Thiamin helps turn the carbohydrates you eat into usable energy, and it keeps your nervous system working properly. When intake stays low over time, that's associated with tiredness and nerve-related symptoms like tingling in the hands and feet.
It's more commonly short than people expect: research published in The Lancet Global Health estimates around 30% of people don't get enough to meet the recommended intake. Because thiamin isn't stored in large amounts, a steady daily supply matters more than the occasional top-up.
Adults need around 1.1 mg (1,100 mcg) a day, from whole grains, pork, legumes, nuts, and fortified cereals. There's no established upper limit, since your body clears what it doesn't use.