Functional linseed

A recent review, published in Comprehensive Reviews in Food Science and Food Safety, explored the evidence for the health-promoting activities and safety of linseed lignans. Lignans are compounds in the fibre portion of many plants including whole grains, pulses, nuts and seeds, and some fruits and vegetables. The major linseed lignan is known as SDG (secoisolariciresinol diglycoside). A tablespoon of ground linseed contains about 152 mg of SDG and a tablespoon of high-lignan linseed oil has about 14 mg.

When people eat linseed meal or high-lignan linseed oil, the fibre portion moves through the digestive tract and its lignans are digested by bacteria in the large intestine. The resulting compounds are absorbed into the body, providing antioxidant and mild oestrogenic effects, and are eventually excreted in the urine.

Lignans as antioxidants

According to the review, active compounds made in the body from linseed lignans have demonstrated strong antioxidant activity. The reviewers noted that these properties are believed to contribute to linseed’s anticancer and anti-inflammatory effects, and might also play a role in its beneficial effects on high cholesterol levels, atherosclerosis, and diabetes. They point out that the breakdown products from lignans have been found to have one to five times as much antioxidant activity as vitamin E.

Lignans as phytoestrogens

Linseed lignans also affect health by modulating oestrogen activity in the body. Compounds made from lignans are able to bind to oestrogen receptor sites, and by preventing oestrogen from binding at those sites, they can act as anti-oestrogens in reproductive age women with normal oestrogen levels. In postmenopausal women and women with low oestrogen levels, however, their weak activation of cells’ oestrogen receptor sites makes their overall action slightly pro-oestrogenic.

Linseed & disease prevention

Early evidence suggesting that linseed lignans might protect against breast, prostate, colon, and skin cancers was outlined in the review. “In general, linseed may be a valuable tool in the fight against various cancers,” the authors of the review said.

Ground linseed, high in soluble fibre, has a low glycaemic index and adding it to the diet has been shown to decrease glycaemic load. Based on such findings, the reviewers contended that supplementing a healthy diet with 40 to 50 grams (1 to 2 tablespoons) of linseed meal per day may improve blood sugar control and prevent type 2 diabetes.

Safely eating linseed

The review points out a few lignan safety considerations and why they are not cause for concern:

• Linseed contains cyanide-forming glycosides (similar to those found in brassica vegetables like broccoli and cabbage), however, a person would need to consume a kilogram (about 8 portions) of linseedmeal in a sitting to achieve a toxic cyanide level.
• Linseed also contains linatine, a compound that reduces vitamin B6 levels, but no changes in vitamin B6 levels have been observed in people eating 45 grams of linseed per day for five weeks.
• Phytates (fibre components that bind some nutrients and prevent their absorption) are common to many high-fibre foods, and can make mineral absorption especially challenging, but deficiencies can easily be prevented by taking mineral supplements and eating mineral-rich foods when taking a fibre supplement such as linseed.
• Though linseed contains trypsin inhibitors (chemicals that interfere with protein digestion), other commonly eaten foods containing soya and canola have higher anti-trypsin effects than linseed. The review authors suggest that supplemental linseed has little effect on protein digestion.

Although this review focused on lignans and did not address issues having to do with linseed fatty acids, it is well known that ALA is easily damaged by heat and oxidation. ALA is preserved in whole linseed but ground linseed and linseed oil are best eaten fresh, and should be stored cold and eaten unheated.

(CRFSFS 2010;9:261–9)