Dietary beta-hydroxy-beta-methylbutyrate supplementation influences performance differently after immunization in broiler chickens

Dietary addition of the leucine metabolite beta-hydroxy-beta-methylbutyrate (HMB) promotes growth in various species. In addition, HMB is described to enhance immune responses which might be associated with metabolic costs. We elaborated further on the role of HMB in growth, metabolism and immunity of meat-type chickens using the following parameters: zootechnical performance, blood chemistry and a specific immune responses after immunization with a human serum albumin (HSA)/Freund's (in) complete adjuvant combination. The chickens received commercial feeds either unsupplemented or supplemented with 300 mg HMB/kg feed. beta-hydroxy-beta-methylbutyrate-supplemented chickens were significantly heavier at 2 weeks of age but this difference was attenuated at later ages. Compared with their unsupplemented controls, cumulative feed conversion was significantly lower in HMB-supplemented chickens. There were no differences in blood chemistry between both dietary treatments. After immunization, HMB significantly attenuated the acute phase protein response at day 1 of post-immunization compared with that of their unsupplemented counterparts. After day 7 post-immunization, body weight gain of the immuno-challenged HMB-supplemented chickens was significantly depressed, but their specific anti-HSA IgG response was significantly enhanced compared with that of their immuno-challenged unsupplemented counterparts. The underlying mechanisms and signalling pathways for these phenomena need to be elucidated. Nevertheless, we are able to conclude that HMB is beneficial for performance under normal circumstances. On the other hand, HMB stimulates the immune response after an immunological challenge, though at the cost of reduced growth.