Background
Dietary fats and oils represent a significant percentage of the daily caloric intake in the United States comprising >33% of total calories (1). A large body of scientific evidence has implicated the quantity and/or quality of dietary fats in the development of several diseases, including cardiovascular disease (CVD),2 some cancers and arthritis (2). Not surprisingly, organizations including the USDA, American Heart Association and National Academy of Sciences/Institute of Medicine have made dietary recommendations recently that focus not only on the quantity but also on the types of fats in the diet, and generally recommend substituting monounsaturated and polyunsaturated fats for saturated fatty acids (SFA) (3).
Because PUFA are not synthesized de novo in mammals, they must be derived from the diet. Once ingested, they are further metabolized and the resulting Long Chain PUFA populate cellular membranes and serve as precursors for hormone-like eicosanoids. Dietary PUFA are almost exclusively plant derived. In plants PUFA are derived from SFA. SFA are progressively desaturated to form monosaturated fatty acid, oleic acid (OA) [18:1(n-9)] and the PUFA, linoleic acid (LA) and 
-linolenic acid (ALA) [18:2(n-6) and 18:3(n-3), respectively]. Depending on the position of the first double bond in the fatty acid molecule, polyunsaturates are classified as either omega-6 (n-6) or omega-3 (n-3) fatty acids (Fig. 1). The PUFA biosynthetic pathway occurs in virtually all plant cells, hence, omega-6 and omega-3 fatty acids are present in varying proportions in leaves, seeds and oil, and from there are incorporated into the diet. In the U.S. polyunsaturated fats constitute ~7% of total energy intake. LA compromises up to 89% of total PUFA energy intake, whereas ALA typically comprises only about 10% total PUFA energy in adult diets (4). This relative disparity in (n-3) to (n-6) intake is reflected in the (n-3) to (n-6) ratio of the most widely consumed vegetable oil in the U.S. diet, soybean oil, which constitutes ~83% of the vegetable oil intake (5) (Fig. 2).
Omega-3 fatty acids, known to be essential for growth and development, have been positively associated with health and the prevention and treatment of heart disease, arthritis, inflammatory and autoimmune diseases and cancer (
2). Accordingly, there are now dietary recommendations and guidelines for omega-3 fatty acid intakes. For example, in a recent Scientific Statement, the AHA Dietary Guidelines suggest Americans consume at least two servings of fish per week, and include in the diet vegetable oils rich in the omega-3 fatty acid, ALA (
6). It also recommends that 1.3–2.7 g/d total omega-3 fats be consumed. Despite these recommendations, it is estimated that actual dietary intakes of omega-3 fatty acids, and eicosapentaenoic (EPA) and docosahexaenoic acids (DHA) specifically, are as low as one-tenth of these levels.
There is consensus for the need for increased dietary omega-3 intakes among populations where intakes are below recommended levels. However, it remains unknown how to achieve this for large sectors of the population, especially where dietary preferences exclude fish. The concept of enriching a wide variety of foods with omega-3 fatty acids, so that consumers can chose omega-3 enriched foods that suit their individual preferences, is logical and has been proven in concept as an effective means to increase omega-3 intakes on a wide scale (7). Presently, sources of omega-3 fats suitable to enrich foods include vegetable and nut oils, which can contain up to 50% ALA, marine oils and EPA and DHA in varying amounts (4) and single-cell oils that are derived from the fermentation of microalgae, which contain DHA and/or EPA. However, there are limitations to each of these sources. The bioconversion of dietary ALA to EPA, which is necessary for the therapeutic and preventative benefits of omega-3 fats, is extremely inefficient with as little as 0.2% of plasma ALA undergoing conversion to EPA (8). Nonhydrogenated fish oils, on the other hand, provide EPA and DHA, the most effective forms of dietary omega-3 fatty acids for decreasing CVD risk and improving overall health. However, persistent questions exist about the sustainability of global fisheries. It is estimated that to achieve the recommended levels of EPA and DHA, a fourfold increase in fish consumption in the United States is necessary (4). Yet, yields from global fisheries have been reported to be stagnant or declining (9), and although aquaculture is a rapidly growing source of fish, the dietary requirements of omega-3 containing farmed fish for EPA and DHA requires fish meal and fish oil be provided in their diets. Hence, the same supply pressures that are facing wild stocks of fish will likely impact farmed fish (10). In addition to issues of sustainability, there is increasing alarm over levels of methyl mercury in some species of long-lived fish, that has prompted warnings to limit consumption of certain fish species, including swordfish, mackerel and shark, and for some at risk groups, avoid consumption all together (Environmental Protection Agency; National Academy of Sciences; Institutes of Medicine). Oil from fermentation of microalgae can be very high in EPA or DHA. Once extracted, it is suitable for enrichment of food, however, it is not currently being produced in sufficient quantities for wide-scale impact.
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