**3. Cysteine requirements**

An animal's amino acid requirement is made up of the total requirement for protein accretion and maintenance. Due to faster growth rate and earlier market age of modern commercial broilers, the requirement for maintenance function becomes reduced [53]. Consequently, the relative need for protein accretion to maintenance varies for individual amino acids. Therefore, the requirement for cysteine and other amino acids with high maintenance needs relative to lysine will reduce [35]. In broiler poultry nutrition, optimum amino acid density must be maintained when considering the balance between energy and proteins in their feed, indicating a higher ratio of essential amino acids to energy in modern broilers [13]. This conforms to the fact that modern commercial broilers are different from those offered by the poultry industry in the 90s when the NRC nutrient requirement for poultry was published. Genetic selection, management practices, and changes in feed are believed to be partially responsible for this [55, 56].

The requirement for total SAAs recommended by [37] were 0.9, 0.69, and 0.57 for 0–3, 3–6, and 6–8 weeks, respectively, as against weekly requirements of 0.94, 0.9, 0.82, 0.78, 0.74, 0.71, and 0.67% for 1–7 weeks of age in modern broilers as presented by [57]. However, proper assessment of amino acid requirements have remain unresolved owing to the difficulty posed by underestimation and overestimation by the oxidation and nitrogen balance methodologies, respectively [51]. Biosynthesis of cysteine occurs in animals and plants via the trans-sulfuration pathway from methionine, in the presence of adequate nitrogen and sulfur [58]. However, since cysteine is synthesized from methionine via the trans-sulfuration pathway, its requirement is usually considered together with methionine [59].

#### **Figure 3.**

*Extracellular and intracellular L-cysteine/L-cystine balance and L-cysteine/L-cystine transport systems. Glu, L-glutamate; Cyss, L-cystine; Cys, L-cysteine; GSH, glutathione; Trx, thioredoxin. Source: Baker [50].*

Although there is adequate physiological concentrations of cysteine, many cells still rely on the trans-sulfuration pathway for a minimum of 47% of their cysteine requirements [60]. Cysteine requirement is therefore subsumed in the total SAAs requirement as captured by [35, 55] for different age ranges in poultry. Commercial diets are traditionally formulated to meet broiler requirements for methionine + cystine (Met + Cys), based on the assumption that amounts of dietary Met are converted into Cys [61].
