Lignans are naturally occurring chemicals widespread within the plant and animal kingdoms. Several lignans—with intimidating names such as secoisolariciresinol—are considered to be phytoestrogens, plant chemicals that mimic the hormone estrogen. These are especially abundant in flaxseeds and sesame seeds. Bacteria in our intestines convert the naturally occurring phytoestrogens from flaxseed into two other lignans, enterolactone and enterodiol, which also have estrogen-like effects. In this article, the term lignans refers to these two specific lignans as well as the phytoestrogen kind, but not to the wide variety of other lignans.

Lignans are being studied for possible use in cancer prevention, particularly breast cancer. Like other phytoestrogens (such as soy isoflavones), they hook onto the same spots on cells where estrogen attaches. If there is little estrogen in the body (after menopause, for example), lignans may act like weak estrogen; but when natural estrogen is abundant in the body, lignans may instead reduce estrogen's effects by displacing it from cells. This displacement of the hormone may help prevent those cancers, such as breast cancer, that depend on estrogen to start and develop. In addition, at least one test tube study suggests that lignans may help prevent cancer in ways that are unrelated to estrogen.1

The richest source of lignans is flaxseed (sometimes called linseed), containing more than 100 times the amount found in other foods! 11Flaxseed oil, however, does not contain appreciable amounts of lignans.12Sesame seed is an equally rich source.44Other food sources are pumpkin seeds, whole grains, cranberries, and black or green tea.13