Constraints and potentials for the nutritional modulation of the fatty acid composition of ruminant meat

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. Simplified summary of the main drives that could limit the c9,t11-CLA deposition in meat of ruminants fed high forage or high concentrate diets.