The science behind dietary omega-3 fatty acids

CMAJ
Marc E. Surette, PhD
Marc Surette is Professor and Canada Research Chair in Cellular Lipid Metabolism, Département de Chimie et Biochimie, Université de Moncton, Moncton, NB

Correspondence to: Dr. Marc Surette, Département de Chimie et Biochimie, Université de Moncton, Moncton NB E1A 3E9; fax 506 858-4541; marc.surette@umoncton.ca

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 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? The goal of this review is not to comment on the extent to which dietary omega-3 fatty acids affect health and disease, but rather it is to give an overview of the nature of these dietary components and to present some of the mechanisms by which they may modulate cellular functions.

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 -linolenic acid, which is a metabolic precursor of the omega-3 fatty acids found in fish and fish oils (Figure 1). Although we are able to convert dietary -linolenic acid into 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 -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 -linolenic acid to the fatty acids found in fish. In addition, dietary oils that are rich in -linolenic acid do not, for the most part, reproduce the biological activity associated with dietary fish oils.3 The balance of this review will address the types of omega-3 fatty acids typically found in fatty fish and fish-oil supplements.

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