Nutritional and immunomodulatory function of methionine in poultry diets – a review

Methionine (Met) plays many important metabolic functions in humans and animals, and therefore may be classified as a functional amino acid (AA). Functional AAs are defined as those AAs that participate in and regulate key metabolic pathways to improve health, survival, growth, development, and reproduction of organisms. As the first-limiting AA in poultry diets, Met affects poultry production parameters such as body weight gains, feed conversion ratio and carcass quality. The results of many experiments on chickens fed diets with different levels of Met (from 0.3 to 1.2% in the starter period, and from 0.3 to 0.9% in the grower period) indicate that commercial broiler chickens do not require more than 0.50 and 0.38% Met in starter and grower diets, respectively, for optimum growth and feed efficiency, whereas higher inclusion rates of Met are needed to stimulate immune responses. The results of recent experiments on chickens are insufficient to define the optimal dietary levels of Met, which has been shown to exert immunostimulatory activity. A few experiments on layer hens have demonstrated that Met requirements for immune competence are higher than for optimum production, but the inclusion levels of this AA needed to stimulate the immune system of birds have not been defined. In the absence of such research, it remains unknown whether feeding growing turkeys diets supplemented with Met above NCR recommendations, as suggested by B.U.T. (British United Turkeys), stimulates the immune system of birds.