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Little is known about the uptake or metabolism of essential fatty acids (EFAs) in various mammalian organs. Thus, the distribution of deuterated alpha-linolenic acid (18:3n-3) and linoleic acid (18:2n-6) and their metabolites was studied using a stable isotope tracer technique. Rats were orally administered a single dose of a mixture (20 mg each) of ethyl D5-18:3n-3 and D5-18:2n-6, and 25 tissues per animal were analyzed for D5-labeled PUFAs at 4, 8, 24, 96, 168, 240, 360, and 600 h after dosing. Plasma, stomach, and spleen contained the highest concentrations of labeled precursors at the earliest time points, whereas other internal organs and red blood cells reached their maximal concentrations at 8 h. The time-course data were consistent with liver metabolism of EFAs, but local metabolism in other tissues could not be ruled out. Brain, spinal cord, heart, testis, and eye accumulated docosahexaenoic acid with time, whereas skin accumulated mainly 20:4n-6. On average, approximately 16-18% of the D5-18:3n-3 and D5-18:2n-6 initial dosage was eventually accumulated in tissues, principally in adipose, skin, and muscle. Approximately 6.0% of D5-18:3n-3 and 2.6% of D5-18:2n-6 were elongated/desaturated and stored, mainly in muscle, adipose, and the carcass. The remaining 78% of both precursors was apparently catabolized or excreted.

An important question for mammalian nutrition is the relative efficiency of C18 versus C20 essential fatty acids (EFAs) for supporting the tissue composition of n-3 and n-6 pathway end products. One specific question is whether C22 EFAs are made available to tissues more effectively by dietary alpha-linolenic acid (18:3n-3) and linoleic acid (18:2n-6) or by dietary eicosapentaenoic acid (20:5n-3) and dihomo-gamma-linolenic acid (20:3n-6). To address this question in a direct manner, four stable isotope compounds were given simultaneously in a novel paradigm. A single oral dose of a mixture of 2H5-18:3n-3, 13C-U-20:5n-3, 13C-U-18:2n-6, and 2H5-20:3n-6 was administered to rats given a defined diet. There was a preferential in vivo conversion of arachidonic acid (20:4n-6) to docosatetraenoic acid (22:4n-6) and of 22:4n-6 to n-6 docosapentaenoic acid (22:5n-6) when the substrates originated from the C18 precursors. However, when the end products docosahexaenoic acid (22:6n-3) or 22:5n-6 were expressed as the total amount in the plasma compartment divided by the dosage, this parameter was 11-fold greater for 20:5n-3 than for 18:3n-3 and 14-fold greater for 20:3n-6 than for 18:2n-6. Thus, on a per dosage basis, the total amounts of n-3 and n-6 end products accreted in plasma were considerably greater for C20 EFA precursors relative to C18.

The conversion of alpha-linolenate (18:3n-3) to docosahexaenoate (22:6n-3) in the presence of low and high dietary levels of linoleate (18:2n-6) is reported in young rats using [U-13C]-alpha-linolenic acid (18:2n-3*) and high precision gas chromatography-combustion isotope ratio mass spectrometry (GCC-IRMS). After consuming an 18:3n-3-deficient diet for 4 weeks, dams were bred and assigned to one of three diet groups: a) 2 g 18:3n-3/kg diet and 17 g 18:2n-6/kg diet (Lo-18:2), b) 2 g 18:3n-3/kg diet and 140 g 18:2n-6/kg diet (Hi-18:2), or c) essential fatty acid-deficient diet (EFAD). Pups were weaned to the maternal diets. At 42 days of age, pups were gavaged with 1 mg 18:3n-3*, and killed 48 h later. Fatty acid composition of liver reflected the diets to a greater extent than did the brain, and 22:5n-6 replaced 22:6n-3 in the brain. About 80% of the label in liver, brain, and plasma was found as 22:6n-3* for the replete groups. The enrichment pattern was similar in liver and plasma except for 18:3n-3, which was higher in liver. Total label detected was 4-fold higher in the EFAD livers and 2-fold higher in the EFAD brains than in the other groups, which were indistinguishable. Conversion of 18:3n-3* to 22:6n-3* was greater in livers from the Hi-18:2 group than from the Lo-18:2 group (P < 0.05). Estimates of overall label recovery in liver and brain were consistent with literature values. These data indicate that high dietary levels of 18:2n-6 do not inhibit conversion of a single dose of 18:3n-3 to 22:6n-3 in young rats, and demonstrate the applicability of high precision GCC-IRMS to fatty acid tracer studies.