Does vitamin B1 help fatigue? One good trial, and one that failed to replicate

Vitamin B1 sells on a simple promise. It is the vitamin your cells use to turn food into energy, so more should mean more energy. The trials do not support that. They support something narrower.

The best evidence here comes from one research group that ran the same experiment twice. It worked. Then it did not.

What happened when the same team ran the same trial twice?

In 2021, Bager and colleagues gave high-dose oral thiamine to adults with quiescent inflammatory bowel disease and chronic fatigue. Fatigue fell 4.5 points while placebo drifted slightly upward. In 2024, the same team ran the same protocol at the same doses in a different fatigued population. Nothing.

Both were randomized, double-blind, placebo-controlled crossover trials, meaning every participant took both thiamine and placebo in random order and served as their own comparison. That design catches real effects in small groups.

The 2021 trial, in Alimentary Pharmacology and Therapeutics, enrolled people whose IBD was in remission and whose exhaustion was not. Doses run 600 to 1,800 mg of oral thiamine a day, hundreds of times the recommended dietary allowance of 1.2 mg for men and 1.1 mg for women.

The 2024 trial, in PLoS One, tested whether that held anywhere else. Same team, same randomized, double-blind, placebo-controlled crossover design, same doses. This time the population was primary biliary cholangitis, a chronic liver disease where disabling fatigue is a defining complaint. Flat.

The negative trial is the more valuable of the two. A positive result plus an honest replication attempt tells you where an effect lives, and most supplement ingredients never get that second study. Read together, the pair says the mechanism is probably not "thiamine gives you energy." It is closer to "correcting a shortfall in someone who has one relieves the fatigue it was causing."

Does the rest of the research point the same way?

Mostly, yes. Across very different diseases, thiamine trials come back weak or negative overall, with whatever signal exists concentrated in people deficient at baseline. Baseline status, not dose and not form, keeps deciding the outcome.

In diabetic neuropathy, the randomized benfotiamine trial of Stracke and colleagues, 2008, in Experimental and Clinical Endocrinology and Diabetes, hit significance per-protocol, counting only people who completed the regimen as assigned, but not by intention-to-treat, which counts everyone randomized. Intention-to-treat is the stricter read. That split means it did not clearly work.

In diabetic nephropathy, the randomized benfotiamine trial of Alkhalaf and colleagues, 2010, in Diabetes Care, was negative. Doses across that work ran 300 to 900 mg a day, not a token amount.

In Alzheimer's disease, Gibson and colleagues, 2020, in the Journal of Alzheimer's Disease, gave benfotiamine 600 mg a day for twelve months. It missed its primary clinical endpoint and moved a brain imaging secondary. That justifies more study, not a clinical claim.

In septic shock, the randomized thiamine trial of Donnino and colleagues, 2016, in Critical Care Medicine, was negative overall, with a signal only in a pre-specified subgroup who were deficient on enrollment. Pre-specified matters: they said in advance they would look there, rather than hunting afterward.

Four diseases, four teams, one shape of answer. And the most circulated thiamine-for-fatigue material online, Costantini's work, is open-label case series, not controlled evidence.

Why would B1 affect energy at all?

This is mechanism, not clinical evidence. It explains why an effect is plausible, not that one exists.

Thiamine becomes thiamine diphosphate, the required cofactor for two enzymes at the center of how a cell burns glucose: pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase. Cofactor means the enzyme does not run without it. No partial credit. When thiamine diphosphate runs low, both slow, and so does the cell's ability to pull energy from glucose.

Tissues that run heavily on oxidative glucose metabolism and hold little reserve feel that first. Nerve tissue qualifies. So does gut tissue. That is why early thiamine trouble shows up as neurological and gastrointestinal, why B1 recurs in what vitamin B1 actually does in the gut and nervous system, and why fog after eating gets blamed on it more than the evidence warrants.

Body stores are commonly cited at 25 to 30 mg, depletable in two to three weeks. Treat that as a textbook figure, not a measured constant. The buffer is short.

Who is actually at risk of running short?

Not most people, and this part costs us money to write. Average US intake including supplements runs about 4.9 mg a day, roughly four times the recommended dietary allowance. Only about 6 percent of the population falls below the Estimated Average Requirement.

That requirement is the intake covering half of a healthy population. Ninety-four percent of Americans clear it. If you eat a varied diet, you are probably not short.

Where thiamine status genuinely deserves thought:

• Alcohol use disorder. Reduced intake, impaired absorption, and impaired storage at once.
• Bariatric surgery, and malabsorption generally.
• Persistent vomiting, including hyperemesis.
• Older adults, where intake and absorption both drift down.
• Diabetes and HIV.
• Long-term diuretic use, which raises urinary losses.

GLP-1 drugs come up constantly here, and the honest status is thin: case reports plus a narrative review by Urbina and colleagues, 2026, in Clinical Obesity. Plausible mechanism, no controlled prevalence study. To rule yourself out, learn the signs of a real thiamine shortfall.

What to do

1. See a clinician about fatigue lasting more than a few weeks. The differential is long: anemia, thyroid disease, sleep apnea, depression, medication effects, malignancy. Most of it matters more than your B1 status, and none of it is fixed by a supplement.
2. Decide honestly whether you are in a risk group. If none apply, adjust your expectations down before you spend anything.
3. If the question feels real, ask about measuring it. A clinician can tell you whether checking your thiamine status is worth doing.
4. Look at the pattern, not the day. Tiredness that lands at the same hour has different causes, and the afternoon crash has specific non-vitamin explanations worth ruling out.
5. Fix the inputs with more evidence than any vitamin. Sleep, alcohol, and meal timing move fatigue further than 75mg of anything.

Common questions

How much thiamine did the trial that worked actually use?

Oral thiamine at 600 to 1,800 mg a day, hundreds of times the recommended dietary allowance, in a supervised research protocol. No daily supplement gets near that, and none should claim to.

Does the form of B1 matter?

Somewhat, and less than marketing suggests. Allithiamine is genuinely lipid-soluble. Benfotiamine is not, despite how often it is sold that way, and absorbs by a different route. Both beat plain thiamine salts in humans. Between the two, the human comparisons split: Greb and Bitsch, 1998, in the International Journal of Clinical Pharmacology and Therapeutics ranked benfotiamine higher, while Park and colleagues, 2016, in Clinical Therapeutics found them bioequivalent. Neither makes one superior. More in allithiamine versus benfotiamine.

If I am not deficient, will more B1 give me energy?

Nothing in the controlled literature predicts that. The signals that exist cluster in people short at baseline, and the one clean replication attempt in a fatigued population failed.

Is high-dose thiamine dangerous?

The trial safety record is reassuring. Benfotiamine at 600 mg a day for twelve months was safe, and high-dose oral thiamine in inflammatory bowel disease was well tolerated with only mild side effects. Anaphylaxis is documented, but with injected thiamine, not oral.

The most useful thing in the thiamine literature is not the trial that worked. It is the one that did not.