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The effect of protein level and amino acid balance on thyroid activity, efficiency of feed utilization and tissue respiration in the chick Pastro, Kenneth Ralph


The present investigation was conducted to determine the extent to which the effects dietary protein level and amino acid balance have on rate of growth and efficiency of feed conversion can be related to the effects they have upon thyroid activity and tissue respiration. Thyroid activity and body weight were studied in chicks fed diets containing 17.5% and 26% of protein and variously supplemented with amino acids. The basal diets were deficient in methionine and glycine. Growth was stimulated at both protein levels by the addition of methionine or methionine plus glycine to the basal diets. Even when supplemented with amino acids, the low-protein diet was inadequate for maximum growth. Thyroid weight, thyroid weight relative to body weight and I¹³¹ uptake per chick were increased when the protein level was increased from 17.5% to 26% but uptake of I¹³¹ per unit weight of thyroid gland was not affected. Supplementation of the diets with lysine significantly reduced thyroidal uptake of I¹³¹ per chick. Addition of methionine or methionine plus glycine reduced uptake of I¹³¹ per unit weight of thyroid gland. The effect of various levels of dietary lysine, methionine and tryptophan on body weight, efficiency of feed conversion, nitrogen balance and metabolizable energy were studied in relation to concurrent effects of diet on thyroid activity and tissue respiration in chicks. Respiration of liver slices and fiber bundles of skeletal muscle was measured with a Warburg respirometer. Feed and feces were analyzed for nitrogen by the macro Kjeldahl method and for combustible energy with a Parr oxygen bomb calorimeter. Addition of lysine to diets deficient in lysine significantly improved body weight, efficiency of feed utilization, and nitrogen retention in chicks fed the experimental diets. Addition of 0.30% lysine to diets containing 1.09% of lysine improved body weight significantly but had no significant effect on the other parameters mentioned. Addition of methionine to a diet containing 1.39% of lysine improved feed efficiency in birds 17 to 22+ days of age. Amino acid balance had no effect on metabolizable energy. Birds fed lysine deficient diets had lighter thyroids and lower thyroidal uptake of I¹³¹ but had higher uptake of I¹³¹ per mgm. of thyroid gland than those fed diets containing 1.09% or more of lysine. Amino acid balance had no effect on oxygen consumed by muscle tissue, but a deficiency of lysine significantly increased oxygen consumed by liver slices taken from 6 and 8 week old chicks. It is concluded that amino acid composition of dietary protein, as well as the protein level in the diet, affects thyroid activity in the chick. Amino acid imbalance caused by dietary lysine deficiency results in high thyroid activity, low body weight, poor efficiency of feed conversion, and poor nitrogen retention but apparently high respiration of liver slices. Possible shifts in methionine and lysine requirements during the first 25 days after hatch were investigated. It was found that the requirement of the chick for methionine did not change during this time hut that the requirement of the chick for lysine was higher in birds 0 to 20 days of age than in older birds.

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