Vitamin B1 (or thiamine) is one of the water-soluble B vitamins. Thiamine is found naturally in some foods, is added to others, and is available as a dietary supplement. This vitamin plays a critical role in energy metabolism and, therefore, in cell growth and function. Humans store thiamine mainly in the liver, but in very small quantities. The vitamin has a short half-life, so people require a constant supply of it from the diet.