Thiamin (Vitamin B1)

What it is

Thiamin was the first B vitamin to be identified. Lack of it causes beriberi, a deficiency disease known since antiquity.^[1] Chemically it is two rings, a pyrimidine and a thiazole, joined by a methylene bridge. In the body it is mostly found in phosphorylated forms, chiefly thiamin pyrophosphate, the active coenzyme.^[1]

Thiamin pyrophosphate drives two kinds of reaction: the decarboxylation of alpha-keto acids such as pyruvate and alpha-ketoglutarate, which are steps in turning food into usable energy, and transketolation among sugar phosphates.^[1] That is why thiamin is usually described as an energy-metabolism vitamin. It is a cofactor in the process, not a source of energy itself.

Storage is small. The adult body holds about 30 mg of thiamin, and its biological half-life is estimated at 9 to 18 days.^[1] Absorption happens mainly in the jejunum, by an active carrier at low intakes and by passive diffusion at higher ones. Only a small share of a large dose is absorbed, and the excess is excreted in urine.^[1]

In the US diet, most thiamin comes from enriched, fortified or whole-grain foods: bread, grain-based mixed dishes and ready-to-eat cereals. Pork and ham are other sources.^[1] When portions of grain foods shrink, thiamin intake shrinks with them. Our guide to what to eat when you are eating less covers how to keep small plates dense in nutrients.

Supplements use salts of thiamin, most often thiamine hydrochloride or thiamine mononitrate. Replenish uses thiamine mononitrate.

How much adults need

Source: Institute of Medicine, Dietary Reference Intakes for Thiamin, Riboflavin, Niacin, Vitamin B6, Folate, Vitamin B12, Pantothenic Acid, Biotin, and Choline (1998).^[1] ND = not determined: the data on adverse effects were too limited to set a UL. RDA = Recommended Dietary Allowance; UL = Tolerable Upper Intake Level.

Amount in Replenish: confirmed at launch

The RDA is set to cover 97 to 98 percent of healthy people in each group.^[1] When the 1998 report was written, median US intake of thiamin from food was about 2 mg a day, above the RDA.^[1] The report names groups who may need more: people on hemodialysis or peritoneal dialysis, people with malabsorption, and women carrying or nursing more than one baby.^[1] The absence of a UL does not mean any dose is proven safe. It means there were not enough data to calculate one.

What the evidence says

Low intake and heavy drinking. In industrialized countries, severe thiamin deficiency is mostly linked to heavy alcohol use combined with limited food intake, where it can present as Wernicke-Korsakoff syndrome.^[1] Early deficiency produces vague signs that are easy to miss: apathy, poor short-term memory, confusion, irritability and muscle weakness.^[1]

After bariatric surgery. A 2008 systematic review gathered 84 cases of Wernicke encephalopathy after bariatric surgery. In 94% the hospital admission came within 6 months of the operation, and frequent vomiting, lasting a median of 21 days, was a risk factor in 90%. Almost half the patients recovered only partly.^[2] The pattern is what matters here: a sharp fall in food intake combined with persistent vomiting.

Eating less on GLP-1 medications. A 2025 joint advisory from four obesity and nutrition societies lists vitamin B1 among the nutrients at risk of running low in people taking GLP-1 medications, because they eat substantially less.^[3] That is a risk based on reduced intake, not a measured rate of deficiency. Our article on vitamins on a GLP-1 sets thiamin beside the other nutrients on that list.

Heart failure. A 2015 meta-analysis found thiamin deficiency more common in people with heart failure than in controls (odds ratio 2.53), and pointed to diuretic use, diet changes and altered absorption as possible causes.^[4] Supplementing thiamin to improve heart function has not held up. A 2024 meta-analysis of 7 randomized, double-blind trials with 274 patients found no significant effect on ejection fraction, walking distance or symptom class, although thiamin did correct deficiency where it existed.^[5]

Where the evidence is thin. Thiamin is often marketed for energy or tiredness. Its coenzyme role in energy metabolism is well established, but that is not evidence that extra thiamin raises energy in people who already get enough. For a broader look at why people feel tired when they eat less, see why GLP-1 medications can make you tired.

Safety and interactions

No adverse effects from thiamin taken by mouth in food or supplements had been reported when the DRI committee reviewed the evidence, which is why no UL exists.^[1] The committee noted that supplements of up to 50 mg a day were widely sold, but that the effects of that much or more had not been studied systematically.^[1] Rare serious reactions have been reported with thiamin given by injection, which is a different route from a drink mix.^[1]

Loop diuretics. In a 1991 pilot study, 21 of 23 people with heart failure on long-term furosemide had biochemical signs of thiamin deficiency, against 2 of 16 controls. Their urine contained more thiamin than expected, pointing to urinary loss.^[6] The study was small, but the 2015 meta-analysis also named diuretics as a likely mechanism.^[4] If you take a loop diuretic, thiamin status is a reasonable thing to raise with your prescriber.

Alcohol and sustained vomiting both raise the risk of deficiency, as described above.^[1][2] Persistent vomiting needs medical attention in its own right; a supplement is not a substitute.

If you take prescription medicine, ask your pharmacist or prescriber before adding a supplement.

In Replenish

Replenish contains thiamin as thiamine mononitrate, one of the 6 vitamins in the formula. With the other vitamins and minerals it provides key vitamins and minerals^† on days when meals are small. The amount per serving is published with the Supplement Facts panel on the Replenish page. It sits alongside vitamin B6, folate and vitamin B12.