiovascular disease, by lowering lipoprotein (a) levels, a primary predictor of heart disease and stroke. This may represent a unique opportunity since current cardiovascular medications do not alter lipoprotein (a) levels; only estrogens and other sex steroids do – and have been found to decrease levels by 35%. Although clinical applications of phytoestrogens are still in their infancy, they have already proven their potential in the treatment of hypercholesterolemia and heart disease, as well as menopause, osteoporosis and cancer. Phytoestrogens may, in fact, be the dietary factors that afford protection against cancer and heart disease in vegetarians. Dietary intervention with phytoestrogens is an affordable and attractive means of preventing chronic disease, with enormous advantages.