Bioequivalence of docosahexaenoic acid intake to the dietary requirement of alpha-linolenic acid in growing rats using non-esterified oxylipins and fatty acids

Alpha-linolenic acid (ALA) is an essential n-3 fatty acid for which dietary requirements remain undefined. ALA can be converted to docosahexaenoic acid (DHA), which by itself can also meet the dietary n-3 fatty acid requirement. However, the amount of dietary DHA that is bioequivalent to ALA remains unknown. The study objective was to assess the dietary ALA requirement in female and male rats in order to estimate the dose of DHA that is equivalent to the requirement of ALA, using non-esterified oxylipins and fatty acids. For this study, 168 male and female rats were assigned to 9 groups containing graded doses of dietary ALA (0.10 to 2.0g/100g diet), and 5 groups with graded levels of DHA (0.07 to 1.3g/100g diet). All diets contained 2g of linoleic acid/100g diet and were based on the AIN93G. Non-esterified fatty acids and oxylipins were analyzed in serum, liver, heart, and brain using solid phase extraction and quantification by HPLC-MS/MS. Using piecewise regression, breakpoints were calculated for ALA diets using DHA/arachidonic acid (ARA) and hydroxy-DHA/hydroxy-ARA oxylipins (DHAOH/ARAOH). Our results indicate that the dietary ALA requirement is 0.95g ALA/100g diet [0.72, 1.17] in female and 0.55g ALA/100g diet [0.31, 0.80] in male rats. Serum appears to be a suitable indicator of the ALA requirement. The amount of DHA required to achieve the same effect as ALA on DHAOH/ARAOH is 4.6-4.8-fold lower, at 0.20g DHA/100g diet [0.14, 0.28] for females and 0.12g DHA/100g diet [0.07, 0.22] for males. Our results indicate a higher ALA requirement than is provided by the standard AIN93G diet, suggesting that reformulation may be required. In addition, dietary DHA is ~4.5 times more potent than ALA in meeting the n-3 dietary requirement, with the effects of DHA being almost entirely additional in those with usual ALA intake.