Thiamine HCl (vitamin B1) is a water-soluble essential nutrient that all bodily tissues need to function. Thiamine has the designation as vitamin B1 because it was the first B vitamin that scientists discovered. It is necessary to generate adenosine triphosphate (ATP) molecules. These molecules are responsible for transferring energy between cells. It is also a cofactor for several enzymes that help metabolize glucose, amino acids, and lipids.