*2.1. Methyl Donation*

It is universally accepted that betaine serves two primary roles in mammalian physiology [12,17]. The first is that of a methyl donor. As the name indicates, trimethylglycine (i.e., betaine) has three methyl groups, which can serve as reagents for transmethylation reactions. If this occurs, betaine is converted into dimethylglycine, or further catabolized into sarcosine, ultimately adding to the amino acid pool as glycine [12,18]. Notably, betaine metabolism supplies methyl groups for the conversion of homocysteine to methionine [19,20], and aids in the synthesis of key metabolic proteins such as creatine [21,22] and carnitine [23], especially during periods of hypertonicity [24]. These findings have led to several lines of research that examine betaine's role in health and disease prevention [12,19,25–27] as well as human performance [28–32].
