Trimethylglycine, commonly called betaine, is a nutrient known as a methyl donor. Methyl donors carry and donate methyl molecules, which is important for cell reproduction and other chemical processes in the body, explains the University of Maryland Medical Center, or UMMC. Trimethylglycine occurs in beets, broccoli, spinach, grains and shellfish. Supplements also are available, derived from sugar beets. Consult a qualified health care provider before taking trimethylglycine supplements.