Betaine's dual role: osmolyte and methyl donor
Betaine (trimethylglycine) has two distinct functions that often get conflated. As an organic osmolyte it accumulates inside cells and helps them hold water and resist osmotic stress, which is the basis for the idea that it supports cell hydration or volume in working muscle. Separately, betaine is a methyl donor: it gives up a methyl group in the reaction that converts homocysteine back to methionine, feeding the methylation cycle.
These two roles operate through different biochemistry, so a diagram showing both is describing two jobs the same molecule happens to do.
Methylation links to creatine and homocysteine
Because methionine derived from that reaction supplies methyl groups used in creatine synthesis and many other processes, betaine intake can lower circulating homocysteine, an effect that is fairly well documented. The performance and body-composition claims are less settled, with mixed trial results, so they should be read as plausible rather than proven.
This is general educational information, not medical advice. Consult a qualified professional before using betaine, particularly if you have kidney or cardiovascular concerns.