Vitamin B1 (Thiamin)Vitamin

• Correcting a diagnosed thiamin deficiencyLikely helps

  This is thiamin's established medical use — repletion in people whose deficiency has produced beriberi or Wernicke-Korsakoff syndrome [1][2].
  
  It is also given in maple syrup urine disease and Leigh syndrome, two inherited metabolic disorders [1][2]. ^[1]

• Wernicke-Korsakoff in alcohol use disorder — dose unsettledPossibly helps

  That the vitamin is the right substance here is not in question; how much, how often and by what route is [1].
  
  A 2013 Cochrane Review found only two eligible randomized trials, 177 people in total, and concluded the evidence cannot guide dose, frequency, duration or route [1].
  
  The European Federation of Neurological Societies still recommends 200 mg three times daily, preferably IV, until symptoms resolve in at-risk patients [1]. ^[1]

• Glucose control in type 2 diabetesPossibly helps

  Small studies found 150–300 mg/day of oral thiamin lowered glucose levels in people with type 2 diabetes or impaired glucose tolerance [1].
  
  NIH ODS notes those studies never assessed whether the change was clinically meaningful [1]. ^[1]

• Diabetic neuropathy, using benfotiaminePossibly helps

  The trials disagree, and we are not settling it for you.
  
  Three small randomized trials of benfotiamine at 120–900 mg/day reported less severe neuropathy and lower urinary albumin excretion than placebo [1].
  
  Another randomized trial at 900 mg/day found no effect on urinary albumin or on kidney injury molecule-1 [1]. ^[1]

• Heart failurePossibly helps

  Poor thiamin status is common in heart failure patients — reported at 21% to 98% depending on the study [1].
  
  A meta-analysis of two randomized placebo-controlled trials found supplementation significantly improved left ventricular ejection fraction, though clinical significance again went unassessed [1]. ^[1]

• Alzheimer's diseasePossibly helps

  Deficiency has been proposed as a contributing factor, on the strength of animal work and reduced activity of thiamin-dependent brain enzymes found at autopsy [1].
  
  A 2001 Cochrane Review of three small trials using 3 g/day oral thiamin could not draw a conclusion, given the sample sizes and thin reporting [1]. ^[1]

• Where NIH ODS lands on all of the abovePossibly helps

  Outside of correcting an actual deficiency, none of these uses is established [1].
  
  ODS's own summary is that larger, well-designed trials are needed before thiamin counts as established therapy for diabetes complications, heart failure or Alzheimer's disease [1]. ^[1]

• Early signs — weight loss, loss of appetite, confusion, short-term memory loss, muscle weakness [1][3]
• Cardiovascular signs early on, including an enlarged heart [1][3]
• Wet beriberi — high-output cardiac failure with edema, enlarged heart, warm extremities, fast heart rate, respiratory distress [1][3]
• Dry beriberi — peripheral neuropathy with muscle weakness, impaired reflexes, and seizures in severe cases [1][3]
• Congestive heart failure from wet beriberi in rare cases, which can be fatal [1]
• Wernicke's encephalopathy, the acute phase — peripheral neuropathy, ataxia, eye-movement abnormalities, up to 20% mortality if left alone [1][3]
• Korsakoff's psychosis, the chronic phase in survivors — severe short-term memory loss, disorientation, confabulation [1][3]
• Roughly one-quarter of patients at the Korsakoff stage do not recover even with parenteral thiamin [1]
• Also linked to deficiency: optic neuropathy, Leigh's disease, African/Nigerian seasonal ataxia, central pontine myelinolysis [2]^[1]

• Thiamin mononitrateThe default in multivitamins and in food fortification .

  Elemental content

  A stable, water-soluble thiamin salt [1].

  Absorption

  Uptake from any single dose saturates above roughly 5 mg [1].
  
  That ceiling applies to the water-soluble salts generally, not to this one in particular.

  Best for

  The default in multivitamins and in food fortification [1].

• Thiamin hydrochloridePrescription tablets, and the 50–100 mg over-the-counter strengths .

  Elemental content

  The other stable water-soluble salt, and the reference form in bioavailability work [1][5].

  Absorption

  Same saturable uptake as mononitrate [1].
  
  It is the comparator benfotiamine was measured against [5].

  Best for

  Prescription tablets, and the 50–100 mg over-the-counter strengths [9][10].

• BenfotiamineThe form used in the diabetic-neuropathy trials, where the results conflict .

  Elemental content

  A synthetic S-acyl derivative, often called fat-soluble — though it is in fact almost insoluble in organic solvents [1][4].

  Absorption

  Intestinal alkaline phosphatases convert it to lipophilic S-benzoylthiamine, which crosses the gut lining and becomes free thiamin in red cells and liver [1][4].
  
  A randomized crossover study in healthy volunteers measured about 11-fold (1147%) higher plasma thiamin bioavailability than an equal oral dose of thiamin hydrochloride [5].
  
  Erythrocyte thiamin diphosphate, the active cofactor, came out about 196% higher [5].

  Best for

  The form used in the diabetic-neuropathy trials, where the results conflict [1].

• Fursultiamine and sulbutiamineSulbutiamine was sold in some countries as the drug Arcalion for weakness and low energy .

  Elemental content

  Lipophilic thiamin-disulfide analogues developed in Japan in the 1950s–60s, originally against widespread beriberi [2][4].

  Absorption

  Built for lipid solubility the water-soluble salts do not have [4].
  
  Human research is very thin — for sulbutiamine, no positive or negative finding has been replicated [6].

  Best for

  Sulbutiamine was sold in some countries as the drug Arcalion for weakness and low energy [6]. It now turns up in cognitive, fat-burner and pre-workout products, and is not approved as a drug in the US [6].

• Side effects of thiamin itself

  Thiamin is not known to interact with medications, and oral excess is excreted in urine [1][3].
  
  Uncommon reported effects: nausea, hives, lethargy, ataxia and impaired gut motility [2][3].
  
  Rare allergic reactions, including anaphylaxis, have occurred with repeated injectable doses [2][3]. ^[3]

• Sulbutiamine, specifically

  Reported side effects at up to 600 mg/day for as long as 2 months include nausea, headache, insomnia, diarrhea, tremor, agitation, palpitations and cough [6].
  
  It sits on the US Department of Defense's Prohibited Dietary Supplement Ingredients list, on grounds of insufficient safety and efficacy evidence [6]. ^[6]

• Glucose given before thiamin can trigger Wernicke's

  Order matters in an emergency. IV glucose given to a thiamin-deficient person can trigger or worsen Wernicke encephalopathy, because glucose metabolism consumes what little thiamin remains as a cofactor [3].
  
  Clinical guidance is to give thiamin before or together with glucose in anyone suspected of deficiency, such as people with alcohol use disorder or malnutrition [3]. ^[3]

• Alcohol use disorder

  Ethanol cuts gastrointestinal absorption of thiamin, depletes liver stores, and impairs phosphorylation into the active cofactor [1].
  
  Up to 80% of people with chronic alcoholism become deficient, and Wernicke-Korsakoff syndrome is 8 to 10 times more common than in the general population [1].
  
  Chronic alcohol use disorder is the leading cause of thiamin deficiency in highly industrialized countries [1]. ^[1]

• After bariatric surgery

  About 27% of bariatric surgery patients experience vitamin B1 deficiency, per a meta-analysis [2].
  
  NIH ODS lists this group among those at elevated risk, alongside people with alcohol dependence and older adults [1]. ^[2]

• Other groups at elevated risk

  Older adults — 20–30% show laboratory evidence of some deficiency, especially in institutional care [1].
  
  People with HIV/AIDS — nearly 10% of AIDS autopsy cases showed Wernicke's encephalopathy in one study [1].
  
  People with diabetes — plasma thiamin measured 50–76% lower than in healthy volunteers in small studies [1]. ^[1]

• Furosemide and long-term diuretics

  Furosemide, a loop diuretic used for edema and hypertension, is linked to lower thiamin concentrations — possibly to deficient levels — through increased urinary loss [1][3].
  
  Whether supplementation reliably keeps long-term loop-diuretic patients out of deficiency still needs clinical-trial confirmation [1][3].
  
  Long-term diuretic use in general has been associated with varying degrees of thiamin insufficiency [2]. ^[1]

• Fluorouracil (5-FU) chemotherapy

  Case reports link this drug, used in colorectal and other solid cancers, to beriberi and Wernicke's encephalopathy [1].
  
  The proposed route is increased thiamin metabolism plus blocked formation of the active cofactor, thiamin diphosphate [1].
  
  NIH ODS notes thiamin supplements may undo some of those effects [1]. ^[1]

• Macrolide antibiotics, erythromycin especially

  Electronic health record data associate them with decreased thiamin levels [3].
  
  The likely route is inhibition of the SLC19A2 thiamin transporter [3]. ^[3]

• Patiromer

  This potassium-binding drug can reduce absorption of orally administered thiamin [3].
  
  How to space the two doses is a question for the prescriber — these sources do not specify an interval. ^[3]

Whole grains, pork, fish, legumes, and fortified or enriched cereal, bread and infant formula [1][2].

In the US, cereals and bread are the biggest dietary contributors, with pork close behind. Dairy and most fruit carry very little [1].

USDA figures: fortified breakfast cereal 1.2 mg (100% DV) per serving, enriched cooked egg noodles 0.5 mg per cup, a 3 oz bone-in pork chop 0.4 mg [1].

Also 0.4 mg each for 3 oz cooked trout and half a cup of boiled black beans, and 0.3 mg for an enriched English muffin [1].

About half of US thiamin intake comes from foods that naturally contain it. The rest arrives through fortification [1].

Cooking costs you some of it. Bread holds 20–30% less thiamin than its raw ingredients, and pasteurization can cut milk's already-small amount by up to 20% [1].

Because thiamin is water-soluble, a meaningful share goes down the drain with discarded cooking water [1]. Unenriched white rice has roughly one-tenth the thiamin of unenriched brown rice [1].

Two more spoilers. Sulfite preservatives split the molecule at its pyrimidine-thiazole bridge, faster under acidic conditions [2].

Some fish and shellfish carry heat-labile thiaminase enzymes that break thiamin down, though cooking inactivates them [2].

Thiamin is water-soluble, so it needs no dietary fat and goes down with or without food, at any hour [7][8].

Morning is the usual suggestion for B vitamins — partly for consistency, partly because their role in energy metabolism can feel mildly stimulating late in the day [7].

The sources split on the stomach question. Cleveland Clinic's general vitamin guidance says water-soluble vitamins are best on an empty stomach with a glass of water [8].

Its own patient guidance for prescription thiamine tablets says the opposite: take the medication with food for best results [9].

No source establishes a strict optimal time. The consistent thread is that food is not required for absorption, but may help tolerability [7][8][9].