Know anyone who has breast cancer? Doing your best to avoid it? Then consider this: Studies are showing that flaxseed can protect against breast cancer and prolong survival in women who have it.

For more than 20 years, the grande dame of flaxseed research, Dr. Lilian Thompson of the University of Toronto, has been studying the effects of flaxseed on cancer, especially in the breast.

How does it work?

Flaxseed is rich in an omega 3 fatty acid called alpha-linolenic acid (ALA), which inhibits inflammatory chemicals that stimulate the growth of malignant cells. It’s also, of all foods, the best source of lignans, compounds that have been shown to reduce the growth and spread of cancer cells.

Lignans, in fact, are phytoestrogens–that is, estrogens from plants–and contrary to what many of us have been led to believe, plant estrogens may protect us from the stronger estrogens our bodies produce and the xenoestrogens found in environmental chemicals. How’s that?

Phytoestrogens have been estimated to be 500-1,000 times weaker than human estrogen, says Dr. Jeanne Wallace, a PhD in nutrition who counsels cancer patients on diet and supplements. “By docking on estrogen receptors in the body, phytoestrogens may prevent activation of these receptors by our own estrogen and xenoestrogens.”

Plus, flaxseed increases Sex Hormone Binding Globulin, a compound that binds estrogen, thus potentially reducing the amount of freely circulating estrogen that can act on breast tissue.

All estrogen, in fact, is not created equal. The liver metabolizes estrogen and changes it into different daughter compounds, “which have differing effects on breast tissue. 2-OH estrogens are favorable, offering protective effects, whereas 16-OH estrogens are unfavorable, and a preponderance of 16-OH estrogen is linked with an increased risk of breast cancer,” says Wallace.

Cruciferous vegies, soy foods and fish oil shift the balance toward the good 2-OH estrogens, according to Wallace.

So does flaxseed, Thompson says.

“Flaxseed (10 to 25 g/daily) has been shown to increase the ratio (of 2 to 16 estrogens) in pre- and post-menopausal women, indicating a protective effect,” explains Thompson, in a chapter she wrote recently for “The Encyclopedia of Dietary Supplements.”

Other mechanisms are also possibly at work here, she adds, and include reducing the activity of enzymes such as aromatase that are involved in estrogen synthesis and the expression of some genes, proteins, hormones and other growth factors that stimulate tumors.

Let’s look at the studies thus far.

The animal studies: Several studies indicate that flaxseed has anti-cancer effects and does not promote breast tumor growth, Thompson writes, in article published this year by the US Flax Institute.

The epidemiological studies (studies of human populations with and without disease) suggest that lignans can reduce the risk of breast cancer and prolong survival of those living with it, including pre and especially postmenopausal women, she points out. High lignan intake has been associated with tumors with more favorable prognostic factors. And while lignans appear to be effective on both estrogen positive and estrogen negative tumors, one study showed they have a stronger effect on ER-PR- than on ER+PR+ ones, she says.

It’s the clinical studies in humans that are considered the gold standard, however, assuming they’re conducted at their best (i.e., subjects include a control group, are randomly allocated and double-blinded so that nobody knows if she’s taking the flaxseed or placebo.)

Several years ago, Thompson conducted the first clinical trial of flaxseed–on postmenopausal women with breast cancer. She fed 25 g flaxseed or placebo for approximately 5 weeks to women awaiting surgery. The results were promising, showing significant changes for the better in the tumors of those who ate flaxseed.

“Clinical studies on breast cancer patients or premenopausal women with high risk of getting breast cancer are very limited,” says Thompson, “but those that have been conducted suggest that flaxseed is able to reduce the growth of breast tumors in postmenopausal women and that SDG (the main lignan) may also reduce the risk of getting breast cancer.”

Today, several clinical trials continue to investigate the almighty seed.

So, based on the evidence thus far, would it be reasonable to incorporate flaxseed into your diet?

If you don’t have breast cancer, yes, eat it, Thompson said in an email interview.

If you do have breast cancer, her answer is “Maybe, based on our clinical trial but more clinical trials with larger number of patients may be needed to confirm it.”

And what if you’re taking other anti-cancer drugs or doing radiation? Again, it’s too early for the wise doctor to answer. “Animal studies suggest that flaxseed does not interfere with tamoxifen treatment but rather enhances its effectiveness,” she said in the interview, and flaxseed oil has been shown to increase the effectiveness of herceptin, but clinical trials have not yet been done. One is in progress, however, on the interaction of flaxseed with aromatase inhibitors, a primary drug used to treat the disease. But no study has looked at radiation and the seed. Meanwhile, you’ll have to make your own decision: Talk to your doctor; weigh the evidence and the risks.

What kind and dose of flaxseed is best?

Choose either brown or yellow flaxseed but be sure you’re getting the most potent kind: One form of the yellow flaxseed called solin has been genetically engineered to have low ALA.

Health guru Dr. Andrew Weil recommends 1-2 tablespoons of ground flaxseed a day for healthy women, and Thompson agrees it’s a reasonable dose–even for those with breast cancer, she said in the interview, based on her qualifiers discussed above.

Thompson’s breast cancer studies have used 25 grams–or 2 ½ T–of ground flaxseed per day. “It’s unknown what the effects of greater than 25 grams/day might be,” she said. “Considering current data, we know that up to 25 g (2.5 T) did not cause adverse effect on breast cancer patients in short term studies. In healthy postmenopausal women, up to 40 g also did not show adverse effects.”

Because pulverizing the seeds releases the lignans and fragile fatty acids, it’s good to grind daily (Yes, a coffee grinder works) — and keep any leftover powder in the fridge or freezer in a small, dark airtight container. Chew well, and drink plenty of water to help move the fiber through your system. Underscore that latter point. In addition to its anti-cancer effects, flaxseed is a laxative, so start small and gradually increase your dose.