Potential and Constrains to Modulate the Fatty Acid Composition of Ruminant Meat
Rui J.B. Bessa, Faculty of Veterinary Medicine, University of Lisbon 
Lisbon, Portugal

Fatty acid (FA) composition of ruminant meat varies largely with feeding regime and degree of intramuscular fat deposition. In general, the main targets when improving the FA profile of meat are to increase rumenic acid or n-3- PUFA. 
The rumen microbial biohydrogenation of PUFA is the major determinant of the peculiar fatty acid composition of ruminant meat, including also the appearance of rumenic acid. The biological role of rumen biohydrogenation is not fully understood but its outcome is largely modulated either by basal diet, lipid supplementation and other individual unknown responses. In the intensively finished ruminants, the biohydrogenation pathways are shift from the usual production of 18:1t11 to the production of 18:1t10 and related intermediates. The major consequence is that intensively produced meat will have low 18:1t11 and 18:2c9t11 contents and a high content of 18:1t10 which is not desirable. Besides its clear association with high starch diets, very little is known about this t10-shift. Conversely, when animals are finished on forages, 18:1t11 is the major biohydrogenation intermediate available and delta-9 desaturase, that endogenously converts 18:1t11 into rumenic acid, is downregulated. Moreover, forage-finished ruminants usually present low intramuscular fat deposition which limits the enrichment of rumenic acid due to its preferential esterification in triacylglycerol’s.
Regarding the enrichment of meat with n-3 PUFAs, and very especially long-chain (LC) PUFAs, the major restrictions are the rumen biohydrogenation of dietary n-3 PUFA, the low conversion of 18:3n-3 to n-3 LC PUFA, but also the strict limitation to its incorporation in muscle membranes. Apparently, the FA composition of muscle membrane phospholipids maintains a fairly constant sum of cis double bonds which seem to be a limitation for the n-3 LC PUFA enrichment in meat.
Potential strategies and difficulties to either prevent the t10-shift in intensively-fed animals or to upregulate the delta-9 desaturase in forage-fed animals plus how to achieve n-3 PUFA accumulation will be discussed in order to address the target of producing a more desirable ruminant meat.