Fig. 6.

The mechanisms of AA metabolism and transport in old laying hens. In Exp. 1, higher absorption and reabsorption capacities were found in 75 weeks old old laying hens compared with the 35 weeks old young hens. Small amounts of AA synthesized in liver were found. The four sources of AA (intestinal absorption, renal re-absorption, and synthesis in kidney (primary) and liver (small quality)) facilitate the higher AA contents in the liver and spleen and the greater antioxidant capacity of the old laying hens. In Exp. 2, supplementation with exogenous AA contributed to the enhanced retention of AA in the spleen and the increased portion of CD4⁺ T lymphocytes and IgG concentration in the sera of 78 weeks old laying hens, suggesting a strengthened immune status. Dietary supplementation of AA improved antioxidant capacity and immune status, indicating the enhanced health condition of the old laying hens