Growth and homocysteine remethylation in the chicken: Impact of age, methionine source and dietary levels of sulfur amino acids and related compounds

Commercial poultry diets are limiting in sulfur amino acids (SAA), methionine (MET) and cysteine (CYS), and there is a large market for MET supplements, especially DL-methionine (DLM) and 2-hydroxy-4-(methylthio)butanoic acid (HMB), the synthetic analogue. Choline (CHO) and betaine (BET) are two other compounds closely associated with SAA metabolism that have biological and economic relevance in poultry nutrition. The efficacy of HMB still remains a controversy even after three decades of research. Dietary source and levels of SAA along with CHO or BET could impact remethylation of homocysteine (HCY), a major metabolite of MET. A research program was undertaken with the objective of using growth assays and an in vitro stable isotope technique to assess the impact of dietary source and level of MET, deficiency or adequacy of CYS, and addition of excess CHO or BET on growth and hepatic HCY remethylation in broilers during the starter, grower and finisher periods. Efficacy of HMB as a source of MET was evaluated during the growth periods under varying dietary conditions and effort was made to see if the differences in efficacy were correlated with hepatic HCY flux. The results indicated that in broilers, the folate-dependent remethylation pathway is more predominant under most dietary conditions than the BET-dependent pathway, especially during the starter and grower periods. Remethylation through the BET-dependent pathway comprised less than 50% of total remethylation under nearly all of the dietary treatment conditions studied. Folate-dependent remethylation appeared to be more responsive to dietary changes in CHO or BET. Impact of SAA levels, especially MET, on remethylation was found to be minimal, but CYS adequacy resulted in some increase in the magnitude of remethylation. During the starter period HMB efficacy improved with CYS adequacy in a SAA-deficient diet. Excess CHO and BET improved efficacy when added to diets deficient in MET and CYS, but not to diets deficient in MET alone. Efficacy of HMB was not significantly different from DLM during the grower and finisher periods and was not affected by the level of CYS in the diet. No correlation was observed between HCY remethylation and HMB efficacy.