Vitamin B1

Thiamin is not known to interact with any medications, but certain medications can have an adverse effect on thiamin levels. Furosemide (Lasix), a loop diuretic, has been linked to decreased thiamin concentrations as a result of urinary thiamin loss. Fluorouracil (5-fluorouracil; Adrucil) chemotherapy might increase thiamin metabolism and block the formation of TDP, the active form of thiamin. — per the NIH Office of Dietary Supplements; general information, not medical advice.

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Because of the lack of reports of adverse effects from high thiamin intakes (50 mg/day or more) from food or supplements, the Food and Nutrition Board did not establish a Tolerable Upper Intake Level (UL) for thiamin; the body excretes excess thiamin in the urine. The apparent lack of toxicity may be explained by the rapid decline in absorption at intakes above 5 mg, although the FNB noted that excessive intakes could nonetheless have adverse effects — per the NIH Office of Dietary Supplements.

Groups most likely to have inadequate thiamin status include people with alcohol dependence (up to 80% of people with chronic alcoholism develop deficiency), older adults, people with HIV/AIDS or diabetes, and people who have undergone bariatric surgery. In its early stage, deficiency can cause weight loss and anorexia, confusion, short-term memory loss, muscle weakness, and cardiovascular symptoms; its most common effect is beriberi (peripheral neuropathy and wasting), and a more common U.S. manifestation is Wernicke-Korsakoff syndrome — per the NIH Office of Dietary Supplements.

Food sources of thiamin include whole grains, meat, and fish, and in the U.S. breads, cereals, and infant formulas are fortified with thiamin. The most common sources in the U.S. diet are cereals and bread, with pork another major source, while dairy products and most fruits contain little thiamin. Among the highest-providing foods listed are fortified breakfast cereals, enriched egg noodles, pork chops, trout, and black beans — per the NIH Office of Dietary Supplements.