From Dr Pepi on Vitamin B1

Thiamine (B-1) is the “anti-stress” vitamin. Not only does B-1 reduce stress, it can also make you feel happier. It can improve your ability to think clearly by improving brain function. B-1 is an immune system booster and protects against aging. It is essential for your body to metabolize (break down) fats, carbs and proteins. B-1 helps preserve the health of your heart, muscles and nervous system. Thiamine is a great energy producer. Deficiencies affect cardiovascular, nervous, muscular and gastrointestinal tissues.
As vitamin B-1 does not store in your body, it is depleted quickly and easily. For instance, one upset can really deplete your supply of B-1! These things also deplete your B-1: drugs, antibiotics, alcohol, cigarettes, sodas, caffeine, sugar and chocolate. (I know, I know, this is upsetting – go take some B-1.)