That's according to a French study published in the Journal of the National Cancer Institute.
The study included about 58,000 postmenopausal French women who were followed for an average of more than seven years.
When the study began between 1993 and 1997, the women were in their early 50s, on average, and didn't have breast cancer. They completed surveys about their diet and lifestyle, including how often they consumed 208 foods and beverages.
Researchers included Francoise Clavel-Chapelon, PhD, of France's Institut National de le Sante et de la Recherche (INSERM), which is France's institute for biological, medical, and public health research.
Levels of Lignans
Using the completed dietary surveys, Clavel-Chapelon and colleagues calculated the women's daily intake of lignans, which are a type of plant estrogen found in many plants.
On average, the women consumed 1 milligram per day of lignans, with daily lignan intake ranging from 0 to 5 milligrams.
The women got their lignans from fruits, vegetables, tea, coffee, and grain products including bread, breakfast cereals, rice, and pasta.
Flaxseed, which is particularly high in lignans, isn't part of the traditional French diet, but it may be becoming more common in French multigrain bread, the researchers note.
More Lignans, Less Breast Cancer
A total of 1,469 women in the study group were diagnosed with breast cancer during the follow-up period.
Women with the highest lignan intake were 17% less likely to be diagnosed with breast cancer during the study than those with the lowest intake.
Those findings take other breast cancer risk factors into account.
However, the study didn't track soy consumption. Soy contains isoflavones, which are plant estrogens that aren't lignans.
Lignans were only linked to a reduced risk of breast cancers that are sensitive to the hormones estrogen and progesterone.
One More Reason to Eat Plants, Grains
The French study didn't directly test lignans against breast cancer, so the findings don't prove that lignans lower breast cancer risk.
But Clavel-Chapelon and colleagues say their findings support recommendations for women to eat diets consisting largely of fruits, vegetables, and grain products, which are all good sources of lignans.