Food
Sources of Alpha-Linolenic Acid (ALA)
Alpha-linolenic acid (ALA)* is the
parent compound
of the omega-3 fatty acid family. ALA is a precursor to the
long-chain fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic
acid (DHA) and is an essential fatty acid for humans because
it cannot be synthesized from dietary precursors.
What Is the Availability
of Omega-3 Fatty Acids in the Diet?
Omega-3 fatty acids constituted a minuscule portion
(<1%) of the total fatty acids in the U.S. food supply in
1985, whereas the omega-6 fatty acids constituted a significant
majority. Thus, the level of omega-3 fatty acids in the U.S.
food supply is low relative to the omega-6 fatty acid level.1 This
fatty acid imbalance has led some nutrition experts to recommend
replacing some omega-6 fatty acids in the diet with those of
the omega-3 family.2 Concern about the current high
level of omega-6 fatty acids in the food supply arises from
studies of their health effects in humans. Omega-6 fatty acids
interfere with the conversion of ALA to its long-chain metabolites,
EPA and DHA; and one omega-6 fatty acid, arachidonic acid,
is converted to thromboxane A2 and other eicosanoids that tend
to enhance atherosclerosis by promoting vasoconstriction and
platelet aggregation. In contrast, the omega-3 fatty acids
inhibit the conversion of linoleic acid to arachidonic acid,
thereby reducing the biosynthesis of arachidonic acid and its
eicosanoids.3, 4 Omega-3 fatty acids appear to provide
protection against atherosclerosis, cardiac arrhythmias, hypertension,
inflammatory and autoimmune disorders, and some types of cancer.2,
5, 6 ALA may have particular antithrombotic properties
that reduce the risk of fatal cardiovascular events and stroke.7,
8
Food Sources
What Are the Major Food
Sources of ALA?
Up to 80% of the fatty
acids in leafy green plants is in the form of ALA; but because
their overall lipid content is low, leafy plants do not contribute
significant amounts of ALA to our diets.9 Flaxseed
is by far the richest source of ALA.1 ALA is also
found mainly in the fats and oils of canola, wheat germ and
soybeans; in nuts such as butternuts and walnuts; and in red
and black currant seeds.10 Fish contain only trace
amounts of ALA, although some species of fish, particularly
fatty marine fish such as salmon, mackerel and herring, are
rich in EPA and DHA.11
What Are the
Food Industry Uses of Flaxseed?
Surveys show
that North American consumers want foods with a strong sensory
appeal and healthful image. They perceive a need to improve their
diets and believe they could be doing more to ensure proper nutrition.
Canadian consumers, for example, are generally concerned about
the fat content of foods and worry whether their fibre intake
is adequate.12 The food industry, particularly the
baking industry, is turning to flaxseed to meet consumers' demand
for foods that taste good and provide nutritional and health
benefits. Bakers are adding milled flaxseed to so-called hearth
breads - breads baked in open hearths and not in pans - and to
variety or multigrain breads. In multigrain breads, flaxseed
is mixed with three or more other grains. Milled flaxseed is
also added to mixes, frozen doughs and thaw-and-serve food products.
These products are designed to meet the North American consumers' demand
for convenience and ease of preparation.13 Overall,
flaxseed is a traditional food whose nutritional profile and
good taste is capturing the attention of the food industry.
How Can Consumers
Increase Their ALA Intake?
Consumers can increase their ALA intake by adding
flaxseed to homemade baked goods; sprinkling flaxseed on yogurt,
cereal and salad; purchasing flaxseed breads and muffins at
retail stores; and buying omega-3 enriched eggs. The increased
ALA content of omega-3 enriched eggs is achieved by feeding
hens rations containing flaxseed. These eggs, which contain
about 0.40 g omega-3 fatty acids per large egg, provide roughly
7-12 times more omega-3 fatty acids than regular eggs. One
omega-3 enriched egg provides nearly half of the optimal intake
level of ALA and about one-quarter of the optimal intake level
of EPA and DHA - all together about the same amount of omega-3
fatty acids found in a 3 oz. serving of fish.14 Consumption
of omega-3 fatty acids from eggs has been reported to reduce
serum lipids and blood pressure in humans.15, 16
Flaxseed Uses in
Baking
Consumers can
easily add flaxseed to their favourite yeast and quick breads.
Reddish-brown flaxseeds (which are slightly larger than sesame
seeds) can be bought whole or milled at many bulk food retail
stores. Whole seeds can be stored at room temperature for up
to one year. The seeds can be milled in a clean coffee grinder to a granular, free-flowing
meal. Milled flaxseed should be placed in an airtight, opaque
container and either refrigerated or frozen. Refrigerated milled
flaxseed should be used within 30 days.
For baking purposes, milled
flaxseed can be substituted for the fat used in baking at a ratio
of 3 to 1. For example, 1 1/2 cups of milled flaxseed can replace
1/2 cup of butter, margarine, shortening or cooking oil in a
traditional recipe. Because baked goods made with flaxseed brown
rapidly, suggested cooking times should be followed carefully.17 Flaxseed
can be sprinkled on top of yeast or quick breads prior to baking
for added taste and crunch.
References
1. Raper NR, et al. J
Am Coll Nutr. 1992;11:304-308.
2. Simopoulos AP. Am
J Clin Nutr. 1991;54:438-463.
3. Leaf A and Weber PC. N
Engl J Med. 1988;318:549-557.
4. Dupont J, et al. Am
J Clin Nutr. 1996;63:991S-993S.
5. Nair SSD, et al. J
Nutr. 1997;127:383-393.
6. Emken EA. In: Proceedings
from the Scientific Conference on Omega-3 Fatty Acids
in Nutrition, Vascular Biology, and Medicine. Dallas, TX: American Heart Association,
1995, pp. 9-18.
7. de Lorgeril M, et al. Lancet. 1994;343:1454-1459.
8. Simon JA, et al. Stroke. 1995;26:778-782.
9. Cunnane SC. In: Flaxseed in
Human Nutrition. Cunnane
SC and Thompson LU, eds. Champaign, IL: AOCS Press, 1995,
pp. 99-127.
10. Nettleton JA. J Am Diet Assoc. 1991;91:331-337.
11. Nutrient Data Research Branch, Nutrition Monitoring
Division. Provisional Table on the Content of Omega-3
Fatty Acids and Other Fat Components in Selected Foods
(100 Grams Edible Portion, Raw). Beltsville, MD: U.S.
Department of Agriculture, 1988.
12. Beggs L, et al. J Cdn Diet
Assoc. 1993;54:21-25. 13. Payne
TJ. In: Flax Council of Canada Conference Proceedings. Winnipeg, MB: Flax Council of Canada, 1997, pp.
50-64.
14. Sim JS and Qi G-H. In: Flaxseed in Human Nutrition. Cunnane SC and Thompson LU, eds. Champaign, IL: AOCS
Press, 1995, pp. 315-333.
15. Oh SY, et al. Am J Clin Nutr. 1991;54:689-695.
16. Cunnane SC, et al. Am J Clin Nutr. 1994;61:62-68.
17. Anon. Adding milled flax to your quick breads. Flax Focus. 1996;9:4.
Flax Council of
Canada, 465-167 Lombard Ave., Winnipeg, MB, Canada R3B
0T6, email: flax@flaxcouncil.ca
Web site: http://www.flaxcouncil.ca/
