What are the signs of thiamin (B1) deficiency?

The usual result of thiamin deficiency is beriberi, which damages the nerves in the limbs and wastes the body.

Who is most likely to run low

In wealthy countries, long-term alcohol use disorder is the most common cause of thiamin deficiency.

What this means for you

Thiamin deficiency in a wealthy country is rarely a story about what someone eats. It is usually a story about alcohol, and that changes both who should think about it and what helps.

Established. In highly industrialised countries, chronic alcohol use disorders are the most common cause of thiamin deficiency.

What beriberi actually looks like is nerve damage in the limbs, starting as numbness, tingling or burning in the feet and hands, alongside weakness and weight loss. There is a second form that affects the heart. Neither announces itself as a vitamin problem.

Not for everyone. The symptoms overlap almost completely with other causes of neuropathy, including diabetes and B12 deficiency. This is a page to act on by asking for a test, not by self-diagnosing from a list.

If you drink heavily, or care for someone who does, the useful move is not a supplement from a shop. It is to raise thiamin with a doctor, because in that situation it is treated as a medical problem and often given by injection at first.

For everyone else, whole grains, pork, legumes, seeds and fortified flour cover 1.2 mg a day without any planning, and no upper limit was set because high intakes have not caused harm.

When it reaches the brain

There is a second emergency behind beriberi, and it gets treated on suspicion rather than on a test result. Wernicke's encephalopathy is thiamin deficiency in the brain. The clearest description of it comes from 82 women who developed it after severe vomiting in pregnancy. Eye movement problems appeared in 94 per cent of them, confusion in 77 per cent and an unsteady walk in 74 per cent. Weak reflexes were in half, muscle weakness in 44 per cent and memory trouble in 31 per cent. Tiredness on its own is not what this looks like.

Treated, it does come back. Across 20 published cases, 18 of them, nine in ten, had partial or full recovery of symptoms after thiamine into a vein, usually 500 mg three times a day. Speed is the whole point of that number. This is a hospital problem and nothing on a shop shelf belongs in it.

That dose is worth holding on to, because it keeps reappearing wherever the outcome was good. In published reports of Wernicke's encephalopathy in kidney disease, thiamine into a vein at 500 mg three times a day often gave full recovery, while Korsakoff's syndrome, which is the permanent memory damage version, turned up among the people who were given less. Five hundred milligrams three times a day is around four hundred times the daily requirement. No tablet you can buy comes anywhere near it, and no tablet you can buy is the answer to this.

Weight-loss drugs have started causing it

Not for everyone. A review of published case reports found six people who developed Wernicke's encephalopathy while taking semaglutide for obesity. Four women, two men, average age 47, and an average of about five months on the drug. Every one of them had prolonged gut symptoms and large weight loss before the neurological trouble started. One died. One recovered well. The other four were left with chronic Korsakoff syndrome, and one of those also lost the use of their legs permanently. Six published cases against millions of prescriptions is not a reason to stop a drug that is working for you. Weeks of vomiting, or weeks of eating almost nothing, is a reason to tell whoever prescribed it, and to say the word thiamine out loud while you are there.

The same logic covers kidney disease, where the run-up described in the reports is appetite gone, vomiting, weight loss, abdominal pain and diarrhoea. The pattern behind all of these is the same. Alcohol is the commonest route in a wealthy country. Anything else that stops food going in, or keeps it from staying in, does the same job by a different road.

Two groups where somebody has counted

After weight-loss surgery the risk has been mapped over time. Before the operation about 7 per cent of patients are already low. At three months it peaks near 19 per cent, roughly one patient in five, then falls to 9 per cent at six months and 6 per cent at a year. The three-month figure comes with a very wide range around it, from 0 to 68 per cent, so read it as a sign of when to look rather than as a probability for you.

Unsettled. In anorexia nervosa nobody can tell you how common this is. Across 17 studies the estimates run from none of the patients to 56.7 per cent when thiamin was measured in blood, and from 5.9 per cent to 100 per cent when it was worked out from what people said they ate. That spread is about the measuring rather than about the illness. A food questionnaire and a blood sample are answering two different questions. The usable summary is that thiamin is worth checking in anyone who has been eating very little, with no number attached to how likely it is.

Why the evidence here is thinner than elsewhere on this site

Unsettled. Almost everything above comes from case series and from counting deficiency inside groups at risk, which the usual scales rate as weak evidence. It is worth saying why. Randomised trials sit at the top of those scales because they assign people to one thing or the other, and nobody can assign a person to become thiamin deficient and then watch what it does to their brain. So this question cannot be answered that way, ever. What you are reading is the best evidence the question allows, and the weakness belongs to the design rather than to a lazy search. Where trials are possible, which is in treating people once someone suspects deficiency, they have been run.

What thiamin does not fix

Unsettled. Thiamin gets recommended for conditions that resemble its deficiency, and heart failure is the main one. It has now been pooled twice. Seven trials in 274 people found no gain in how the heart pumped, while reporting that the supplement did correct a deficiency where one existed. Eight trials in 384 people found no benefit on anything measured, and thiamine status was on that list too. So the two reviews agree that it does not help the heart, and disagree about whether it even corrects the deficiency it is given for. In diabetes, a 2025 review of seven clinical studies found the results conflicting and the trials too small to settle anything. Treat a suspected deficiency as a medical question, and treat the rest as marketing.

Thiamin gets recommended for conditions that look like its deficiency, and those claims have been tested. Seven randomised trials in 274 people with chronic heart failure found no gain in how the heart pumped, although the supplement did correct a deficiency where one existed. In diabetes, a 2025 review of seven clinical studies found the results conflicting and the trials too small to settle anything. Treat a suspected deficiency as a medical question, and treat the rest as marketing.

Written from the evidence cards and the data on this page, then checked back against the sources. Bioma Learn has no human medical reviewer at this time and we say so rather than invent one. Information, not medical advice. How we verify.

Where to get it from food

• Bacon, meatless fortified — 6.34 mg per 1 cup (528% DV)
• Corn flour, masa, white fortified — 1.68 mg per 1 cup (140% DV)
• Chicken, meatless, breaded, fried fortified — 1.66 mg per 1 cup, diced (138% DV)
• Sesame seed kernels, toasted — 1.54 mg per 1 cup (129% DV)
• Meatballs, meatless fortified — 1.35 mg per 1 cup (113% DV)

All thiamin (B1) sources ranked →

Full guide: Thiamin: a deficiency that is mostly about alcohol, not about diet