Severe thiamine deficiency may lead to serious complications involving the nervous system, brain, muscles, heart, and stomach and intestines. Dietary sources of thiamine include beef, brewer's yeast, legumes (beans, lentils), milk, nuts, oats, oranges, pork, rice, seeds, wheat, whole-grain cereals, and yeast.hiamine was one of the first compounds recognized as a vitamin. Thiamine is involved in many body functions, including nervous system and muscle function, the flow of electrolytes in and out of nerve and muscle cells, digestion, and carbohydrate metabolism.