Omega-3 Fatty Acids: Chemistry and Nutritional Significance

Fereidoon Shahidi Department of Biochemistry, Memorial University of Newfoundland St. John's, NL, Canada

INTRODUCTION

The importance of polyunsaturated fatty acids (PUFA) in health and nutrition is well recognised. While saturated and monounsaturated fatty acids may be synthesised in the body, polyunsaturated fatty acids cannot be synthesised de-novo and must be supplied in the diet. Hence, PUFA are known as essential fatty acids (EFAs), with linoleic acid (LA) and α-linolenic acid (ALA) serving as the important "parent" components. These fatty acids are metabolised in the body and undergo a series of desaturation and elongation reactions to produce long chain PUFA which are regarded as conditionally essential (Figure 1).

Figure 1. The n-6 and n-3 fatty acids and their metabolites

The essential fatty acids are classified into the omega-3 (or n-3) and omega-6 (or n-6) families based on the position of the first double bond from the methyl end group in the fatty acid chain. This is because the position of the double bond from the methyl end dictates the biological activity of the molecules involved. Thus, LA and ALA are regarded as the parent n-6 and n-3 fatty acids in each series, respectively. The enzymes responsible for desaturation and chain elongation in both n-3 and n-6 families are identical. Imbalance in the intake of n-6 versus n-3 fatty acids may lead to over-produc-tion of eicosanoids with less preferred activities. However, the conversion efficacy from LA and ALA to long-chain PUFA is at best 5% or so in healthy individuals.¹

The first step in metabolism of n-3 and n-6 EFAs requires delta-6-desaturase and is in fact the slowest or rate determining step. This is the enzyme with impaired activity due to aging, disease conditions (arthritis, diabetes, hypertension and inflammation), life-style factors (alcohol consumption, smoking and stress), micronutrient deficiency (zinc, magnesium and vitamin B₆) or certain drugs (e.g. corticosteroids). Thus, consumption of adequate amounts of long-chain PUFA (LC PUFA) is generally recommended in order to help in disease prevention and health promotion.

OMEGA-3 POLYUNSATURATED FATTY ACIDS

The omega-3 family of fatty acids is derived from the parent fatty acid -linolenic acid (ALA). ALA is present in flax and perilla oils and in smaller amounts in canola and soybean oils (Table 1). As noted earlier, the conversion of ALA to LC PUFA is inefficient and hence their direct dietary intake is recommended. Long-chain omega-3 PUFA are synthesised mainly by the uni- and multicellular phytoplanktons and algae and are eventually transferred to the food web and into the lipids of aquatic species. Since fish feed on algae and marine mammals eat fish, they become rich in omega-3 PUFA. Therefore, the best sources of long-chain omega-3 PUFA are the flesh of fatty fish such as herring, mackerel, menhaden, salmon, capelin, anchovy and tuna, among others, and the liver of white lean fish such as cod and halibut. In addition, the blubber of marine mammals such as seals and whales provides fatty acids that are similar to those of fish and fish liver oils (Table 1). The high content of LC omega-3 PUFA in marine organisms is suggested to be a consequence of cold temperature adaptation in which they remain liquid and oppose any tendency to crystallize.

Fatty acid	Canola	Soy	Flax	Seal Blubber	Cod Liver	Menhaden	Algal
Table 1. Major Fatty Acid Composition of Selected Sources of Omega-3 Oils (ω/ω%)

The common LC omega-3 PUFA in marine oils are eicosapentaenoic acid (EPA), docosapentaenoic acid (DPA) and docosahexaenoic acid (DHA). While DPA is present in low levels in fish oils, it is found at about 5% in seal blubber oil. The location of omega-3 fatty acids in the triacylglycerol molecules may further influence their absorption and deposition in the body. While fish oils' omega-3 fatty acids are primarily located in the sn-2 positions, in seal blubber oil there is a dominance of omega-3 fatty acids in the sn-1 and sn-3 position of triacylglycerols. The beneficial health effects of LC omega-3 PUFA are ascribed to their role in modifying the synthesis of eicosanoids. However, these highly unsaturated fatty acids (HUFA) are prone to oxidation, both in-vivo and in-vitro. The body protects itself via antioxidants, particularly vitamin E. Therefore, intake of omega-3 PUFA must be completed by adequate consumption of dietary antioxidants. Antioxidants other than vitamin E are found to augment the in-vivo effects of - tocopherol. Furthermore, stabilisation of such oils by the use of appropriate antioxidants during storage is necessary. Thus, many of the marine oil capsules sold as dietary supplements in the market contain varying amounts of α - or mixed tocopherols in order to stabilise them against oxidation and this would indirectly address the additional burden on the body resulting from high degree of unsaturation of their fatty acid constituents. The high content of HUFA in oils also presents a challenge in delivering them in a form that does not have off-flavours. However, novel microencapsulation processes may address this concern when food and nutraceutical applications of such oils is intended.²

NUTRITIONAL AND HEALTH BENEFITS OF OMEGA-3 FATTY ACIDS

A rapidly growing body of literature in recent years has confirmed the role of omega-3 fatty acids in alleviating cardiovascular disease, type 2 diabetes, inflammatory ailments and autoimmune disorders, among others. These effects are generally rendered by more moderate eicosanoids, namely thromhoxane A₃ (TXA₃) and prostacyclin I₃ (PGI₃) as compared to TXA₂ and PGI₂, from 20:5T3 and 20:4T6, respectively. The TXA₃ and PGI₃ do not trigger platelet aggregation as much as TXA₂ and PGI₂ formed from the omega-6 PUFA. Therefore, long chain omega-3 PUFA may help reduce the tendency for blood to clot. There are, however, two other processes involved in blood clotting and marine oils may not play a role in either of them. These include: a cascade of reactions resulting in the formation of an insoluble protein, fibrin, which traps blood cells and clumps of platelets to form a clot; and fibrinolysis, which involves the dissolution of blood cells by the enzyme plasmin, thus ensuring that blood is coagulated only when necessary.³

However, there are other possibilities for the beneficial effects of marine oils on heart disease. These include the ability to lower plasma triacylglycerol levels, the ability to raise plasma high density lipoprotein (HDL) cholesterol level, the ability to reduce the likelihood of cardiac arrhythmias, which are potentially lethal and often cause sudden death, and the ability to lower blood pressure, particularly in subjects with high blood pressure.

There are many other studies that report on the beneficial effects of omega3 fatty acids on rheumatoid arthritis, diabetes and, more recently, on mental health, including schizophrenia and bipolar disorders. It is also important to note that DHA is essential for the development of the foetal brain and the eye retina. DHA is one of the most abundant fatty acids in the cellular membranes of the brain and its level in the foetal brain increases steadily during the last trimester of pregnancy. Hence it is necessary for pregnant women, especially those with chain pregnancy, to consume an appropriate amount of long-chain omega-3 PUFA. DHA is also present in human milk, particularly those on diets rich in long chain omega-3 PUFA, therefore DHA must be added to infant formula and also in the diet of pregnant and lactating women.⁴ EPA, although quite important, may interfere with the formation of archidonic acid and its metabolites, hence its presence in infant formula has been questioned.

FOOD AND NUTRACEUTICAL AND DIETARY SUPPLEMENT APPLICATION OF OMEGA-3 FATTY ACIDS

Recent advances in the development of functional foods and nutraceuticals have led to novel formulations which often include omega-3 fatty acids. Thus, flaxseed is now used in specialty bakery and cereal products. In addition, long chain omega-3 fatty acids may be acquired from the diet by direct consumption of seafoods, especially fatty fish.⁵ However, much of the population do not usually consume fish, especially fatty fish, and hence the use of long chain omega-3 fatty acids from marine or algal origin is recommended. The products in which such oils may be included are bread, crackers, cereals, cereal bars, milk and dairy products, fruit juices, salad dressing, mayonnaise, spreads, margarine, pasta, meat and lean fish products and baby food and infant formulas. In these cases, depending on the time interval between preparation and consumption, application of antioxidants as well as consideration of appropriate packaging and storage constitution might prove beneficial.

For therapeutic purposes, however, omega-3 concentrates may be required. Production of omega-3 concentrates may be carried out in order to offer a pure fatty acid, such as EPA or DHA or a mixture of omega-3 fatty acids.⁶ These may be produced and consumed in the free fatty acid, simple ackyl ester or triacylglycerol form.⁷ Concentrates containing different levels of total omega-3 fatty acids are now available.

REFERENCES

1. Shahidi, F. and Finley, J.W., Eds. 2001. Omega-3 Fatty Acids: Chemistry, Nutrition and Health Effects. ACS Symposium Series 788. American Chemical Society, Washington, DC.
2. Wanasundara, U.N. and Shahidi, F. 1995. Storage stability of microencapsulated seal blubber oil. J. Food Lipids 2: 73-86.
3. Shahidi, F. and Kim, S-K. 2002. In Quality Management of Nutraceuticals. ACS Symposium Series 803, Ho, C-T. and Chong, Q.Y., Eds. Pp. 76-87. American Chemical Society, Washington, D.C.
4. Simopoulos, A.P. 1991. Omega-3 fatty acids in health and disease and growth and development. Am. J. Clin. Nutr. 54: 438 463.
5. Shahidi, F., Ed. 2000. Seafoods in Health and Nutrition - Transformation in Fisheries and Aquaculture: Global Perspectives. ScienceTech Publishing Co., St. John's, NL, Canada.
6. Shahidi, F. and Wanasundara, U.N. 1998. Omega-3 fatty acid concentrates: nutritional aspects and production technologies. Trends Food Sci. Technol. 9: 230 240.
7. Wanasundara, U.N., Wanasundara, J. and Shahidi, F. 2002. In Seafoods - Quality, Technology and Nutraceutical Applications. Alasalvar, C. and Taylor, T., eds. Pp. 157 174. Springer, Berlin and New York.

BIOGRAPHY

Fereidoon Shahidi, Ph.D., FACS, FCIC, FCIFST, FIFT, FRSC, is a University Research Professor at the Memorial University of Newfoundland. He is the author of over 500 referred research articles and book chapters and author or editor of over 30 books. Dr. Shahidi is the editor-in-chief of the Journal of Food Lipids and serves as the editorial boards of Food Chemistry, Journal of Agricultural and Food Chemistry and Journal of Food Sciences. He is the editor of the 6th Edition of Bailey's Industrial Oils and Fats as well as the editor-in-chief of the Nutraceutical Science and Technology Series. Dr. Shahidi is a founder of the Nutraceutical and Functional Food Division of IFT and an organiser of the International Conference and Exhibition on Nutraceuticals and Functional Foods, which is held annually. Dr. Shahidi is the recipient of the 2005 Stephen S. Chang Award from IFT.