All B vitamins are water soluble which means that the body doesn't have big storages of these but for most of them (everyone except from B12), the body will get rid of the extra intake. This also means that you need to get an intake of these every day.
The B vitamins act as coenzymes in the metabolism. All B vitamins also influence each other in many ways.
You can find thiamine in (examples) fruits, vegetables, meat and fish, sunflower seeds, wheat germ, brazil nuts, dried soy beans and pork. Sifted flour are enriched (sweden) with many B vitamins and thiamine is one of them. About 50 % of the vitamin get lost/destroyed while boiling vegetables and up to 40 % whily baking bread. Different factors in coffee and tea will inhibit the vitamins bioavailability.
The vitamin is very important for the metabolism as we've already mentioned. If you have a higher intake of carbs (example if you work out hard and eat extra carbs), then the need for thiamine will also increase.
The vitamin is also important for the brain and the nerv cells indirect (from the metabolism) as the brain only can use glucose as energy source. Indirect it is also needed for synthesis of DNA and RNA.
Alcoholics often suffer from vitamin B1 deficiency due to several reasons; alcohol inhibits the absorption in the intestines, alcohol is a diuretic (urindrivande) so losses increase, alcohol increases the need for the vitamin and alcoholics usually also eat a more one-sided and inadequate diet.
There are also many people that has a hidden deficiency so this is something that needs to be thought of by many. One reason could be that the food loose so much of the vitamin when we heat up food.
People that has a higher need of vitamin B1 are (more than physical active ppl with increased intake of carbs and alcoholics) older people, pregnant and breastfeeding women. Women that are taking birth controll pills are also having a higher need of vitamin B intake.
Thiamine is not toxic in higher intakes - it has been used as a supplement for many years without any reports of serious side effects.