Thiamin (vitamin B1) is essential for carbohydrate metabolism and neural function. Severe deficiency causes beriberi while milder deficiency causes non-specific symptoms like malaise. Thiamin deficiency has been common in people relying mainly on polished rice. The removal of rice bran during polishing removes thiamin. The richest sources of thiamin are brewer's yeast, pork liver, and whole grains. However, thiamin is unstable when heated or exposed to oxidizing and alkaline conditions.