Vitamin B1

Thiamine is an essential water-soluble vitamin that is involved in carbohydrate metabolism and energy production. It is also plays an important role in the central nervous system through regulation of chloride channels and is needed for synthesis (and possibly release) of acetylcholine. 

Low levels are associated with alcohol intake, fatigue, weakness, cardiac arrhythmias, confusion, memory loss, neuropathy, depression, weight loss, pain and ataxia among other things.  

Severe deficiency is known as the condition beriberi.