Antioxidant compounds found in soy foods have been shown to reduce levels of hormones associated with breast cancer risk in women. A new study suggests that other factors associated with soy may also play a role in lowering cancer risk. The investigators found that nine healthy, pre-menopausal women who consumed a diet containing soymilk in which most of the antioxidant compounds, isoflavones, had been removed, produced less oestrogen and progesterone than they produced before they added soy to their diets. Other reproductive hormones were not affected by the diet, which was also low in animal protein and high in fibre, the researchers report. According to the study published in the July issue of the Journal of Clinical Endocrinology and Metabolism, oestrogen levels fell as soy protein and fibre intake increased. However, Dr. Lee-Jane W. Lu, the study's lead author, stressed that it is not yet clear which dietary compounds caused the change in hormone levels. "The important finding from my study is that it is not too hard to lower a woman's...female hormone," she told Reuters Health. "By replacing, not supplementing, a portion of one's energy intake (with) soy, one can lower one's female hormones." Oestrogen can stimulate the growth of some types of breast cancer cells and is thought to play a role in the development of some cases of breast cancer. Women with a higher lifetime exposure to oestrogen, for instance, those who got their first period at an early age, those who do not have children and women who do not breastfeed, may face a higher risk of breast cancer. The hormone progesterone also contributes to breast cancer risk by helping tumours to grow. "Our results may have implications for breast cancer prevention by soy dietary intervention," according to Lu and her colleagues at the University of Texas Medical Branch in Galveston. In the study, researchers measured levels of oestrogen, progesterone, sex hormone-binding globulin, luteinising hormone (LH) and follicle stimulating hormone (FSH) in women before they began the diet. LH and FSH stimulate ovarian function. The women followed the diet, which included 36 ounces of soymilk containing less than 5 milligrams of isoflavones daily, for one month. The study diet contained more carbohydrate and less protein than the women's usual diets, the report indicates.