Trimethylglycine or TMG is also more commonly referred to as betaine (BEET-ah-een).

Trimethylglycine or TMG is also more commonly referred to as betaine (BEET-ah-een). For the chemists, as the name implies it is a trimethyl derivative of the common amino acid glycine. TMG is commonly found in our diet in beets (that's where the name betaine came from), whole grains, spinach and shellfish. Feb 4, 2015

 (Source: https://ansperformance.com/trimethylglycine-supplementation/ )