NEW YORK (Reuters Health) - Postmenopausal women who eat foods rich in estrogen-like plant chemicals called lignans may have a modestly decreased risk of developing breast cancer, a research review suggests.
In an analysis of 21 studies published in the past 13 years, researchers found that postmenopausal women who reported the highest intakes of dietary lignans were 14 percent less likely to develop breast cancer than those with low intakes.
The same relationship was not seen, however, among premenopausal women.
The findings, published in the American Journal of Clinical Nutrition, add to a conflicting body of research into the relationship between dietary phytoestrogens and breast cancer risk.
Phytoestrogens are plant-based compounds that are structurally similar to estrogen and may have weak estrogen-like, as well as anti-estrogen, activity in the body. Some studies have linked high phytoestrogen intake to a lower risk of breast cancer, but others have suggested that the compounds may help fuel breast cancer growth -- or have no significant effect on a woman's risk of the cancer.
Lignans are one of the three main types of phytoestrogen. The new study focused on lignans, in part because they are the main phytoestrogen in the typical Western diet.
Flaxseed and sesame are particularly high in lignans, and the compounds are also found in whole grains, berries and some other fruits, a number of vegetables such as broccoli and kale, and green tea.
For this study, Dr. Jenny Chang-Claude and colleagues at the German Cancer Research Center in Heidelberg combined the results of 21 previous studies on lignan intake and breast cancer risk. In some of the studies, researchers also took blood or urine samples to measure participants' levels of enterolignans -- compounds created when intestinal bacteria interact with dietary lignans.
Overall, the researchers found no correlation between women's lignan intake and their risk of breast cancer. However, when they separated the women by menopause stage, they found that "high" lignan intake - which they did not define in the study - was related to a somewhat lower risk of breast cancer.
In one study of nearly 60,000 postmenopausal women in France, for example, the one-quarter of women with the highest lignan intake were 17 percent less likely to develop breast cancer during the study period compared with the one-quarter with the lowest intake -- estimated based on dietary questionnaires the women completed at the outset.
Breast cancer is the most common cancer in women worldwide, accounting for around 16 percent of all female cancers. It kills around 519,000 people globally each year.
The researchers on the French study accounted for a number of other factors in breast cancer risk -- including the women's age, family history of breast cancer, weight and history of estrogen exposure from birth control pills or hormone replacement therapy. The relationship between lignan intake and breast cancer risk remained.
Still, the overall findings of the review show only an association between higher lignan intake and lower breast cancer risk -- and do not prove that the compounds themselves confer the protection.
The studies the researchers evaluated had various limitations, such as relying on dietary questionnaires to estimate lignan intake instead of measuring it or watching what subjects ate.
And many were case-control studies, where researchers compared the reported diet histories of women with breast cancer to those of women without the disease; these types of studies are not as strong as prospective studies -- where, for example, researchers would collect diet information at the outset, then follow women over time to see which ones developed breast cancer.
If lignans do have an effect on breast cancer development, these findings suggest it is likely to be "moderate," Chang-Claude told Reuters Health in an email.
Still, foods high in lignans are also generally healthy ones, the researcher noted. "Therefore, it might be advisable for postmenopausal women to include some lignan-rich foods in their diets," she said.
In theory, lignans and other phytoestrogens might protect against breast cancer by inhibiting the body's own estrogen activity, or through other pathways, such as the compounds' antioxidant effects.
It's not clear why lignan intake would have different effects in pre- and postmenopausal women, according to Chang-Claude and her colleagues. One possibility, they note, is that any protective lignan activity is only effective when women's natural estrogen levels are relatively lower, as they would be after menopause.
SOURCE: here American Journal of Clinical Nutrition, May 12, 2010.