We have previously proposed that the omega-3 fatty acid, stearidonic acid (SDA) [18:4(n-3)], a metabolic intermediate between ALA and EPA, could effectively bridge the gap between the sustainability of a land-based oil and the effectiveness of fish oil (11). SDA is an intermediate between ALA and EPA and can represent up to 4% of the total fatty acids in fish oil (12), and thus has been present in the human diet for as long as fish, such as menhaden, have been in the diet. SDA represents the Δ6 desaturation product of ALA and as such, bypasses a rate-limiting step in the conversion of dietary ALA to EPA. The relative effectiveness with which dietary SDA is bioconverted to EPA is thus dependent upon regulation of the subsequent desaturation and elongation reactions (Fig. 1). Recently, the relative efficiencies of dietary SDA, ALA and EPA in elevating plasma EPA was assessed in a clinical study (11). In this study, dietary SDA was found to increase plasma EPA between threefold and fourfold more efficiently than comparable levels of ALA and was approximately one-third as effective as dietary EPA (Fig. 3). At this relative rate of metabolic conversion of SDA to EPA, moderate intakes of SDA-containing oil could positively impact plasma EPA to an extent that would be expected to confer the cardiovascular benefits associated with consumption of EPA. Through incorporation into a variety of widely consumed foods, SDA could significantly increase omega-3 intakes to such an extent that the health benefits of a diet containing adequate omega-3 fats can be realized by a larger sector of the population.
Natural sources of SDA do exist, but are limited such that wide-scale enrichment of foods is currently impossible. In addition to its occurrence in fish oil, SDA is also found in the seed oil and leaves of a number of plants, including evening primrose (Oenothera biennis), borage (Borago officinalis), black currant (Ribes nigrum) and Echium plantagineum. However, none of these species represent crops adapted to wide scale production and yields of oil are low and variable and not economically competitive. One approach to achieving SDA production on a large scale would be to implement an intensive breeding program for a species such as Echium plantagineum for adaptation to commercial scale agriculture, although the feasibility of this is questionable. An alternative approach, and one we have attempted successfully, is to engineer the capacity to produce SDA in to a high oil-yielding oilseed crop where existing commercial cropping and oil extraction technologies are utilized. That approach is presented below.
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