Enhancing fatty acid composition of milk and meat through animal feeding¹

In ruminants, extensive ruminal biohydrogenation of unsaturated fatty acids (FA) results in numerous cis and trans isomers of 18:1 and of conjugated and non-conjugated 18:2, the incorporation of which into ruminant products depends on the composition of the diet (forage vs concentrate) and of dietary lipid supplements. The low amount of 18:3n-3 (α-linolenic acid) absorbed explains its limited incorporation in meat and milk lipids. Its protection against hydrogenation has been an objective for several decades, but only encapsulation in a protein matrix is efficient. In non-ruminants, the FA composition of products is determined by dietary FA, despite minor differences in digestibility and in metabolic activity. Physicochemical differences in intestinal absorption processes between ruminants and non-ruminants can explain the lower FA digestibility in non-ruminants, especially for saturated FA. Unlike in non-ruminants, FA digestibility in ruminants does not depend on FA intake, except for 18:0. The decrease in cow butterfat, especially with concentrate diets, is generally attributed to t10-18:1 or t10,c12-18:2, but the regulation is probably more complex. Differences in terms of butterfat content and FA composition of milk between cow, ewe and goat responses to the amount and composition of ingested lipids are due to between-species variations in mammary metabolism. In animals bred for meat production, dietary 18:3n-3 results in increases in this FA and in n-3 long-chain polyunsaturated FA (20:5n-3, 22:5n-3) in muscles. The extent of this increase depends both on animal and nutritional factors. Grass is a source of 18:3n-3, which contributes to increased 18:3n-3 in muscle of ruminants as well as of pigs. Conjugated linoleic acids are mainly present in fat tissues and milk due to t11-18:1 desaturation. Their concentration depends on tissue type and on animal species. Non-ruminants fed synthetic conjugated linoleic acids incorporate them in significant amounts in muscle, depending on the isomer. All dietary manipulations favouring polyunsaturated FA incorporation in milk and meat lipids increase the risk of lipoperoxidation, which can be efficiently prevented by use of dietary combined hydro- and lipophilic antioxidants in the diet. Putative effects on organoleptic and technological quality of products deserve further studies.