A Rich
Source of Lignans
Flaxseed is one of the richest sources of lignans,
a type of phytoestrogen. Phytoestrogens are a diverse group
of plant-derived compounds that can interfere with estrogen
metabolism in animals and humans. In fact, phytoestrogens
may have contrary biological effects, exhibiting both estrogen
and antiestrogen activity.1
Lignans have numerous
biological properties, including antimitotic, antifungal
and antioxidant activities.2 Lignans from pine
cones3 and the creosote bush4 have
been shown to inhibit replication of the human immunodeficiency
virus in vitro. A newly-identified lignan, cinnamophilin, inhibits thromboxane
synthase, which decreases thromboxan A2 production and thereby reduces platelet aggregation
and vasoconstriction.5 Flaxseed and other lignans
are currently being investigated for their anticancer properties.
What Are the Major Food Sources
of Lignans?
Lignans are widely
distributed in the plant kingdom, being found in most unrefined
grains such as barley, buckwheat, millet, and oats; legumes
such as soybeans; and some vegetables such as broccoli, carrots,
cauliflower and spinach. The richest source of lignans is flaxseed.
Flaxseed contains high levels of the plant lignan precursor,
secoisolariciresinol diglycoside (SDG), and provides 75-800
times more plant lignans than most other foods found in vegetarian
diets.6 Most flaxseed lignans are removed during
processing and thus are not found in appreciable quantities
in flaxseed oil.7
Are Lignans the Same as Lignin?
Lignans and lignin are related structurally - they have
in common a cinnamic acid residue and may be synthesized
by a similar pathway8 - but they differ in their
biological effects. Lignans are
diphenolic structures formed from joining two cinnamic acid
derivatives.8 Hundreds of structurally diverse
compounds are classified as lignans, some of which have been
investigated for their antiestrogen effects. Lignin is a type of insoluble dietary fibre. Like cellulose,
lignin is a structural polymer found in plant cell walls
that is resistant to hydrolysis by the digestive enzymes
of humans.9 Cinnamic acid is one precursor of
lignin.10
What Is the
Relationship between Plant and Mammalian Lignans?
Plant lignans are precursors
of mammalian lignans. For example, the main lignan precursor
in flaxseed -secoisolariciresinol diglycoside or SDG - is converted
by bacteria in the colon to the main lignans found in humans
and other animals: enterodiol and enterolactone. Enterodiol and
enterolactone are called animal or mammalian lignans to distinguish
them from SDG and other plant lignan precursors.
What Is the Metabolic Fate of
Ingested Lignans?
Ingested plant lignans
are converted to mammalian lignans by gut bacteria. The mammalian
lignans, enterodiol and enterolactone, have two metabolic fates:
1) They can be excreted directly in the feces; or 2) After
being absorbed from the gut, they enter the enterohepatic circulation
where they are conjugated mainly with glucuronate and then
excreted in urine and bile.7, 11 The concentration
of enterodiol and enterolactone in urine is related to the
concentration of dietary plant lignans - large intakes of plant
lignans result in large amounts of enterodiol and enterolactone
excreted in the urine of rats and humans.12, 13
Do Lignans Protect against Cancer?
Lignans may protect against
certain cancers, particularly hormone-sensitive cancers such
as those of the breast, endometrium and prostate, by interfering
with sex hormone metabolism. Lignans have been shown to stimulate
hepatic synthesis of sex hormone binding globulin (SHBG), thus
enhancing the clearance of circulating estrogen,12 and
to bind to estrogen receptors on SHBG in a dose-dependent manner,
thereby inhibiting estrogen and testosterone binding.14 As
SHBG is found in breast cancer cells, the binding of mammalian
lignans to SHBG may interfere with estrogen-mediated tumorigenic
processes.
Do flaxseed lignans have
anticancer properties? Based on animal studies, the answer is,
Yes. In one study, female rats fed a high-fat basal diet supplemented
with either milled flaxseed or defatted flaxseed meal for four
weeks showed decreased epithelial cell proliferation and nuclear
aberrations in mammary gland tissue compared with rats fed
the basal diet alone.2 In another study, rats with
existing mammary tumours were fed a basal diet alone or the basal
diet plus SDG, flaxseed oil or flaxseed (2.5% or 5%). Tumour
volume was lower in rats fed the basal diet plus SDG or flaxseed.
Moreover, established tumour volume was reduced significantly
when the rats were fed flaxseed oil, which contains alpha-linolenic
acid.15 Flaxseed has also been shown to have a protective
effect against colon cancer in rats.16
Do Flaxseed and Other Lignans
Have Anticancer Properties in Humans?
The answer to
this question is, Perhaps. Lignans have been shown, for example,
to suppress the differentiation and growth of cultured human
leukemic cells, possibly by interfering with DNA, RNA and/or
protein synthesis. Moreover, lignan cytotoxicity on normal immune
cells appears to be low.17, 18 Population studies
of diet and disease risk also suggest an anticancer role for
lignans and other phytoestrogens. Populations with high intakes
of phytoestrogens - such as the Japanese and Chinese, who typically
consume a low-fat, high-fibre diet rich in isoflavonoids from
soybeans and lignans from vegetables and grains - have lower
incidence and mortality rates of breast, endometrial and prostate
cancers. Western populations tend to consume low-fibre, high-fat
diets and to have a higher risk of these cancers.19 Population
differences can be seen in plasma levels of isoflavonoids, which
are higher among Japanese men than European men,20 and
in urinary levels of mammalian lignans and isoflavonoids, which
are higher among vegans and lactovegetarians than omnivores.12 Studies
of the anticancer effects of flaxseed and other lignans are suggestive
but not conclusive. Not all studies, for example, have shown
an effect of flaxseed consumption on plasma androgen and SHBG
levels.21 Long-term studies of flaxseed effects in
women with breast cancer are underway.
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Flax Council of
Canada, 465-167 Lombard Ave., Winnipeg, MB, Canada R3B
0T6, Email: flax@flaxcouncil.ca,
Web site: http://www.flaxcouncil.ca/
