The objective was to determine whether rats could synthesize longer chain polyunsaturates from hexadecadienoate (16:2n-6) and hexadecatrienoate (16:3n-3). Rats were gavaged with uniformly 13C-labelled hexadecadienoate or hexadecatrienoate, euthanized 24 h later, and total lipids were extracted from liver and carcass. Gas chromatography/combustion/isotope ratio mass spectrometry was used to measure 13C levels in individual liver, carcass, and whole body fatty acids. 13C Enrichment was present in desaturated and chain-elongated polyunsaturates, including linoleate, arachidonate, alpha-linolenate, and docosahexaenoate at 12-13% of the dose of tracer given. 13C Enrichment from hexadecatrienoate was highest in carcass and liver alpha-linolenate, representing 3.5 and 17.9% of the total alpha-linolenate pool, respectively. For linoleate, arachidonate, or docosahexaenoate, the contribution of 13C did not exceed 0.2% of the total body pool. Green leafy vegetables common in the human diet were shown to contain up to 1.2% of total fatty acids as hexadecadienoate and 11.6% as hexadecatrienoate. Hence, humans consuming green vegetables probably synthesize a small proportion of their total body content of linoleate and alpha-linolenate.

Polyunsaturated fatty acids are needed for normal neonatal brain development, but the degree of conversion of the 18‐carbon polyunsaturated fatty acid precursors consumed in the diet to their respective 20‐and 22‐carbon polyunsaturates accumulating in the brain is not well known. In the present study, in vivo 13C nuclear magnetic resonance spectroscopy was used to monitor noninvasively the brain uptake and metabolism of a mixture of uniformly 13C‐enriched 16‐and 18‐carbon polyunsaturated fatty acid methyl esters injected intragastrically into neonatal rats. In vivo NMR spectra of the rat brain at postnatal days 10 and 17 had larger fatty acid signals than in uninjected controls, but changes in levels of individual fatty acids could not be distinguished. One day after injection of the U‐13C‐polyunsaturated fatty acid mixture, 13C enrichment (measured by isotope ratio mass spectrometry) was similar in brain phospholipids, free fatty acids, free cholesterol, and brain aqueous extract; 13C enrichment remained high in the phospholipids and cholesterol for 15 days. 13C enrichment was similar in the main fatty acids of the brain within 1 day of injection but 15 days later had declined in all except arachidonic acid while continuing to increase in docosahexaenoic acid. These changes in 13C enrichment in brain fatty acids paralleled the developmental changes in brain fatty acid composition. We conclude that, in the neonatal rat brain, dietary 16‐and 18‐carbon polyunsaturates are not only elongated and desaturated but are also utilized for de novo synthesis of long‐chain saturated and monounsaturated fatty acids and cholesterol.

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