Metabolism
of Omega-3 and Omega-6 Fatty Acids
Fatty Acid Profile of
Flaxseed
Flaxseed contains
a mix of fatty acids. It is high in polyunsaturated fatty acids
(73%), moderate in monounsaturated fatty acids (18%), and low
in saturated fatty acids (9%). The saturated fat level of flaxseed
is similar to that of canola. Flaxseed is a rich plant source
of alpha-linolenic acid (ALA), an essential fatty acid in the
human diet and the parent fatty acid of the omega-3 family,
as shown in Figure 1. ALA is converted to two main long-chain
fatty acids, eicosapentaenoic acid (EPA) and docosahexaenoic
acid (DHA), in a series of enzymatic reactions (below). ALA
has been shown to modulate eicosanoid synthesis,2 and
its concentration in breast milk exceeds that of DHA, suggesting
a particular requirement for ALA by infants.3, 4
Omega-6/Omega-3 Fatty Acid Ratio
of Flaxseed
ALA comprises about
57% of the total fatty acids in flaxseed, whereas the omega-6
fatty acids comprise about 16%, giving an omega-6/omega-3 ratio
of 0.3:1.1 Because the typical Western diet is high
in linoleic acid and low in omega-3 fatty acids, some experts
recommend replacing 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
omega-3 fatty acid intake. This improves the dietary omega-6/omega-3
ratio.5
Major Families of Unsaturated
Fatty Acids
There are four major families of unsaturated
fatty acids (see Table 1). Oleic acid is the most prevalent
fatty acid in nature and can be synthesized in the body from
dietary stearic acid (18:0).6 Palmitoleic acid can
be synthesized from dietary palmitic acid (16:0). Oleic and
palmitoleic acids are not essential in human nutrition because
they can be formed from dietary precursors. Two fatty acids
are required in the diets of humans because our bodies cannot
manufacture them from dietary precursors: Alpha-linolenic acid,
the parent compound of the omega-3 fatty acid family, and linoleic
acid, the parent compound of the omega-6 family. Arachidonic
acid, a metabolite of linoleic acid, is considered an essential
fatty acid (EFA) only when a linoleic acid deficiency exists.7
Desaturation
and elongation. Alpha-linolenic acid and linoleic acid
are converted to their respective metabolites by a series
of alternating desaturations and elongations. The desaturations
add a double bond by removing hydrogen, while the elongations
add two carbon atoms.
The first
step in the metabolism of both EFA families is desaturation,
catalysed by delta-6-desaturase. This step is followed by elongation,
then desaturation (catalysed by delta-5-desaturase), then elongation
and, finally, desaturation (catalysed by delta-4-desaturase).
The desaturation steps tend to be slow, while the elongation
steps are rapid. Thus, the tissue concentrations of gamma-linolenic
acid (GLA) (18:3n-6) and stearidonic acid (18:4n-3) tend to
be low, because they are formed slowly by desaturation and
then quickly elongated to other metabolites.8, 9
Competition between families. Mammals cannot
interconvert the omega-3 and omega-6 fatty acids. Furthermore,
their metabolism requires the same desaturation enzymes,
resulting in competition between the two families. An excess
of one family of fatty acids can interfere with the metabolism
of the other, reducing their incorporation into tissue lipids
and altering their biological effects.8, 10
Metabolism of Alpha-Linolenic
Acid (ALA)
Dietary ALA appears
to have two metabolic fates. It can be desaturated and elongated
to its long-chain metabolites or it can undergo ß-oxidation.
Although studies of ALA ß-oxidation in humans have not been conducted,
studies in animals suggest that the metabolism of ALA by ß-oxidation
may contribute significantly to energy production.11 Additional
research is needed to clarify the relative importance of these
metabolic pathways.
Conversion
of ALA to EPA and DHA. About 15% of ALA
is converted to EPA and about 5% is converted to DHA in a
process that is somewhat slow in humans.5, 11 Conversion
of ALA to its long-chain metabolites is affected by various
dietary factors. A diet rich in linoleic acid, for example,
has been shown to reduce ALA conversion by as much as 40%,10 and
a high maternal intake of linoleic acid lowers EPA and DHA
levels in umbilical plasma, suggesting reduced ALA conversion
and availability of omega-3 fatty acids to the developing
fetus.12 Saturated and trans fatty
acids also interfere with ALA desaturation and elongation,13,
14 and in rats, ethanol inhalation results in a significant
loss of liver DHA, indicating reduced ALA conversion.15 DHA
can be converted back into EPA in a reaction called retroconversion.
This is believed to be a minor metabolic pathway in humans.16
References
1. Canadian Grain Commission. Flaxseed
Export Quality Data. Winnipeg,
MB.
2. Ferretti A and Flanagan VP. Prostaglandins
Leukot Essent Fatty Acids. 1996;54:451-455.
3. Ratnayake WMN and Chen Z-Y. Lipids. 1996;31(Suppl):S279-S282.
4. Budowski P, et al. World Rev
Nutr Diet. 1994;75:105-108.
5. Nutrition Advisory Panel Meeting:
Executive Summary. Winnipeg, MB: Flax Council of Canada,
1995.
6. Food Fats and Oils. Washington, DC: Institute of Shortening and Edible
Oils, Inc., 1994.
7. Food and Nutrition Board, National
Research Council. In:Recommended Dietary Allowances, 10th
ed. Washington, DC: National Academy Press, 1989, pp.
44-51.
8. Horrobin DF and Manku MS. In: Omega-6 Essential Fatty Acids. Horrobin
DF, ed. New York, NY: Alan R. Liss, 1990, pp. 21-53.
9. Salem Jr N and Pawlosky RJ. World
Rev Nutr Diet. 1994;75:114-119.
10. 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.
11. Cunnane SC.
In: Flaxseed in Human Nutrition. Cunnane
SC and Thompson LU, eds. Champaign, IL: AOCS Press, 1995,
pp. 99-127.
12. Al MDM, et al. J
Am Coll Nutr. 1996;15:49-55.
13. Ackman RG and
Cunnane SC. In: Advances in Applied
Lipid Research. Padley FB, ed.
London: JAI Press Ltd., 1992, pp. 167-215.
14. Houwelingen AC v and Hornstra G. World Rev Nutr Diet. 1994;75:175-178.
15. Salem Jr N, et al. Lipids. 1996;31(Suppl):S153-S156.
16. Brossard N, et al. Am J Clin
Nutr. 1996;64:577-586.
Flax Council of Canada, 465-167 Lombard Ave., Winnipeg, MB Canada R3B
0T6, Email: flax@flaxcouncil.ca
Web site: http://www.flaxcouncil.ca/
