Flaxseed may protect postmenopausal women against breast cancer, according to a new study by researchers at the University of Minnesota, the American Chemical Society reports this week. According to the study's lead researcher, Dr. Joanne Slavin at the Department of Food Science and Nutrition, University of Minnesota, their recent study is believed to be the first to show that flax may be protective against breast cancer in humans. Slavin and her associates studied 28 postmenopausal nuns in a convent in central Minnesota, chosen primarily because of their strict dietary practices. The volunteers were given daily dietary supplements of either zero, five or ten grams of ground flaxseed for seven week cycles over the course of a year. Consumption of five or ten grams of flax significantly decreased blood levels of certain types of estrogen that are characteristic of postmenopausal women. Since previous studies have shown that increased levels of these estrogens (estrone sulfate and estradiol) may increase a woman's risk of developing breast cancer, reducing levels of these hormones is thought to be protective against breast cancer, according to the researchers. The exact mechanism by which flaxseed exerts its effect is not known. Flaxseed is considered the most concentrated food source of lignan, a type of plant hormone that is structurally similar to estrogen. Lignan may lower estrogen in humans by inhibiting enzymes that are involved in estrogen synthesis, Slavin says. Although Slavin believes that lignans are likely the most active chemical component of flax affecting hormone levels, she adds that other components are also thought to lower the cancer risk, including omega-3 fatty acids and soluble fibre. Further studies are needed to determine whether these or an undetermined chemical contributes to the beneficial effects of flax and to determine the chemical mechanisms underlying this effect, she says. Flaxseed comes from the flax plant and its seeds are ground into a powder and used to make breads and cereals. It is also sold as an oil. Dr. Slavin was keen to stress that further research was required in order to thoroughly examine the effect that flaxseed may have of reducing breast cancer in postmenopausal women.