Body Composition and Nutrient Utilization of Chicks Fed Amino Acid Diets Containing Graded Amounts of Either Isoleucine or Lysine

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Abstract
A fivefold increase in the concentration of carcass fat occurred when chicks were ad libitum fed a crystalline amino acid diet containing graded amounts of isoleucine (0 to 0.6%). Weight gain, feed consumption and gain to feed ratio paralleled the trend in carcass fat. Comparable results were noted when lysine was the variable amino acid except that carcass fat was maximized at a lower concentration of dietary L-lysine·HCl (0.71%) than was needed to maximize weight gain and feed consumption (0.95%). Carcass fat of chicks fed a nitrogen-free diet was approximately four and two times that of chicks fed the isoleucine- and lysine-devoid diets, respectively. In all instances retention of protein was a linear function of the limiting amino acid consumed. Energy retention and metabolizable energy consumed were linearly related in a positive manner. In contrast to results noted with ad libitum feeding, carcass fat concentration declined progressively when chicks were equally fed diets containing graded amounts (0.4, 0.5, 0.6 and 0.7%) of isoleucine. The inverse relationship noted between carcass fat and weight gain is attributed to the greater intake of “effective” protein per se since caloric intake was held constant.