CHICAGO (American Chemical Society) -- Flaxseed, sold in health food stores as a dietary supplement, may protect postmenopausal women against breast cancer, according to a new study by researchers at the University of Minnesota in St. Paul.

While the study appears promising, more long-term studies are needed before recommending the supplement to women in general, the researchers caution. They presented their findings today at the 222nd national meeting of the American Chemical Society, the world's largest scientific society.

Previous studies in cell and animal models of breast cancer have suggested that flaxseed may reduce chemical markers associated with an increased risk for the disease. The current study is believed to be the first to show that flax may be protective against breast cancer in humans, says Joanne Slavin, Ph.D., R.D., lead investigator in the study and a nutritionist at the university's department of food science and nutrition.

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 fiber. 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, which has been used for thousands of years for both food and fabric. As a food, its seeds are ground into a powder and used to make breads and cereals. It is also sold as an oil. Fibers from the flax plant are used to make linen.

Flaxseed is not commonly used in women's diets in this country, although it is being increasingly used as part of a new wave of specialty food products that target women. A growing number of women take flaxseed to relieve symptoms of menopause and decrease the risk of heart disease and osteoporosis, although these effects are unproven. Estrogen replacement therapy is also used for the same purpose but has been associated with an increased risk of breast cancer.

Breast cancer is the most common type of cancer among women in the United States. More than 180,000 new cases are diagnosed each year in this country, according to the National Cancer Institute (NCI), which cites early detection as the key to a good prognosis.