Histidine Status in the Chick as Measured by Growth Rate, Plasma Free Histidine and Breast Muscle Carnosine

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Abstract
Three assays with crystalline amino acid diets were conducted to delineate the histidine requirement of chicks of various ages, and to compare weight gain, plasma free histidine and breast muscle carnosine as parameters of response. Data from assays 1 and 2 indicated a histidine requirement for maximal weight gain of close to 0.32% of the diet for all age periods between hatching and 29 days posthatching. Graded increments of L-histidine between 0 and 0.57% were fed to chicks between 8 and 20 days posthatching in assay 3. Least squares analysis indicated a linear response in weight gain up to 0.285 ± 0.002% histidine, with no additional response above this point. Muscle carnosine concentration remained constant at all dietary levels of histidine below 0.268 ± 0.041%; between this level and 0.368 ± 0.028%, however, carnosine concentration increased linearly from 750 µg/g to 3,500 µg/g wet tissue and then plateaued. Plasma free histidine concentration remained low and unchanged until dietary histidine reached 0.410 ± 0.011% at which point it began to increase markedly. Administration of low levels of L-carnosine to histidine-deprived chicks increased growth rate but had little effect on either plasma histidine or muscle carnosine. These results suggest that a) muscle carnosine is labile and readily metabolized to histidine and β-alanine, b) protein retention has priority over muscle carnosine accumulation when deficient levels of histidine are fed, and c) accumulation of free histidine in plasma does not occur until the histidine need for both optimal weight gain and muscle carnosine synthesis has been satisfied.