Omega-3 fatty acids are being increasingly promoted as important dietary components for health and disease prevention.1,2 These fatty acids are naturally enriched in fatty fish like salmon and tuna and in fish-oil supplements. An increasing number of foods that are not traditional sources of omega-3 fatty acids, such as dairy and bakery products, are now being fortified with small amounts of these fatty acids.2
My omega-3 breakfast example
This recent promotion of omega-3 fatty acids has likely been driven by recommendations for omega-3 fatty acid consumption made by scientific groups such as the American Heart Association.3 The search for the molecular and cellular mechanisms by which omega-3 fatty acids affect health and disease has led to a large body of evidence which suggests that these dietary lipids modulate numerous processes, including brain and visual development, inflammatory reactions, thrombosis and carcinogenesis. An obvious question that someone unfamiliar with omega-3 fatty acids might ask is: How can these nutrients affect so many seemingly unrelated processes in different cell types and tissues?
What are omega-3 fatty acids?
Our diet contains a complex mixture of fats and oils whose basic structural components are fatty acids. We generally consume at least 20 different types of fatty acids, which are classified as saturated, monounsaturated and polyunsaturated. Fatty acids have many fates in the body, including β-oxidation for energy, storage in depot fat or incorporation into phospholipids, which form the major structural components of all cellular membranes.
Not all dietary fatty acids are created equally. Because humans do not have the enzymatic machinery required to synthesize omega-3 fatty acids, they must be obtained from the diet (termed “essential fatty acids”). Even among dietary polyunsaturated fatty acids, there are different families of compounds, and this is at the heart of the difference between omega-3 fatty acids and other dietary lipids. Omega-3 fatty acids generally account for a small fraction of the total daily consumption of fatty acids in Western societies.2,4
Fish such as tuna, trout and salmon are especially rich sources of these fatty acids. Fish-oil supplements are also a rich source, as they typically contain 30%–50% omega-3 fatty acids by weight. Small quantities of omega-3 fatty acids are naturally present in meats like beef, pork and poultry. Despite containing small quantities of omega-3 fatty acids, meats contribute to the overall intake of these fatty acids simply because of the large amounts consumed in Western societies.4
Omega-3 fatty acids from fish and fish oils are not to be confused with those from plant sources, such as flax and canola oil. These plant oils are enriched in an omega-3 fatty acid called α-linolenic acid, which is a metabolic precursor of the omega-3 fatty acids found in fish and fish oils. Although we are able to convert dietary α-linolenic acid into more beneficial omega-3 molecules such as, eicosapentaenoic, docosapentaenoic and docosahexaenoic acids (which are found in fish and fish oils), this conversion is not efficient in people who consume a typical Western diet.
Consequently, following the consumption of foods containing α-linolenic acid, our tissues are exposed to very little of the types of omega-3 fatty acids found in fish and fish oils. Some beneficial biological activity has been attributed to plant-derived omega-3 fatty acids; however, the associated health benefits are likely independent of the conversion of α-linolenic acid to the fatty acids found in fish. In addition, dietary oils that are rich in α-linolenic acid do not, for the most part, reproduce the biological activity associated with dietary fish oils.3
Dietary intake
Among people who consume typical Western diets, the average intake of the types of omega-3 fatty acids found in fish is about 150 mg per day. This is equivalent to consuming about 1 fish meal every 10 days.5 This falls well below the combined intake of dietary eicosapentaenoic and docosahexaenoic acids (650 mg/d) recommended at a workshop held in 1999 on essential fatty acids at the US National Institutes of Health.6 Similarly, the International Society for the Study of Fatty Acids and Lipids recommends an intake of at least 500 mg daily.7 The American Heart Association recommends that people without coronary heart disease have 2 fish meals each week (at least 300 mg of omega-3 fatty acids daily), and they recommend that patients with documented coronary heart disease receive 1000 mg daily.3
In addition, the US Food and Drug Administration recommends that the average daily intake of omega-3 fatty acids from fish should not exceed 3000 mg because of possible adverse effects related to glycemic control, increased bleeding tendencies and elevations in low-density lipoprotein cholesterol associated with very high intake of omega-3 fatty acids.8 Whether these concerns are warranted remains to be determined; however, it is not clear if these higher doses would provide additional health benefits.
Although the recommendations for the intake of omega-3 fatty acids are based on extensive literature, in practice, these intakes have been difficult to achieve because dietary habits are well entrenched and are difficult to change.
Consequently, most people are reluctant to regularly include several weekly servings of fish in their diets. Additionally, concerns exist that fish contain environmental contaminants such as heavy metals, methyl mercury and organochlorides.3 The consumption of dietary fish-oil supplements is an effective way to increase omega-3 fatty acid intake without changing dietary habits; however, compliance is a problem because 1–3 fish-oil capsules must be taken daily to achieve the recommended intake.
Despite these obstacles, the demand for fatty acids has been increasing. In recent years, an increasing number of foods have become available that are fortified with omega-3 fatty acids mostly from fish oils. Overall, the increased awareness of omega-3 fatty acids from fish and fish oils raises the question of whether fish-oil producers will be able to meet the impending worldwide demand.
New Sources of Omega-3
Consequently, a number of alternative sources of omega-3 fatty acids have been or are being developed. For example, oils naturally enriched in docosahexaenoic acid are being extracted from cultured microorganisms like the algae Crypthecodinium cohnii and are now being used to fortify a number of products, including baby formula. Similarly, novel plants like Echium plantagineum are now being cultivated because their seed oil naturally contains stearidonic acid, an intermediate in the metabolism of omega-3 fatty acids.
Finally, a number of companies have developed transgenic varieties of common plants such canola, soybean and safflower that can produce seed oils that are highly enriched with stearidonic, eicosapentaenoic and docosahexanoic acids.
The increased diversity and availability of sources of omega-3 fatty acids will likely allow the continued expansion of food products fortified with these fatty acids, a trend that may result in the attainment of the recommended dietary intake of these nutrients.
Given the extensive literature supporting the health-promoting effects of these dietary components, the gentle shift to reduced cell and tissue reactivity may be accompanied by a subtle, but relevant, shift in the overall health and well-being of the population.
REFERENCES
1. Lee KW, Lip GY. The role of omega-3 fatty acids in the secondary prevention of cardiovascular disease. QJM 2003;96:465-80. [Abstract] 2. Whelan J, Rust C. Innovative dietary sources of n-3 fatty acids. Annu Rev Nutr2006;26:75-103. [Abstract] 3. Kris-Etherton PM, Harris WS, Appel LJ. Fish consumption, fish oil, omega-3 fatty acids, and cardiovascular disease. Circulation 2002;106:2747-57. [Abstract] 4. Taber L, Chiu CH, Whelan J. Assessment of the arachidonic acid content in foods commonly consumed in the American diet. Lipids 1998;33:1151-7. [Abstract] 5. Kris-Etherton PM, Taylor DS, Yu-Poth S, et al. Polyunsaturated fatty acids in the food chain in the United States. Am J Clin Nutr 2000;71:179S-88S. [Abstract]
6. Simopoulos AP, Leaf A, Salem N Jr. Essentiality of and recommended dietary intakes for omega-6 and omega-3 fatty acids.Ann Nutr Metab1999;43:127-30. [Abstract]
7. Cunnane S, Drevon CA, Harris W, et al. Recommendations for intakes of polyunsaturated fatty acids in healthy adults.ISSFAL Newsl2004;11:12-25.
8. Department of Health and Human Services. US Food and Drug Administration. Substances affirmed as generally recognized as safe: menhaden oil. Washington: Federal Register; 1997. 1997;62:30751–7 (21 CFR Part 184 [Docket No. 86G-0289]). Available:http://frwebgate.access.gpo.gov/cgi-bin/getdoc.cgi?dbname=1997_register&docid=fr05jn97-5(accessed 2007 Nov 26).