Thiamine (vitamin B1) is the cofactor your cells need to turn food into usable energy. It sits at the entry point of mitochondrial metabolism, where glucose and fats get converted into ATP. Without enough of it, every cell in your body is running slow, and the ones most affected are the ones with the highest energy demand: brain, heart, nerves, and the autonomic nervous system that runs digestion, blood pressure, and temperature regulation.

Frank deficiency (beriberi, Wernicke's encephalopathy) is rare in developed countries but functional shortfalls are not. The people who tend to benefit most from supplementation are those with persistent fatigue, brain fog, exercise intolerance, autonomic symptoms (dizziness on standing, sluggish digestion, temperature dysregulation), or diabetic nerve symptoms. Alcohol use, gut dysfunction, diuretics, metformin, high-carb diets, and chronic stress all deplete it. If your bloodwork looks fine but you feel like the lights are dim, thiamine is one of the more underrated levers to pull.

The wrinkle is that standard thiamine HCl, the form in most B-complexes, is poorly absorbed at higher doses and doesn't cross cell membranes or the blood-brain barrier efficiently. The fat-soluble derivatives, benfotiamine and TTFD, were developed specifically to get around this and are what most of the higher-dose protocols use.

Knowing whether you're actually low is harder than it should be. Serum thiamine, the test most doctors run, is close to useless, it reflects recent intake, not what's inside your cells. Whole-blood thiamine (RBC thiamine diphosphate) is the better test but rarely available in standard labs, you'll usually need a functional medicine panel or send-out test. Most people end up figuring it out backwards: they have the phenotype (persistent fatigue, brain fog, autonomic symptoms, often with a history of high-carb diet, gut issues, alcohol, or chronic stress), they run a properly cofactored protocol for 4-8 weeks, and they either feel meaningfully better or they don't.

Deep-dive

Dosage:

• Standard maintenance (B-complex or standalone): the RDA is 1.1-1.2 mg/day. Most quality B-complexes deliver 25-100 mg of thiamine HCl, which is plenty for general use and covers anyone eating a varied diet
• Higher-dose thiamine HCl: 500-1500 mg/day for women, 1000-1800 mg/day for men, taken in divided doses with food. This is the Costantini-style protocol and the form used in the IBD and PBC fatigue RCTs. Cheap, well-tolerated, less cellular penetration per milligram than the fat-soluble derivatives
• Benfotiamine: 150-300 mg twice daily (300-600 mg/day) is the standard range. Go up to 1200 mg/day for diabetic neuropathy or cognitive use, matching trial doses. Taken with food. Better tolerated than TTFD because it doesn't cross the blood-brain barrier as readily, fewer central effects in the first few weeks
• TTFD: start at 50 mg once daily with food, work up to 100-300 mg/day in divided doses over 2-4 weeks. This is the form for brain, autonomic, and energy symptoms. Stronger and more activating, more likely to produce paradoxical reactions, requires careful cofactor support
• Cofactors are not optional at higher doses. Magnesium glycinate or malate at 200-400 mg elemental daily, a quality B-complex (especially riboflavin, B6, B12), and adequate dietary potassium. Without these, TTFD especially will pull cofactors from their stores and create new bottlenecks, the classic paradoxical reaction
• Timing: with food, earlier in the day. TTFD can be mildly activating and is best avoided in the evening. Benfotiamine and HCl are generally neutral on sleep
• Older adults: start at half the standard dose and titrate slower. Sensitivity is higher and cofactor stores are often lower
• Cycling: no firm consensus. Continuous protocols are common at the lower end. Some run 3 months on, 1 month off at higher TTFD doses. Reasonable middle ground: titrate up, hold at the dose that produces the response, drop to a maintenance dose (50-100 mg TTFD or 150-300 mg benfotiamine) once symptoms stabilise
• Pregnancy and breastfeeding: standard prenatal thiamine intake is adequate. Pharmacologic doses haven't been studied in pregnancy and should be a medical decision, not a self-experiment. Hyperemesis gravidarum is a separate medical issue

Here's what you can expect:

At standard maintenance doses (a B-complex or 25-100 mg HCl), you'll notice nothing specific unless you were genuinely low to start with, in which case fatigue and brain fog tend to lift over 2-4 weeks.

At higher pharmacologic doses (300+ mg benfotiamine or 50+ mg TTFD), if you're the right phenotype (persistent fatigue or brain fog not explained by sleep, thyroid, or iron), the first two weeks often produce a noticeable lift in energy, cleaner thinking, and a sense that the autonomic system has settled, steadier heart rate, better tolerance of standing, more reliable digestion. Some people describe it as the lights coming back up. The full effect, particularly for fatigue and neuropathy symptoms, tends to land around 4-8 weeks.