Nutritional
Components
Flaxseed has been used
in the diets of humans for thousands of years. The Babylonians cultivated
flaxseed as early as 3,000 B.C., and millennia later, in 650
B.C., Hippocrates used flaxseed for the relief of intestinal
discomfort. Flaxseed was so important for the health of his
subjects that the 8th century king, Charlemagne, passed laws
and regulations governing its consumption.1 Today,
consumers are turning to flaxseed for its many health benefits
and pleasant, nutty flavour - a distinctive addition to baked
products. Flaxseed provides essential nutrients, including
protein, essential fatty acids, and vitamins and minerals;
and it contains both soluble and insoluble dietary fiber. Moreover,
flaxseed is a rich source of lignans, a type of phytoestrogen.
Phytoestrogens are compounds that may protect against certain
types of cancer by interfering with the effects of estrogen.
Additional information about the nutritional components of
flaxseed is given in the following sections.
Major Nutritional Components of Flaxseed
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The major nutritional
components of flaxseed are oil and protein, as shown
in Table 1. A 100 g portion of flaxseed provides 1,890
kJ or 450 kcal.
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TABLE 1
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Composition
of Flaxseed As a Food
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Fata
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41%
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Total dietary fibre
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28%
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Protein
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20%
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Moisture
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7%
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Ash
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4%
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aAnalysed by the American Oil Chemists' Society's
(AOCS) Official Method
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Am 2-93, which is
based on the Federation of Oils, Seeds and Fats Associations
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Ltd. (FOSFA) Official
Method. The American Organization of Analytical Chemists
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(AOAC) Method 996.06
will produce a lower fat content.
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A Mix of Fatty Acids
Flaxseed is naturally low
in saturated fat and provides a moderate amount of monounsaturated
fat (see Figure 1). Roughly 73% of the fatty acids in flaxseed
are polyunsaturated. Flaxseed is particularly rich in alpha-linolenic
acid (ALA), an omega-3 fatty acid; it contains a lesser amount
of linoleic acid (LA), an omega-6 fatty acid. Because of its
high ALA content, flaxseed has an omega-6/omega-3 fatty acid
ratio of 0.3:1.2 The growing concern that the linoleic
acid content of the typical Western diet is too high has led
some experts to recommend replacing dietary omega-6 fatty acids
with those from the omega-3 family. Consuming flaxseed or foods
rich in ALA, such as omega-3 enriched eggs derived from hens fed
flaxseed, increases the omega-3 fatty acid content of the diet
and improves the dietary omega-6/omega-3 fatty acid ratio.3
Essential
Fatty Acids
Flaxseed is a very rich
plant source of alpha-linolenic acid, an essential fatty acid
(EFA) of the omega-3 family.2 Omega-3 fatty acids
have been shown to regulate gene transcription and expression,
thus altering enzyme synthesis,4 and to modify several
risk factors for coronary heart disease, including reducing serum
triglycerides and blood pressure.5 They also protect
against thrombosis and certain types of cancer and modify immune
and inflammatory reactions.6-8 EFAs are required for
maintaining the structure of cell membranes and the permeability
of the skin, as precursors for eicosanoids such as prostaglandins
and thromboxanes, and in cholesterol transport and metabolism.9 There
are two EFAs in the human diet: Alpha-linolenic acid (18:3n-3)*,
the parent fatty acid of the omega-3 family, and linoleic acid
(18:2n-6), the parent fatty acid of the omega-6 family.
Lignans
Lignans are phytoestrogens - compounds
found in plants that have weak estrogen activity in animals.
[Other common phytoestrogens include isoflavonoids such as daidzein
and genistein found in soybeans, some other beans, seeds and
whole grain products.10] The richest source of lignans
is flaxseed, which contains high levels of the lignan precursor
secoisolariciresinol diglycoside (SDG). Lignans are also widely
distributed in the plant kingdom, being present in small amounts
in most unrefined grains such as barley, buckwheat, millet, and
oats; legumes such as soybeans; and some vegetables. Lignans
are not present in appreciable amounts in flaxseed oil.
The flaxseed lignan
prescursor SDG is converted by gut bacteria to the main lignans
found in humans
and other mammals - enterodiol and enterolactone. (Enterodiol and
enterolactone are called animal or mammalian lignans to distinguish
them from SDG and other plant lignan precursors.) The concentration
of enterodiol and enterolactone in urine is related to the concentration
of plant lignans in the diet - large intakes of plant lignans result
in large amounts of enterodiol and enterolactone excreted in the
urine of rats and humans.11 Populations with high fibre
diets and high intakes of phytoestrogens such as lignans, tend to
have lower rates of hormone-dependent cancers such as breast cancer
than Western populations with low fibre intakes.12 Lignans
are believed to protect against hormone-sensitive cancers by inhibiting
certain enzymes involved in hormone metabolism, reducing the availability
of estrogen and interfering with tumour cell growth.13
Fibre
Flaxseed is a source of dietary fibre, providing
about 28 g total dietary fibre/100 g dry weight. About two-thirds
of flaxseed fibre is water insoluble and consists of non-starch
polysaccharides such as cellulose and lignin.14 Insoluble
fibre helps improve laxation and prevent constipation, mainly
by increasing fecal bulk and reducing bowel transit time.15 In
one study of 10 healthy, young adults, bowel movements per
week increased by 30% while subjects consumed two muffins containing
50 g/flaxseed daily.16 The water-soluble fibre fraction
of flaxseed comprises about one-third of total dietary fibre.
The main soluble fibre in flaxseed is mucilage gum.14 Water-soluble
fibre helps maintain blood glucose levels and lower blood cholesterol
levels.15, 17 North American populations are advised
to consume more fibre-rich foods,18, 19 as high-fibre
foods appear to offer protection against certain types of cancer.20 Epidemiologic
studies show that people living in Asia, Africa and eastern
Europe have lower rates of certain cancers, particularly hormone-dependent
cancers such as cancer of the breast, endometrium and prostate,
than people living in Western countries. Their lower cancer
rate may be due in part to their high-fibre diet, which helps
lower blood levels of lipids and some hormones.12 The
low-fibre, high-fat diet typical of Western populations tends
to raise blood estrogen levels, which may contribute to cancer
development by stimulating tumour cell growth.21
*18:3n-3
is the designation for alpha-linolenic acid, an omega-3 or
n-3 fatty acid with 18 carbons and 3 double bonds (denoted
18:3). 18:2n-6 is the designation for linoleic acid, an omega-6
or n-6 fatty acid with 18 carbon atoms and 2 double bonds.
Protein
Protein constitutes
about 20 g/100 g dry weight of flaxseed. The amino acid profile
of flaxseed protein resembles that of soybean flour.14
Vitamins and Minerals
Flaxseed contains
essential vitamins and minerals. It is particularly rich in
potassium, providing about seven times as much as a banana
on a dry-weight basis. Flaxseed provides minor amounts of magnesium,
iron, copper and zinc.22
References
1. We Are Flax Bread.
Winnipeg, MB: Flax Council of Canada.
2. Bhatty RS. In: Flaxseed in
Human Nutrition. Cunnane SC and Thompson LU, eds. Champaign, IL: AOCS
Press, 1995, pp. 22-42.
3. Nutrition Advisory Panel Meeting:
Executive Summary. Winnipeg, MB: Flax Council of Canada,
1995.
4. Clarke SD and Jump DB. Lipids. 1996;31(Suppl):S7-S11.
5. Schmidt EB, et al. In: Proceedings from the Scientific Conference on Omega-3
Fatty Acids in Nutrition, Vascular Biology, and Medicine. Dallas,
TX: American Heart Association, 1994, pp. 208-211.
6. Simopoulos AP. Am
J Clin Nutr. 1991;54:438-463.
7. Weksler BB. World
Rev Nutr Diet. 1994;76:47-50.
8. Kremer JM. Lipids. 1996;31(Suppl):S243-S247.
9. Food and Nutrition Board, National Research Council.
In:
Recommended Dietary Allowances, 10th
ed. Washington, DC: National Academy Press, 1989, pp. 44-51
10. Kelly GE, et al. Proc Soc Exp Biol Med. 1995;208:40-43.
11. Adlercreutz H, et al. J Steroid
Biochem. 1987;27:1135-1144.
12. Rose DP. Annu Rev Publ Health. 1993;14:1-17.
13. Adlercreutz H. Scand J Clin Lab
Invest.
1990; 50(Suppl 201):3-23.
14. Vaisey-Genser M. In: Flaxseed:
Health, Nutrition and Functionality. Winnipeg, MB: Flax Council of Canada, 1994.
15. Spiller RC. Pharmacol Therap.
1994;62:407-427.
16. Cunnane SC, et al. Am J Clin Nutr. 1994;61:62-68.
17. Salmerón J, et al. J Am Med Assoc. 1997;277:472-477.
18. Health and Welfare Canada. Canada's
Food Guide to Healthy Eating.
Ottawa, ON: Minister of Supply and Services Canada, 1992.
19. U.S. Department of Agriculture. Nutrition and Your Health: Dietary Guidelines for
Americans. Washington, DC: U.S. Government Printing Office, 1995.
20. Colditz GA, et al. Harvard report on cancer prevention. Cancer Causes Control. 1996;7(Suppl 1):1-59.
21. Adlercreutz H. Gastroenterology. 1984;86:761-766.
22. Carter JF. Cereal Foods World. 1993;38:753-759.
Flax Council of Canada,
465-167 Lombard Ave., Winnipeg, MB, Canada R3B 0T6, E-mail:
flax@flaxcouncil.ca Web site: www.flaxcouncil.ca 05.98PD0200
