Vitamin B1, or thiamine, is a vitamin that the body requires for energy metabolism and for cell growth, function, and development. Thiamine is also necessary for the proper function of the brain. It is found in meat, fish, and whole grains. Breakfast cereals are often fortified with vitamin B1. Women who are pregnant or breastfeeding require greater amounts of thiamine. People with certain conditions, including HIV, diabetes, and alcohol dependence often have low levels of this vitamin. People who undergo bariatric surgery may suffer from thiamine deficiency due to malabsorption. Symptoms of thiamine deficiency include weight loss, memory loss, muscle weakness, enlarged heart, and mental signs and symptoms.