A principal function of thiamin in all cells is as the coenzyme cocarboxylase or TPP. The tricarboxylic acid cycle (TCA; citric acid cycle; Krebs cycle) is responsible for production of energy in the body. In this cycle, breakdown products of carbohydrates, fats and proteins are brought together for further breakdown and for synthesis.
Little is known of thiamin functions in nervous tissue. However, evidence has accumulated for a specific role of thiamin in neurophysiology that is independent of its coenzyme function.
Brewer's yeast is the richest known natural source of thiamin. Cereal grains and their by-products, soybean meal, cottonseed meal and peanut meal are relatively rich sources of thiamin. Since the vitamin is present primarily in the germ and seed coats, by-products containing the latter are richer than the whole kernel, while highly milled flour is very deficient.
The level of thiamin in grain rises as the level of protein rises; it depends on species, strain, and use of nitrogenous fertilizers. The content in hays decreases as plants mature and is lower in cured than in fresh products. Since thiamin is water soluble as well as unstable to heat, in certain cooking operations large losses result.
