Phenylalanine requirement, imbalance, and dietary excess in one-week-old chicks: growth and phenylalanine hydroxylase activity

Assessment of limiting dietary amino acids in broiler chickens offered reduced crude protein diets

As lowering crude protein (CP) in poultry diets continues to minimize amino acid excess, it is important to understand the limiting order of amino acids and the impact of their deficiencies. Therefore, a pair of experiments were conducted to observe the effects of individual amino acid deletions on growth performance, carcass traits, and nutrient utilization. Both experiments involved 3 control diets based on wheat and soybean meal, including a 210.0 g/kg CP industry control (IC), 186.7 g/kg CP positive control (PC) supplemented with feed-grade amino acids to match the IC amino acid profile, 186.7 g/kg CP negative control (NC) with reducing N corrected apparent metabolizable energy (AME_N) by 0.5 MJ/kg and removing feed-grade amino acids beyond L-Lys-HCl, DL-Met, and L-Thr from PC. Ten deletion diets where the following supplemented amino acids were individually removed from the PC: Val, Ile, Leu, Trp, Arg, His, Phe + Tyr, glycine equivalence (Gly_equi), Pro, and Energy (0.5 MJ/kg reduction in AME_N of the PC). All diets were formulated to contain similar concentrations of digestible Lys, total sulfur amino acid (TSAA) and Thr. Experimental diets were offered to broiler chickens from 15 to 22 d post–hatch in a cage study (Exp. 1) to gain digestibility and nutrient utilization data; whereas they were offered from 15 to 35 d post–hatch in a floor-pen study (Exp. 2) to gain performance and carcass yield data. The removal of supplemented Val, Arg, and Ile resulted in reduction on broiler performance (P < 0.05), and the removal of Val, Arg, Ile, and Gly_equi negatively influenced carcass traits (P < 0.05). Results from both experiments indicate that Val and Arg are co-limiting in wheat-soybean meal diets, but that Ile and Gly_equi may potentially limit breast and thigh development.

Progress of amino acid nutrition for diet protein reduction in poultry

There is growing interest among nutritionists in feeding reduced protein diets to broiler chickens. Although nearly a century of research has been conducted providing biochemical insights on the impact of reduced protein diets for broilers, practical limitation still exists. The present review was written to provide insights on further reducing dietary protein in broilers. To construct this review, eighty-nine peer reviewed manuscripts in the area of amino acid nutrition in poultry were critiqued. Hence, nutritional research areas of low protein diets, threonine, glycine, valine, isoleucine, leucine, phenylalanine, histidine, and glutamine have been assessed and combined in this text, thus providing concepts into reduced protein diets for broilers. In addition, linkages between the cited work and least cost formation ingredient and nutrient matrix considerations are provided. In conclusion, practical applications in feeding reduced protein diets to broilers are advancing, but more work is warranted.

Phenylalanine-pyruvate aminotransferase activity in chicks subjected to phenylalanine imbalance or phenylalanine toxicity

Two experiments were done to determine the influence of Phe imbalance and excess on Phe-pyruvate aminotransferase (PAT) activity in the chick. Five replicates of 3 chicks (experiment 1) or 2 chicks (experiment 2) of a commercial brown egg layer strain were fed a semipurified diet for 1 wk and then received experimental diets for 10 d. Three diets were used in experiment 1: the basal diet contained 0.46% Phe; the imbalance diet was similar to the basal diet except that it contained a 10% mixture of indispensable amino acids lacking Phe (IAA - Phe) to create a Phe imbalance; the imbalance corrected diet was similar to the imbalance diet except that it was supplemented with 1.12% Phe to correct the imbalance. A 3 x 2 factorial arrangement of treatments in experiment 2 provided 3 dietary levels (0.46, 1.58, and 2.46%) of Phe and either no supplement or 10% supplement of IAA - Phe. Nonfasted chicks were killed and livers were sampled in experiment 1, and livers, kidneys, brains, and pectoralis major muscles were sampled in experiment 2. In experiment 1, liver PAT activity per gram of liver was 80 and 55% higher (P < 0.01) in chicks fed the imbalance and imbalance corrected diets than in chicks fed the basal diet. In experiment 2, the livers and kidneys, but not brains and muscles, of chicks that received the 10% supplement of IAA - Phe had higher activities of PAT per gram of tissue per minute and per milligram of tissue protein extract per minute than chicks that did not receive IAA - Phe (P < 0.001). No effect of dietary Phe on PAT activity was detected (P > 0.05). Phenylalanine-pyruvate aminotransferase activity appears to be regulated in response to dietary content of indispensable amino acids but not by the dietary level of Phe.

Phenylalanine hydroxylase activity and expression in chicks subjected to phenylalanine imbalance or phenylalanine toxicity

Experiments were performed to investigate the activity of hepatic Phe hydroxylase (PAH) and plasma amino acid concentrations under conditions of Phe imbalance or toxicity in chicks fed on experimental diets from 7 to 14 or 16 d of age. In experiment 1, Phe imbalance was created by adding 10% of a mixture of indispensable amino acids lacking Phe (IAA - Phe) to a basal diet containing 0.46% Phe. The activity of PAH was not significantly affected by the imbalance. Correcting the imbalance by adding 1.12% Phe to the diet prevented the growth impairment and increased the activity of PAH. In experiment 2, growth was reduced by the addition of excess (2%) Phe to the basal diet. Correcting the excess by adding the IAA - Phe to the diet prevented the growth reduction. The activity of PAH was not significantly affected by 2% Phe, but it increased in chicks fed the corrected diet. The levels of PAH mRNA were not affected by the dietary treatments. A factorial arrangement of treatments with 3 dietary levels of Phe (0.46, 1.58, and 2.46%) with or without the IAA - Phe was used in experiment 3. The effects on growth were similar to those of the same treatments in experiments 1 and 2. The addition of Phe significantly increased hepatic PAH activity, but there was no detectable main effect of the IAA - Phe and no interaction. Plasma Phe concentration was increased by dietary Phe and decreased by the IAA - Phe mixture. We conclude that hepatic PAH activity in chicks variably increases in response to Phe or a 10% dietary supplement of indispensable amino acids including Phe but does not increase in response to IAA - Phe when the amino acids are added to a diet that is marginally adequate in Phe. The increased activity does not involve changes in PAH mRNA. The effects of IAA - Phe on plasma Phe concentrations appear to be independent of hepatic PAH activity as measured in vitro.