METHIONINE: The First Limiting Amino Acid in Typical Indian Poultry Diet

Ram Singh Bibyan, Raman Malik, Harneet Kour, Sarita Kaushal and Pramod Kumar

Methionine (Met) was first isolated by JH Müller, scientist at Columbia University (USA), in 1922. Its formula and structure were described three and six years later by S Odake; G Barger and FP Coyne, respectively and the essentiality of Met for animal was established in 1930. The first technically feasible synthesis of DL-Met was achieved by W Schwarze, H Wagner, and H Schulz as pharmaceutical grade to treat chronic protein deficiency suffered by soldiers returning home from World War II in 1948.  Amino acids are units or building blocks of protein. They contain at least one carboxyl group (-COOH) and at least one amino (-NH2) group. Amino acids can exist in two forms referred to as D- or L-isomers. The amino acids are in L-form. Except DL- Methionine, birds utilize only L-form of amino acids. Therefore, only L-form of amino acids and DL-Methionine are supplemented in diet. In 1953, animal feeding trials were conducted and showed positive results which led to start of Met feeding to animals. Presently, DL-Met is being produced by Evonik in Americas, Europe and Asia; having world-class Met production plants situated at Mobile (USA), Antwerp (Belgium) and Singapore, supporting the global demand for Met. Since Met is limited in plant protein sources and is required at high level for feather growth and protein synthesis, Met is always classified as the first limiting amino acid in commercial poultry diet. Sulfur-containing amino acids are amino acids that contain sulfhydryl groups and play crucial roles in protein structure, metabolism, immunity, and oxidation. Met is a sulfur containing amino acid and is recognized as a vital molecule for optimum nutrient metabolism impacting animal health and development. Apart from being a building block for proteins like feathers, muscle, enzymes, hormones etc., it acts as a precursor for other sulfur containing amino acids and their derivatives, e.g. cystine, homocysteine, and glutathione, and serves as the main methyl group donor to epigenetic pathways in the form of S-adenosylmethionine (SAM). Being essential amino acid, Met impacts several crucial functions in poultry, which include protein synthesis, lipid metabolism, immune system regulation, digestive functions, oxidative stress status and altering the level of SAM, thus, changing gene expression by DNA methylation reactions. Met delivers resilience to heat stress as its dietary level higher than that required for growth performance improves the antioxidant status in liver and muscle of broilers exposed to heat stress. Met reduces oxidative stress in the body by increasing antioxidative compounds such as glutathione. Glutathione plays an important role in antioxidant defence. It is capable of preventing damage to intestinal cellular components caused by reactive oxygen species (ROS) such as free radicals, peroxides and heavy metals. Especially, increased glutathione concentration affects the development of small intestinal villus. Both the height and width of the villi were improved in the Met supplementation group.

A deficiency of Met has negative impacts on the growth performance, carcass traits, meat quality, and oxidative status of breast muscle in poultry. Several studies have reported that met deficiency had adverse effects on body weight gain, amino acid metabolism, and intestinal development in layers affecting their subsequent development during the growing period, suggesting an appropriate level of Met supplementation in the diet of layers during their early life. A Met deficiency typically leads to poor feed conversion, retarded growth in meat birds, and reduced egg production in layers and breeders. Met is required to provide the building blocks for immune cells and tissues. This includes the nonspecific mechanisms such as the skin and mucosa, and the specific mechanisms that include T and B lymphocytes. This is particularly important for newly hatched chicks that are highly susceptible to infection during the first two weeks of life. Met is a major component of feathers. Met and cysteine (another sulfur-containing amino acid that is not essential in the diet) are critical to feather formation. A deficiency of Met results in poor feathering and increased incidence of feather pecking. A methionine-deficient bird will tend to eat feathers to obtain enough Met. Feather pecking may turn into cannibalism in a flock. Several studies conducted in author’s laboratory proved that Met levels higher than that of its requirement, resulted in amelioration of mycotoxicosis in broiler chickens, turkeys, quails and ducks. Aflatoxins are metabolized to highly reactive epoxides and phenolates that can bind and interfere with nucleic acid and proteins. The epoxides and phenolates are normally conjugated with glutathione that serves to protect vital macromolecules from these toxin intermediates. Hepatic necrosis is thought to result when glutathione reserves have been drastically depleted by conjugation with toxin intermediates so that the toxin intermediates are free to bind covalently to vital cellular macromolecules. Therefore supplementing Met, which helps to increase hepatic GSH concentration may aid to protect liver against aflatoxicosis. In authors’ laboratory, the effect of inclusion of DL-Met in diet containing 1.0 ppm total aflatoxin (AF: 76.45% AFB1, 10.52% AFB2, 9.89% AFG1 and 3.14% AFG2) in broiler chickens (day-old to 42 days of age in coloured broiler chickens) was investigated. Inclusion of slightly higher methionine (0.025 or 0.05%) in feed was beneficial to counteract the adverse effects of aflatoxin partially. In another study, inclusion of DL-Met at 500 ppm level in aflatoxin (500 ppb AFB1) contaminated diet ameliorated the adverse effects of aflatoxin on growth performance, feed conversion efficiency, yield of different organs and blood biochemicals in growing Japanese quails.

As DL-Met synthesis is costly, alternate sources of met, such as Methionine Hydroxy Analogue Free Acid (MHA-FA) and Methionine Hydroxy Analogue Calcium salt (MHA-Ca), were developed and are being offered by companies around the world to economise poultry feeding. These alternate sources differ in their structure and bioavailability to the animals. Earlier literature reported that DL-Met is considered 100% bioavailable, whereas MHA-FA is considered 63-70% bioavailable relative to DL-Met to poultry. Recent studies have reported a bioefficacy of 65% for MHA-FA and MHA-Ca for most poultry and aqua species in any production system. This means that 100 parts of MHA products can be replaced by 65 parts of DL-Met in feed without impacting performance allowing for substantial cost savings. In authors’ laboratory, studies were undertaken to determine the efficacy of DL-Methionine and Methionine Hydroxy Analogue to ameliorate the toxic effects of 500 ppb aflatoxin B1 contaminated in poultry feed. The inclusion levels of MHA were selected on the basis of 100, 80 and 65% activity equivalent of 500 ppm DL-Met considering that DL-Met is 100% bioavailable. Dietary addition of aflatoxin B1 at the rate of 500 ppb resulted in depressed body weight gain, poor feed conversion efficiency and lesions in liver morphology. Dietary inclusion of MHA at 500 and 625 ppm could not ameliorate the adverse effects of aflatoxicosis, whereas, inclusion of DL-Methionine at 500 ppm or MHA at 769 ppm levels in 500 ppb aflatoxin B1 contaminated diet ameliorated those adverse effects of aflatoxicosis caused by 500 ppb of aflatoxin B1 in Japanese quails. Both supplementation of DL-Methionine at 500 ppm and MHA at 769 ppm level in aflatoxin B1 contaminated diet was equally efficacious in ameliorating the adverse effects of aflatoxicosis in growing Japanese quails. The present investigation clearly demonstrated that 100 units of MHA led to equivalent body weight gain and feed efficiency as that of 65 units of DL-Methinine. These results were in agreement with a large number of broiler trials reported in literature wherein 100 units of MHA were compared with 65 units of DL-Methionine. In the present study, 100 units of MHA were equal to 65 units of DL-Methionine in ameliorating adverse effects of aflatoxicosis caused by 500 ppb aflatoxin B1. The study further revealed that MHA, being a cheap source of methionine, can be used in any production system for economising feed cost and without impacting the performance of poultry birds. However, while choosing a methionine source, its bioavailability must be established to fulfil the genetic potential of poultry birds as methionine is the first limiting amino acid in typical Indian poultry diet.