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The objective was to characterize the fatty acid (FA) composition of lamb meat with emphasis on biohydrogenation intermediates (BI) induced by dietary sunflower and linseed oil and to test if a synergistic effect on meat trans-11 18:1 and cis-9,trans-11 18:2 concentrations could be obtained with a blend of both oils. Thirty two lambs were assigned to four groups and fed for 6 weeks one of the following diets: pelleted dehydrated lucerne (Control); and Control supplemented with 7.4% of sunflower oil (SF), linseed oil (LS) or a blend of sunflower and linseed oils (2 : 1 vol/vol) (SFLS). Longissimus thoracis muscles were analyzed for FA. LS increased n-3 PUFA due to contribution of 18:3n-3 but not of very long n-3 PUFA. Total conjugated linoleic acids were similar in oil-supplemented lambs, but the cis-9,trans-11 18:2 was higher with SF than with LS. No synergistic effects on trans-11 18:1 or cis-9,trans-11 18:2 were observed when both oils were fed together. Oil supplementation increased the concentrations of most BI in meat. However, the BI patterns were different for LS and SF. Some FA were only found in lambs fed linseed oil, including the unusual cis-12,cis-15 18:2 which is proposed as a new intermediate of the 18:3n-3 biohydrogenation pathway.

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The meat from ruminants can be a valuable dietary source of rumenic acid (c9,t11-CLA) and of n-3 long chain polyunsaturated fatty acids (n-3 LC-PUFA). Nevertheless, commonly ruminant meats present a fairly low content of both c9,t11-CLA and n-3 LC-PUFA. Most of c9,t11-CLA in meat is derived from the delta-9 desaturation of t11-18:1 catalyzed by stearoyl-CoA desaturase (SCD) and deposited in triacylglycerides (TAG). Thus, optimizing c9,t11-CLA in meat implies: a high substrate supply, a high SCD activity, and a high intramuscular fat (IMF) deposition. High rumen t11-18:1 outflow requires high forage diets which down-regulate SCD activity and IMF deposition. Conversely, as most animals are finished with cereal rich concentrate diets, the SCD activity and IMF deposition are promoted but the rumen outflow of t11-18:1 frequently is replaced by t10-18:1 (t10-shift) which cannot be converted to CLA. Thus the occurrence of the rumen t10-shift is probably the main constraint to c9,t11-CLA enrichment in meat, as is reviewed in detail. The constrains to n-3 LC-PUFA enrichment of ruminant meat are mostly associated to the extensive rumen biohydrogenation of PUFA, the low elongation and desaturation of 18:3n-3 into n-3 LC-PUFA, and the capacity of muscle lipids to incorporate n-3 LC-PUFA. Considering the almost exclusive esterification of n-3 LC-PUFA in membrane phospholipids, the limits to n-3 LC-PUFA incorporation in muscle are discussed considering the amount of and type of available phospholipids as well as the possibility of its esterification in TAG.

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Lipid metabolism in the rumen is responsible for the complex fatty acid profile of rumen outflow compared with the dietary fatty acid composition, contributing to the lipid profile of ruminant products. A method for the detailed dimethylacetal and fatty acid analysis of rumen contents was developed and applied to rumen content collected from lambs fed lucerne or concentrate based diets supplemented with soybean oil. The methodological approach developed consisted on a basic/acid direct transesterification followed by thin-layer chromatography to isolate fatty acid methyl esters from dimethylacetal, oxo- fatty acid and fatty acid dimethylesters. The dimethylacetal composition was quite similar to the fatty acid composition, presenting even-, odd- and branched-chain structures. Total and individual odd- and branched-chain dimethylacetals were mostly affected by basal diet. The presence of 18∶1 dimethylacetals indicates that biohydrogenation intermediates might be incorporated in structural microbial lipids. Moreover, medium-chain fatty acid dimethylesters were identified for the first time in the rumen content despite their concentration being relatively low. The fatty acids containing 18 carbon-chain lengths comprise the majority of the fatty acids present in the rumen content, most of them being biohydrogenation intermediates of 18∶2n−6 and 18∶3n−3. Additionally, three oxo- fatty acids were identified in rumen samples, and 16-O-18∶0 might be produced during biohydrogenation of the 18∶3n−3.