Response of Plasma Concentration of Free Amino Acid to Change of Dietary Protein and Amino Acid Levels in Adult Thoroughbreds

The response of the plasma concentration of free amino acids to the change in dietary levels of them is a parameter available to predict the amino acid requirements in some species. In order to confirm whether the response of plasma concentrations of free amino acids is also available in horses, three experiments were conducted repeatedly on four adult male Thoroughbreds. The diets were supplied twice a day at morning and evening. In Experiment 1, to determine the timing of blood sampling, blood samples were taken at 0, 1.5, 3, 5 and 7 hr after feeding the 14.9% CP diet. Plasma concentrations of all free amino acids except Arg, Gln and Gly increased by 1.5 hr and then gradually decreased to the levels before the start of the morning meal. In Experiment 2, to examine the quickness and the stability of response of plasma concentrations of free amino acids to the abrupt change in dietary Lys and Val levels, the horses were supplied with diets with 14.9% CP and different levels of Lys and Val successively at 10 day interval. Blood samples were taken at 3 and 7 hr after the morning meal at 0, 1, 3, 5 and 10 days after switching diets. Plasma concentrations of Lys and Val responded to the change in dietary levels of them by 3 days, and the responses were maintained for 10 days after switching diets. Those of all amino acids except Lys and Val did not respond. In Experiment 3, to confirm whether or not plasma concentrations of each amino acid respond to the change in dietary CP and/or amino acid levels, the horses were successively supplied with diets with different CP levels successively at 5 day intervals. Blood samples were taken at 3 hr after the morning meal 5 days after changing diets. Plasma concentrations of Cys, His, Ile, Leu, Phe, Tyr and Val responded, but those of other amino acids did not respond to changes in dietary CP and/or amino acid levels. These results indicated that the response of plasma concentrations of free amino acids which responded to the change in dietary levels might be available as a parameter to predict requirements of them for adult male thoroughbreds within a short period.