Effect of leucine on performance, blood parameters, fat and protein metabolism and carcass quality and quantity in high fat diet fed broilers under heat stress

Introduction: Studies have shown the negative effect of stress (e.g., climatic, environmental, nutritional, social and psychological stress) on animal welfare, performance, and immunological parameters. Following the increase in consumption of poultry products and the widespread production of these products, stressors can have a negative impact on the production and health of chickens, thereby affecting human nutrition. Heat stress is one of the most challenging environmental conditions affecting commercial poultry. Compared with other species of domestic animals, broiler chickens are more sensitive to high ambient temperatures. High temperature imposes severe stress on birds and leads to important economic losses in the poultry industry. Heat stress, one of the concerning problems in poultry industry, results in poor welfare, low performance, inferior meat quality, and high mortality of chickens. High ambient temperature is a major problem in many parts of the world such as Iran, especially during summer. Exposure to high temperature has been reported to cause undesirable changes in growth rate, meat yield and breast protein content of broilers. In-ovo administration of L-leucine on embryonic day (ED) 7 has caused the hypothermia at hatching, and thermotolerance in young broiler chicks under heat stress. Branched-chain amino acids, especially leucine, potentially activate the mTOR signaling pathway and activate protein synthesis by initiating translation. As an essential amino acid, leucine has certain biological properties, such as providing energy, regulating protein, carbohydrate and lipid metabolism, adjusting immune function and mRNA translational origination (by activating the mTOR signaling pathway). Leucine inhibits the expression of lipogenic enzymes (fatty acid synthetase and acetyl-CoA carboxylase) in adipose cells and enhances muscle fatty acid oxidation (Sun and Zemel 2007). Therefore, considering the role of leucine in activating the mTOR signaling pathway and the metabolism of protein and fat, it is expected that leucine will improve the performance and quality of meat in broiler chickens under heat stress. Material and methods: Two hundred, one-day-old, male broiler chicks (Ross 308) were obtained from a commercial hatchery. The birds were initially weighed, so that the pens had similar initial weight distribution. Birds were randomly assigned to five experimental treatments, with five replicates of 8 chicks per pen. Experimental diets were the control diet (corn-soybean based diet), control diet + 0.1% (w/w) leucine, control diet + 0.2% (w/w) leucine, control diet + 0.3% (w/w) leucine and control diet + 0.4% (w/w) leucine. Lighting program was used according to the guidelines of the Ross 308. Environmental temperature was maintained at 32 ± 1°C throughout the experiment. The diet was based on corn and soybean meal and formulated according to Ross 308 requirements for three period of starter (1-10 d), growth (11-24 d) and finisher (25-42 d) days. Body weight gain and feed intake was measured periodically and calculated during the whole experiment on pen basis, and the feed conversion ratio was subsequently calculated. At day 42 of age, two birds per pen were randomly selected, weighed, and slaughtered. After slaughter, percentage of carcass, breast, thigh, abdominal fat, pancreas, liver, heart, bursa of fabricius and spleen were calculated as a ratio of the live weight. To determine the meat quality, 50g of left breast muscle was collected in plastic bags and stored at a negative temperature of 20°C. Meat quality parameters including water holding capacity, moisture, ash, pH, fat, protein, malondialdehyde and total antioxidant activity were measured. Data were analyzed using the general Linear Model procedures of SAS 9.1. When the analysis of variance was significant, Duncan's multiple-range test was used to separate the means. Statements of statistical significance were based on P<0.05. Results and discussion: There was no significant difference between feed intake, body weight gain and feed conversion ratio of experimental diets (P>0.05). Our results did not agree with the observations of Erwan et al. (2009) reporting the positive effect of leucine supplementation (0.5% and 0.67% leucine to the diet containing 20% crude protein) on feed intake, weight gain and feed conversion ratio of broilers. Moreover, relative weights of the different organs and the carcass components (except the abdominal fat) were not affected by the dietary treatments. Abdominal fat of 0.1% and 0.2% leucine fed birds was significantly lower than that of the control birds (P<0.05). Addition of 0.1%, 0.2% and 0.3% dietary leucine resulted in a lower breast fat as compared to 0.4% leucine and control diet fed birds (P<0.05). Possibly leucine increases fat metabolism of adipocytes by increasing fat flow to skeletal muscle and therefore increases the energy substrate to support leucine-stimulated protein synthesis which results in decreased abdominal fat weight. Breast meat protein of 0.4% leucine fed birds was significantly higher than that of control chicks (P<0.05). The enzymatic activities responsible for the metabolism of branched amino acids are found mainly in the skeletal muscle, heart, kidney, but less so in the liver. Thus, branched amino acids do not break down directly in the liver and most are available for metabolism in skeletal muscle and other tissues. No significant differences were observed between the treatments for blood levels of glucose, triglycerides, total protein, uric acid and cholesterol (P>0.05). The use of dietary leucine had no effect on breast meat malondialdehyde and total antioxidant activity of broiler chickens (P>0.05). Conclusion: According to the results of current experiment, dietary leucine supplementation in the diet of broiler chickens under heat stress conditions had no effect on performance of broiler chickens. Leucine supplementation at low levels (0.1 and 0.2%) reduces the meat fat, while higher levels of the amino acid (0.4% leucine) stimulates the protein synthesis.