Betaine and choline are closely related compounds from a structural and functional perspective. Betaine is also called trimethylglycine and choline is also called tetramethylglycine. Both compounds function as methyl donors, and when choline donates a methyl group it becomes betaine. A further donation of a methyl group results in the compound dimethylglycine. One important role for betaine is the conversion of homocysteine to methionine, a process that requires methylation of the homocysteine molecule, the methyl group being donated from betaine. An alternative methylation pathway for homocysteine exists and this required the folic acid derived methyltetrahydrofolate molecule donating a methyl group in a reaction catalysed by the vitamin B12 dependent enzyme methylenetetrahydrofolate reductase. Folic acid, vitamin B12 and betaine are therefore integral in the maintenance of low cellular levels of homocysteine.
As high levels of homocysteine in the cells leads to raised plasma levels of homocysteine, and raised plasma levels of homocysteine are associated with cardiovascular disease, betaine, vitamin B12 and folic acid deficiencies increase the risk of cardiovascular disease significantly. Because dietary choline can be converted to betaine, and because cellular methionine levels increase following administration of high amounts of dietary choline (possibly due to increased synthesis from homocysteine), choline may also be protective of cardiovascular disease at high intakes. Choline is usually classed as a member of the B group of vitamins, and although it can be manufactured in the body, it is officially recognised as an essential dietary nutrient. Choline may also have other protective effects against cardiovascular disease, because it is required for the metabolism of fats from the liver, and without choline fatty acids accumulate in the liver causing metabolic dysfunction.