Enhancement of Cellular and Humoral Immunity in Young Broilers by the Dietary Supplementation of β-Hydroxy-β-Methylbutyrate

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Abstract
As a dietary supplement, β-Hydroxy-β-Methylbutyrate (HMB), a catabolite of leucine, has been shown to reduce broiler mortality. In a series of experiments, male broilers (Experiments 1 and 2, n=576) were grown for 21 days on diets that contained HMB at 0, 0.01. 0.05, and 0.10% of diet. In Experiment 3 (n=240), chicks were fed diets containing 0, 0.05, 0.075, and 0.10% HMB. HMB dietary supplementation did not significantly affect broiler weight gain in any experiment. However, a trend toward increased mean broiler weight gain per bird was observed in Experiments 1 and 3 when HMB was consumed at 0.10% of the diet. Mean feed to gain ratio was not affected by the inclusion of HMB in broiler diets. In Experiment 3, HMB supplemented diets did not affect bursa of Fabricius, thymus, and spleen weights at 21 days of age. Cutaneous basophilic hypersensitivity response against pokeweed mitogen was higher (P⩽0.05) at 48 and 72 hours post-injection in chicks on 0.05% dietary HMB (Experiment 1). In Experiment 2, this increase occurred 24 hours post-injection in chicks fed HMB at 0.01% of the diet. On the contrary, the T-cell mediated response against PHA-P mitogen was comparable between all dietary treatments in multiple experiments. Macrophage hnction profiles were determined at 21 days of age. All chicks in experiments 1 and 2 on HMB supplemented diets showed an increase in the recruitment of Sephadex-G50⩽-elicited abdominal exudate cells (AEC). A 2-fold increase in AEC numbers occurred at the 0.10% HMB level (Experiment 1, P⩽0.05). Although HMB supplementation did not significantly affect the phagocytic potential of the abdominal macrophages, nitrite levels in the macrophage culture supernatants were higher in 0.01% and 0.05% treatment groups as compared to the controls (Experiment 2, P⩽0.04; Experiment 3, P⩽0.05). HMB supplementation did not alter the bird's ability to clear Escherichiacoli or Salmonella arizona from the bloodstream. Beginning 7 days post-hatch, chicks were injected i.v. with a 7% sheep red blood cells suspension. Serum samples were collected to determine the primary and secondary antibody response. Chicks receiving the 0.1% HMB diet in Experiments 1 and 2 exhibited increased IgG and total anti-sheep red blood cell (SRBC) antibody levels during the primary response. During the secondary response, birds consuming the 0.10% HMB diet had elevated IgM levels as well as increased total anti-SRBC levels over the controls in Experiments 1 and 3. These studies show that HMB supplementation improves several immunological functions in young broilers, and such improvement may result in decreased mortality.