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 lignansrefers 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. ^
The richest source of lignans is flaxseed (sometimes called linseed), containing more than 100 times the amount found in other foods! ^ [Flaxseed oil] , however, does not contain appreciable amounts of lignans. ^ Sesame seed is an equally rich source. ^ Other food sources are pumpkin seeds, whole grains, cranberries, and black or green tea. ^
Effective dosages of purified lignans have not been determined. In studies of flaxseed as a source of lignans, flaxseed has been taken at a dose of 5 to 38 g daily.
Cooking flaxseed apparently does not decrease the amount of lignans absorbed by the body.
The most promising use for lignans is in cancer prevention. According to observational studies, people who eat more lignan-containing foods have a lower incidence of breast and perhaps colon cancer. ^ This, however, does not prove that lignans are the cause of the benefit, for other factors in these foods, or in the characteristics of the people who consume these foods, may have been responsible. Double-blind, placebo-controlled studies are necessary to prove that a medical treatment provides benefits, and none have yet been reported for lignans. (For information on why this type of study is so important, see Why Does This Database Rely on Double-blind Studies? )
Nonetheless, animal studies offer additional support for a potential cancer-preventive or even cancer-treatment effect. Several studies showed that lignan-rich foods or lignans found in flax inhibited breast and colon cancer in animals ^ and reduced metastases from melanoma (a type of skin cancer) in mice. ^ Test tube studies have found that flaxseed or one of its lignans inhibited the growth of human breast cancer cells ^ and that the lignans enterolactone and enterodiol inhibited the growth of human colon tumor cells. ^ In many of these studies, it isn't clear whether lignans are responsible for the benefit seen, as flaxseeds contain many other substances. Animal and human studies have begun to examine specific lignans, and results seem to confirm that at least some of the positive effects probably come from the lignans themselves; ^ still, until more and better designed trials are done, we will not know lignans' precise effects on the human body, or the precise dose needed to prevent cancer.