Methionine-balanced diets modulate the fatty acid profile of beef.

Methionine plays a central role in methyl metabolism and glutathione synthesis, and its availability can influence the partitioning of methyl groups toward lipid metabolism and antioxidant defenses. Manipulating methionine supply may therefore affect muscle fatty acid desaturation-elongation pathways and oxidative stability. However, few studies have examined the effects of rumen-protected methionine (RPM) supplementation on the lipid profile and antioxidant status of beef. Therefore, our study investigated how RPM supplementation affects the fatty acid (FA) composition and antioxidant status of meat from young Charolais bulls. We hypothesized that RPM supplementation could modulate muscle lipid metabolism by interfering in the fatty acid desaturation and elongation pathways, thereby affecting both n-6 and n-3 polyunsaturated fatty acid profiles, and potentially improving antioxidant capacity in muscle. Thirty-six weaned Charolais bulls, blocked by initial body weight and age, were randomly assigned to one of four isoenergetic dietary treatments in a 2 × 2 factorial design combining two levels of metabolizable protein (100% vs. 120% of requirements) crossed by two methionine supplementation strategies (with or without RPM). All diets were based on the same forage-concentrate ratio (54% grass silage, 6% straw, 40% concentrate). All animals received isonet energy diets based on a constant forage-concentrate ratio, differing only in metabolizable protein supply and RPM supplementation. To investigate additional biological mechanisms beyond dietary treatments, such as within-diet animal variability, residual feed intake (RFI) was used to assess whether differences in nutrient utilization efficiency within treatments were associated with variations in muscle fatty acid composition and antioxidant status. Rumen-protected methionine supplementation significantly (P < 0.05) reduced several n-6 polyunsaturated fatty acids (PUFA), including total PUFA, C18:2 n-6, C20:3 n-6, and C20:4 n-6 (FDR < 0.05), thereby slightly improving the n-6/n-3 ratio without affecting antioxidant markers or oxidative stability. Muscle nutritional indices such as PUFA/SFA and n-6/n-3 remained stable across treatments. Rumen-protected methionine supplementation showed no effect on antioxidant enzyme activities, vitamin contents, or MDA concentrations (P > 0.10). Residual feed intake showed only weak exploratory associations with muscle fatty acid profile, none of which remained significant after correction for multiple testing. In conclusion, rumen-protected methionine modulated muscle PUFA composition without compromising oxidative stability, while RFI provided additional insight into within-treatment variability in muscle fatty acid profile.