This review focuses on recent progress on the functions of choline and betaine and on the interactions between these two methyl donors and methionine chickens (mainly), pigs, fish (and crustaceans). When chickens receive a diet marginally deficient in methionine, supplementation with choline or betaine does not generally induce a return to the maximum growth rate. In this species betaine potentiates submaximal doses of some anticoccidial compounds, most probably through its osmolytic property. The ‘slimming’ effect which has been observed for betaine in a few assays with chickens and pigs has not been confirmed in several other species and therefore remains to be unequivocally demonstrated. In fish there is a need for phospholipids during the early stages of development. In salmonidae some studies suggest that betaine may facilitate the adaptation to sea water.
