High leucine bioavailability improves beef quality by altering serum metabolism in beef cattle

This study aimed to investigate the effect of dietary L-Leu supplementation on amino acid composition, serum metabolism, and meat quality characteristics in beef cattle. Twenty-four Angus cows of similar initial weight (575.5 ± 22.1 kg) were randomly assigned to two treatment groups with six replicate pens (two cattle per pen). The groups were fed a basal diet (CON) or a basal diet supplemented with 6.0 g/100 kg BW per day of L-Leu for 120 days pre-slaughter. Feeding L-Leu significantly increased average daily gain and decreased feed-to-weight-gain ratio (P < 0.05). L-Leu improved plasma free leucine bioavailability (P < 0.05), increased the concentrations of tyrosine and glutamine (P < 0.05), and decreased the concentrations of threonine and valine (P < 0.05). It also increased the content of total protein in plasma (P < 0.05). Supplementation with L-Leu tended to increase the marbling score (P = 0.06) and decrease subcutaneous fat thickness (P = 0.06), as well as the content of C10:0 (P < 0.05), C14:0 (P = 0.05), C20:0 (P < 0.05), and C18:2n-6 t (P = 0.07) in the longissimus thoracis muscle. However, L-Leu significantly increased the crude protein content in the longissimus thoracis muscle (P < 0.05). Correlation analysis revealed that L-Leu downregulated the relative abundance of metabolites associated with subcutaneous fat thickness and beef fatty acid synthesis (P < 0.01), and upregulated the relative abundance of metabolites associated with crude protein and ether extract in the longissimus thoracis muscle (P < 0.01). In conclusion, dietary L-Leu supplementation increases leucine bioavailability and improves meat quality in fattening beef cattle by altering host serum metabolism.