Metabolism of polyunsaturated fatty acids and their oxylipins

Abstract

Background and aim: The beneficial health effects of omega (n)3 polyunsaturated fatty acids (PUFAs) have been mainly attributed to eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Many of the positive health effects are believed to be mediated via their oxidized metabolites called oxylipins. The intake of EPA and DHA is low in most countries due to low fish intake. Consequently, the omega 3 index, which is defined as relative weight percent of EPA+DHA in red blood cells (RBCs), is low. Furthermore, PUFAs are also subject to a complex metabolism that is not fully understood yet. However, there is consensus that human enzymes can synthesize EPA and DHA from their essential precursor PUFA alpha-linolenic acid (ALA). However, ALA intake and conversion rates have been shown to be quite low. It is currently being discussed that the high intake of the n6 PUFA linoleic acid (LA) might be one reason for low conversion rates. Furthermore, retroconversion of DHA to EPA has been shown to occur upon DHA supplementation. Methodologically, the investigation of the metabolism of PUFAs and their oxylipins in humans is challenging. Previous studies were limited by inhomogeneous study collectives, suboptimal designs and analyses solely based on relative fatty acid blood ratios (instead of absolute concentrations). Moreover, analytical-, inter- and intra-day fluctuations of oxylipin patterns are unknown. The aim of the present thesis was therefore to (i) investigate the metabolism of PUFAs and their oxylipins in human blood with homogeneous study collectives and under controlled conditions, using defined PUFA administrations and (ii) determine analytical-, inter- and intra-day variations of oxylipin patterns. Methods: Four separate human studies were carried out. All studies were conducted with homogenous collectives (n=12-19) regarding dietary habits (especially fish and PUFA intake), sex, age, smoking status, BMI and EPA and DHA concentrations in blood. In two separate studies, the effects of a 12-week ALA and DHA supplementation, respectively, on PUFA and oxylipin patterns were investigated. Several intermediate timepoints (1, 3 and 6 weeks) allowed for the analysis of PUFA and oxylipin concentrations over the course of time. In a further two-week cross-over study, the effects of a low-LA/high-ALA (0.56:1) and a high-LA/low-ALA diet (25.6:1) on PUFA metabolism were compared. In all studies, PUFAs were quantified in RBCs, which have been shown to have the lowest variability compared to other blood sample types. The last study investigated analytical variance of oxylipin concentrations, the effect of standardized and non-standardized diet on fasting oxylipins and inter- as well as intra-day fluctuations of the oxylipin pattern. Results: In all studies, the variability of PUFA and oxylipin data was significantly lower compared to previous studies. ALA supplementation increased ALA, EPA, DPAn3 and decreased DHA concentrations in RBCs, while direct DHA supplementation increased DHA as well as EPA concentrations. In both studies, changes in plasma oxylipin concentrations generally reflected their precursor PUFAs in RBCs. The low-LA/high-ALA diet led to a faster increase of ALA and EPA concentrations compared to simple ALA supplementation while DPAn3 and DHA concentrations remained constant. Analytical variance of oxylipins was comparable to those of other LC-MS based oxylipin quantification methods and for 84 % of analytes between 5 20 %. Fasting plasma oxylipin samples were subject to minor fluctuations between different days irrespective from a standardization of the diet, while oxylipin concentrations during the day fluctuated largely. Conclusion: Modifications of dietary PUFA intakes lead to shifts in the entire PUFA profile in RBCs. An ALA intake 10-fold higher than common in Germany does not lead to an increase of the omega-3 index confirming the low conversion of ALA to EPA and DHA observed in other studies. In contrast, a concomitant reduction of LA intake slightly increases the omega 3 index supporting the inhibitory effect of LA on ALA conversion. Supplementation of preformed DHA leads to a large increase of DHA, but also increases EPA possibly due to retroconversion of DHA to EPA. From a dietary point of view, supplementation of preformed DHA should be preferred over ALA supplementation to significantly increase the status of EPA and DHA. DHA and ALA supplementation causes a shift in the whole oxylipin pattern, which indicates a complex interplay between PUFA and oxylipin metabolism. Analytical variance of oxylipin concentrations of the method used here is low and fasting plasma seems to be suitable for the investigation of oxylipin biology.