Use of Oxylipins to Determine the Dietary Requirement for the Essential Fatty Acid, α-Linolenic Acid (ALA)

Objectives: The dietary requirement for ALA remains unclear, as evidenced by the AI recommendation for this essential fatty acid. In previous studies we observed that the amount of dietary ALA required to maximize DHA oxylipin levels appears to be higher than the amount required to maximize tissue DHA levels. Further, we also observed that dietary ALA could reduce both arachidonic acid (ARA) and its oxylipins. However, only DHA levels have classically been used to estimate the ALA requirement. Since oxylipins mediate many fatty acid functions, this study was designed to examine whether both DHA and ARA oxylipins could be used to more accurately determine the dietary ALA requirement. Methods: Rats and mice were provided a range of 0.1-2.5 g ALA and a consistent level of 2g of linoleic acid per 100g diet. Non-esterified oxylipins in serum, liver, kidney and brain homogenates underwent solid phase extraction and were analyzed by HPLC-MS/MS. Results: In response to increasing levels of dietary ALA, DHA oxylipins initially increased rapidly and then plateaued whereas ARA oxylipins tended to decrease and plateau. Thus, the ratio of DHA/ARA oxylipins synthesized via the major common biosynthetic pathway (lipoxygenase) was calculated and the breakpoint of the transition from increase to plateau was estimated by piecewise regression. In serum, liver and kidney the estimated breakpoint indicated an average ALA requirement of ∼0.7g/100g diet (1.7%energy), which would result in a recommended dietary allowance of ∼1.0g/100g diet (2.4%energy). In contrast, in the brain this oxylipin ratio was constant across all levels of dietary ALA, indicating that the breakpoint occurred at less than 0.1g of ALA/100g of diet, and suggesting that the brain has priority for ALA metabolism. Conclusions: The combination of DHA and ARA oxylipins can be used to estimate the dietary ALA requirement and indicates that the requirement is higher than previously estimated using DHA alone. Further, this ratio could be used as a biomarker of w-3 fatty acid status to address questions such as the bioequivalence of w-3 fatty acids, or the optimal dietary w-6/w-3fatty acid ratio. Funding Sources: This work was supported by the Natural Sciences and Engineering Research Council of Canada [RGPIN-06215-2020 and Vanier Canada Graduate Scholarship CVG-176282].