Vitamin B1 (thiamine)

Updated June 3, 2026

Thiamine sits at the center of glucose metabolism, and that single fact explains most of what it does in the body. It is the required cofactor for pyruvate dehydrogenase, alpha-ketoglutarate dehydrogenase, and transketolase. Those three enzymes decide whether glucose gets burned cleanly through the citric acid cycle or piles up as lactate. Pyruvate dehydrogenase is the gate into the mitochondrion. Without thiamine pyrophosphate, pyruvate cannot feed acetyl-CoA into the Krebs cycle, the electron transport chain runs short on NADH, and ATP output drops. Tissues with high glucose throughput need it most. The brain and the heart are first in line. The brain burns roughly 20 percent of total body glucose at rest and tolerates a shortfall poorly. Classic beriberi shows the pattern: wet beriberi presents as cardiovascular collapse, dry beriberi as peripheral neuropathy and confusion. Wernicke-Korsakoff syndrome in chronic alcohol use is the same biochemistry concentrated in the brain.

There is a neurotransmitter layer most people miss. Acetyl-CoA from the thiamine-dependent step is also the substrate for acetylcholine, which is why cognitive symptoms (memory, attention, slow processing) show up early in marginal deficiency. Transketolase in the pentose phosphate pathway produces NADPH, which the brain uses to regenerate glutathione. Thiamine sits upstream of both cholinergic tone and central antioxidant capacity. Alpha-ketoglutarate dehydrogenase is also notably impaired in Alzheimer's brain tissue at autopsy. The picture is less "vitamin" and more "core mitochondrial cofactor the brain leans on hard."

The form on the bottle is most of what determines whether the dose on the label matters. Plain thiamine hydrochloride saturates intestinal absorption around 5 mg per dose, so going higher orally is mostly excreted in urine. Benfotiamine, sulbutiamine, and TTFD (also called allithiamine) are fat-soluble derivatives that bypass that ceiling and reach different tissues at different concentrations. Pick the form to fit the job.

The RDA is 1.1 mg per day for women and 1.2 mg for men, with no established upper limit because thiamine is water-soluble and excess clears rapidly. That number prevents overt deficiency. It does not optimize anything. Low-grade insufficiency is common in people on high carbohydrate intake (which raises metabolic demand for the cofactor), older adults, people on loop diuretics like furosemide (which actively depletes thiamine), chronic alcohol use, and anyone with malabsorption from gastric bypass, IBD, or long-term metformin use. If you fit one of those buckets, supplementing pays off.

Thiamine hydrochloride is the cheap, water-soluble standard. Doses of 10 to 50 mg per day cover deficiency reliably, and the form tolerates grams in clinical settings. Thiamine mononitrate is what fortified bread and most multivitamins use. More shelf-stable, otherwise similar.

Benfotiamine is the most-studied fat-soluble derivative and the one with the cleanest clinical track record. It absorbs intact through intestinal cells, gets converted back to thiamine in the bloodstream, and reaches much higher plasma levels at the same oral dose. The diabetic neuropathy literature uses 300 to 600 mg per day. Small randomized trials show consistent reductions in pain and improvements in nerve conduction over weeks to months. The mechanism is not just energy rescue. Benfotiamine pushes glucose down the transketolase branch and away from the pathways that drive diabetic complications: hexosamine flux, protein kinase C activation, the polyol pathway, and advanced glycation end product (AGE) formation. Less AGE accumulation means less crosslinking of nerve and vascular proteins. That is the story behind the nerve conduction numbers. It does not cross the blood-brain barrier well, so it works on peripheral tissue and stays there. Fine if peripheral neuropathy is the target.

Sulbutiamine is two thiamine molecules joined by a disulfide bridge, fat-soluble, and crosses the blood-brain barrier easily. It has real human evidence in asthenia, the older clinical term for unexplained fatigue. Typical doses run 400 to 600 mg per day. Animal work shows sulbutiamine upregulates cholinergic and dopaminergic activity in cortex and hippocampus, which fits the profile users report. Tolerance can develop over weeks. A tool for episodes, not a forever supplement.

TTFD (thiamine tetrahydrofurfuryl disulfide), sometimes called allithiamine, is the synthetic cousin of a fat-soluble thiamine found in trace amounts in garlic. It crosses the blood-brain barrier well. Raises both blood and central thiamine efficiently. Derrick Lonsdale has used it for decades in autonomic dysfunction work. His framing: marginal central thiamine status disrupts brainstem regulation of heart rate, gut motility, and temperature. Restore the substrate and the autonomic picture improves. Doses run wide, 50 to 300 mg. The taste is bad. Some users hate the sulfurous odor. Mechanism is sound, case record is real, large rigorous trials are sparse.

Costantini's high-dose protocol, developed in Italy and used in fatigue, Parkinson's, and inflammatory bowel disease contexts, pushes thiamine HCl into gram territory: 1 to 4 g per day, split. Published case series and small uncontrolled trials report meaningful symptom changes in some subsets, with no consistent toxicity. The mechanism proposed is mass-action rescue of partially impaired thiamine-dependent enzymes, restoring mitochondrial throughput. Outside mainstream guidelines, but used in the literature. Run it only with a clinician who knows the territory.

Compared to other B vitamins, B1 is unusual because the form on the label so dramatically changes what the dose does. B12 has cyanocobalamin versus methylcobalamin, but absorption differences there are smaller. B9 has folic acid versus methylfolate, with real differences but bounded. For B1, the gap between a 50 mg thiamine HCl tablet and a 300 mg benfotiamine tablet is not just dose. Different distribution, different use case.

Safety is reassuring. Standard doses up to several hundred milligrams per day are well tolerated. Allergic reactions to injected thiamine exist but are very rare with oral forms. Interactions worth knowing: loop and thiazide diuretics deplete thiamine over time, chronic alcohol use impairs absorption, and metformin lowers B12 over the long run and may also nudge thiamine status. If you are managing diabetes, autonomic dysfunction, alcohol use, are on diuretics or metformin, or are pregnant, talk to a clinician before starting higher-dose thiamine, especially in the fat-soluble forms.

Putting the forms in their lanes. For preventing deficiency, plain thiamine HCl at 10 to 50 mg per day is cheap and effective. For diabetic peripheral neuropathy, benfotiamine at 300 mg per day has real trial support. For central nervous system targets, fat-soluble forms that cross the blood-brain barrier (sulbutiamine, TTFD) matter more than raw dose. For the Costantini high-dose protocol, the path runs through a clinician familiar with it. Match the form to the target tissue and the dose follows.