| Therapeutic Dosages
| Therapeutic Uses
| What Is the Scientific Evidence for Lignans?
| Safety Issues
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
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
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
), 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
(sometimes called linseed), containing more than 100 times the amount found in other foods!
, 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.
A number of preliminary human and
suggest that lignans may be helpful for
, particularly of the colon and breast.
Despite positive preliminary results in animal studies,
studies in humans have yielded mixed results for improving
There is also mixed evidence as to whether or not flaxseeds or lignans decrease menopausal symptoms.
Lignans may improve kidney function in various types of kidney disease (specifically, lupus nephritis and polycystic kidney disease).
: Flaxseed or other treatments for kidney disease should be taken only under a doctor's supervision, due to the serious nature of these disorders.
consists of a constellation of conditions increasing the risk of cardiovascular disease, including obesity, unhealthy cholesterol and triglyceride levels, high blood pressure, and pre-diabetes. In one study, 100 people with metabolic syndrome were randomized to receive flaxseed lignans (4 g/per day) or placebo during a 6-month exercise program.
A number of tests were done, including body composition, cholesterol levels, blood pressure, and bone mineral density. But, no significant differences turned up between the two groups.
Benign prostatic hyperplasia (BPH) causes a range of urinary problems in men. A 4-month randomized, controlled trial involving 78 men with BPH found that those treated with lignans experienced an improvement in their symptoms compared to the placebo group.
Researchers have also explored whether the lignan enterolactone can reduce the risk of heart disease. But, there is only weak evidence to support this.
What Is the Scientific Evidence for Lignans?
The most promising use for lignans is in cancer prevention. According to
studies, people who eat more lignan-containing foods have a lower incidence of breast and perhaps colon cancer.
For example, in 2 systematic reviews of up to 23 studies, researchers found an association between a high intake of lignans and a reduced risk of breast cancer in postmenopausal women.
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?
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.
23Test 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.
In one study, 38 postmenopausal women were randomized to eat 2 slices of bread with 46 mg of lignans or 2 slices of bread with less than 1 mg of lignans (placebo).
At the end of the 12-week study, the women in the high-lignan group did not experience a reduction in hot flashes or other menopausal symptoms.
Another study, though, offers more positive news. Eighty postmenopausal women were randomized to receive a combination treatment (lignans plus
After 3 months, the women receiving the combination product had an improvement in their menopausal symptoms, though it is unclear which component(s) of the combined treatment produced the beneficial effect.
Women who are pregnant or breastfeeding should avoid high intake of flaxseed or purified lignans. One study found that pregnant rats who ate large amounts of flaxseed (5% or 10% of their diet), or a purified lignan present in flaxseed, gave birth to offspring with altered reproductive organs and functions, and that lignans were also transferred to the baby rats during nursing.
In humans, eating 25 g of flaxseed per day amounts to about 5% of the diet.
High intake of lignans may not be safe for women with a history of estrogen-sensitive cancer, such as breast cancer or uterine cancer. A few test tube studies suggest that certain cancer cells can be stimulated by lignans such as those present in flaxseed.
Other studies found that lignans inhibit cancer cell growth.
As with estrogen, lignans' positive or negative effects on cancer cells may depend on dose, type of cancer cell, and levels of hormones in the body. If you have a history of cancer, particularly breast cancer, talk with your doctor before consuming large amounts of flaxseeds.
Other potential concerns are discussed in the safety section of the