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Objective: To investigate the in vivo oxidation of [sup.13]C18:2n-6 and its conversion into longer-chain polyunsaturates (LCPs) in healthy subjects. Design: Blood samples were collected from six subjects before (fasted) and 7, 11 (non-fasted), 24, 48, 72, 168 and 336 h (fasted) after ingestion of a single bolus of 45 mg uniformly labeled [sup.13]C18:2n-6 dissolved in 8 g olive oil. In three subjects, breath was also sampled and C[O.sub.2] production measured every hour during the first 12h. Subjects consumed their habitual diets. Plasma [sup.13]C-enrichments were measured by GC-G-IRMS and fatty acid compositions by GC/FID. Setting: Maastricht University, Department of Human Biology. Subjects: Three men and three women, recruited by local advertisement. Results: The tracer/tracee ratio (TTR) of C18:2n-6 in plasma total lipids was already increased 5 h after tracer intake. The mean peak amount ([+ or -] s.e.m) of [sup.13]C18:2n-6 (3.4 [+ or -] 0.8 mg; 7.6% of dose) was found after about 17 h, [sup.13]C18:3n-6 (0.018 [+ or -] 0.008 mg; 0.04% of dose) after 7-48 h, and [sup.13]C20:3n-6 (0.028 [+ or -] 0.011 mg; 0.06% of dose) after 48-336 h. Time to peak TTRs of C20:4n-6 varied between subjects and were on average 0.022 [+ or -] 0.006 mg (0.05% of dose). The proportion of [sup.13]C18:2n-6 recovered in breath after 12h ranged between 16.8 and 25.1%. Conclusions: These findings suggest that a single bolus of 45 mg U-[sup.13]C18:2n-6 can be used to study the oxidation of [sup.13]C18:2n-6. However, because of the low TTRs for C20:4n-6, a higher dose is recommended for studying the conversion of [sup.13]C18:2n-6 into LCPs. In addition, since only about 35% of the tracer was found in plasma total lipids and as [sup.13]C[O.sub.2] in breath, it might be necessary to study other accessible lipid fractions as well to study the overall conversion of linoleic acid. Descriptors: linoleic acid; metabolism; desaturation; elongation; arachidonic acid; humans; oxidation; stable isotope; GGGIRMS, Introduction Arachidonic acid (C20:4n-6) is a major precursor of eicosanoids, which have important physiological and pathological functions in humans such as in inflammatory processes, immunological reactions and platelet aggregation. Arachidonic [...]

An in vitro study of the influence of fish oil and vitamin E supplementation on copper-catalyzed oxidation of low density lipoprotein (LDL) showed that fish oil increased the ability of LDL to undergo copper-catalyzed oxidazability while vitamin E resisted the process. Fish oil also increased the formation of conjugated dienes during oxidation. Vitamin E, on the other hand, increased the time required to reach the maximum amount.