Ruminant diets usually contain low levels (2-3%) of lipids and fatty acids (FA). Before absorption, dietary lipids undergo extensive modifications by ruminal micro-organisms. These processes and FA absorption are described in detail in Chapter 11. In the present chapter, the origins of muscle FA are summarised in Figure 12.1. Muscle medium chain FA are synthesised de novo from plasma acetate and β-hydroxybutyrate to generate C12:0, C14:0 and C16:0. Long chain muscle FA arise from modified dietary lipids via plasma lipoproteins (C16:0, C18:0, C18:1, C18:2 and C18:3) and are taken up through the action of the lipoprotein lipase (LPL). All saturated FA (SFA, mainly C14:0, C16:0, C18:0) and C18:1 t11 can be desaturated by the stearoyl-coenzyme A desaturase (Δ9 desaturase, SCD) to produce C14:1 c9, C16:1 c9, C18:1 c9 and C18:2 c9t11. Both C18:2 n-6 and C18:3 n-3 are elongated and desaturated to longer n-6 and n-3 polyunsaturated fatty acids (PUFA): C20:4 and C22:4 for the n-6 PUFA and C20:5, C22:5 and C22:6 for the n-3 PUFA.