Thiamin: Benefits, Dosage, and Safety

What thiamin is

Thiamin (also spelled thiamine), known as vitamin B1, is one of the water-soluble B vitamins. It is naturally present in some foods, added to some fortified food products, and available as a dietary supplement. About 80% of the roughly 25 to 30 mg of thiamin in the adult body is stored as thiamin diphosphate (TDP, also called thiamin pyrophosphate), its main metabolically active form. The body stores thiamin primarily in the liver but only in very small amounts, and the vitamin has a short half-life, so a continuous dietary supply is required.

What it does

• Serves as thiamin diphosphate (TDP), an essential cofactor for five enzymes involved in glucose, amino acid, and lipid metabolism
• Plays a critical role in energy metabolism, converting nutrients into usable energy
• Supports the growth, development, and normal function of cells
• Supports normal nervous system and cardiovascular function (deficiency produces neurological and cardiac signs)

How much thiamin per day

These are population reference values from the NIH and the Dietary Reference Intakes. They are targets for healthy people, not personal prescriptions. The last column is the Tolerable Upper Intake Level, the most you should get from all sources combined.

Deficiency and who is at risk

In its early stage, thiamin deficiency can cause weight loss and anorexia, confusion, short-term memory loss and other mental signs and symptoms, muscle weakness, and cardiovascular symptoms such as an enlarged heart. The most common effect is beriberi, characterized mainly by peripheral neuropathy and wasting, with impaired sensory, motor, and reflex functions; in rare cases beriberi causes congestive heart failure with edema of the lower limbs and, occasionally, death. In the United States a more common manifestation is Wernicke-Korsakoff syndrome. Its acute phase, Wernicke's encephalopathy, is usually marked by peripheral neuropathy; without treatment up to 20% of people with it die, and survivors can develop Korsakoff's psychosis, which involves severe short-term memory loss, disorientation, and confabulation.

Who may need more

• People with chronic alcohol dependence, in whom thiamin deficiency is the most common cause in industrialized countries
• Older adults with low dietary intake, multiple chronic diseases, or use of multiple medications
• People with HIV/AIDS at increased risk of deficiency and its sequelae
• People with poorly controlled type 1 or type 2 diabetes, who may have lower plasma thiamin
• People who have undergone bariatric (weight-loss) surgery, for whom thiamin-containing supplements are almost always recommended
• People taking loop diuretics such as furosemide long term

Food sources

Whole grains, meat, and fish. In the United States the most common sources are fortified cereals and bread, with pork another major source. Examples per serving: fortified breakfast cereals (100% DV, 1.2 mg), enriched egg noodles (0.5 mg per cup), broiled bone-in pork chop (0.4 mg per 3 oz), cooked trout (0.4 mg per 3 oz), boiled black beans (0.4 mg per half cup). Dairy products and most fruits contain little thiamin. Because thiamin dissolves in water and is degraded by heat, cooking and processing can reduce food thiamin content.

Forms and absorption

The most commonly used forms of thiamin in supplements are thiamin mononitrate and thiamin hydrochloride, which are stable and water soluble. Multivitamin/mineral supplements typically provide about 1.5 mg thiamin. B-complex and thiamin-only supplements are also available. Benfotiamine is a synthetic thiamin derivative used in some supplements; it is not water soluble and is converted to thiamin in the body. Data on the bioavailability of thiamin from food are very limited, though absorption appears to increase when intakes are low.

Safety, excess and interactions

Too much

The body excretes excess thiamin in the urine. 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 for thiamin. The apparent lack of toxicity may be explained by the rapid decline in absorption of thiamin at intakes above 5 mg. The FNB noted, however, that despite the lack of reported adverse events, excessive intakes of thiamin could have adverse effects.

Interactions to know

• Furosemide (Lasix) and other loop diuretics: research has linked furosemide use to decreases in thiamin concentrations, possibly to deficient levels, as a result of increased urinary thiamin loss
• Fluorouracil (5-fluorouracil, Adrucil) chemotherapy: several cases of beriberi or Wernicke's encephalopathy have resulted from treatment, possibly because the drug increases thiamin metabolism and blocks formation of thiamin diphosphate (TDP), the active form; thiamin supplements might reverse some of these effects

How status is measured

Thiamin status is not reliably indicated by blood thiamin levels. It is most often measured indirectly by assaying erythrocyte transketolase activity, a TDP-dependent enzyme, in the presence and absence of added TDP; the resulting TDP effect is typically 0% to 15% in healthy people, 15% to 25% in marginal deficiency, and above 25% in deficiency. Urinary thiamin excretion is also used to reflect dietary intake (less than 100 mcg/day suggests insufficient intake and less than 40 mcg/day indicates extremely low intake), though it does not reflect tissue stores.

Regulated by the FDA as a dietary supplement ingredient under DSHEA, not as a drug; supplements are not FDA-approved to diagnose, treat, cure, or prevent disease. The FDA Daily Value (DV) for thiamin is 1.2 mg for adults and children age 4 years and older. FDA does not require food labels to list thiamin content unless thiamin has been added to the food.

What the evidence supports

We grade the strength of evidence behind each common reason people take thiamin, mirroring how the NIH describes it.