**4. Cysteine digestibility and bioavailability**

Following a combination of heat treatment and alkaline food processing some alterations occur in the chemical nature of cysteine leading to some effects in its digestibility and subsequent absorption. These two processes are vital in ensuring the assimilation of amino acids by broilers. Heat processing causes the oxidation of a significant portion of protein-bound cysteine to cystine, which has lower digestibility [62]. This may probably be due to the formation of disulphide bridges during the transformation process. Dietary cystine is also converted to lanthionine under the influence of heat and alkali treatment [63]. The reduced SAA activity of lanthionine results in reduced availability of protein-bound cysteine [64]. Since protein metabolism continues even when no protein is being consumed, some of the amino acids released are oxidized and are not available for re-synthesis of new proteins. Feeding a protein-free diet to broilers, therefore, elicit a cysteine response (reduction in body weight loss and improves nitrogen balance) [65], indicating that it could be substantially depleted in the body pool making it the first limiting amino acid for endogenous protein synthesis [53].

All the nutrients ingested by an animal via its diet cannot be utilized by the animal because some are undigested. Furthermore, some are either absorbed in forms that cannot be utilized for physiological and metabolic functions in the body or are not absorbed at all. The available nutrients refer to the portion of the nutrients that are digested, absorbed and metabolized [20]. The same is true of amino acids, which are bioavailable if they occur in forms that can be utilized by the cells for maintenance or production. Digestibility of amino acids, therefore, is the digestion of the amino acids consumed in the diets and their subsequent absorption from the lumen of the small intestine into the bloodstream [66]. The portion of the absorbed amino acids present in chemical forms amenable to protein synthesis indicates their bioavailability [17]. The concept of digestible amino acids is critical to establish ideal protein ratios [67], and broiler diets are now formulated based on digestible proteins and amino acids [68].
