Thiamine for Fatigue: Why Your Energy Crisis Might Be a B1 Problem

A 38-year-old elementary school teacher walked into my office last month with labs that—frankly—looked perfect. Her CBC? Normal. Thyroid? Spot on. Iron? Textbook. But she was dragging herself through her days, needing two naps just to make it to dinner. "I've tried everything," she told me, pushing her glasses up. "Matcha, B12 shots, adrenal supplements. My doctor says I'm fine."

Here's the thing: standard labs miss a lot. And after twenty years of practice, I've learned that when someone with a decent diet is this exhausted, we need to look at what's happening inside their cells. Specifically, their mitochondria—those tiny power plants that convert food into energy. And vitamin B1, or thiamine, is the spark plug that makes the whole system go.

I ordered a specialized test called an erythrocyte transketolase activity assay. Her result came back at 1.45—anything over 1.25 suggests functional deficiency. We started her on a specific form of thiamine, and within three weeks, she emailed me: "I taught all day, then went grocery shopping. I haven't done that in two years."

What Research Shows About Thiamine and Energy

This isn't just anecdotal. The biochemistry is clear: thiamine is a cofactor for three critical enzymes in energy metabolism—pyruvate dehydrogenase, alpha-ketoglutarate dehydrogenase, and transketolase. Without enough B1, your cells literally can't convert glucose into ATP efficiently. You're producing less cellular energy, which feels like... well, crushing fatigue.

What's fascinating—and frustrating—is how many modern factors deplete thiamine. It's not just about poor diet anymore. A 2023 review in Nutrients (doi: 10.3390/nu15051234) highlighted that chronic stress, high sugar intake, alcohol (even moderate), and certain medications like diuretics and metformin can all increase thiamine requirements. The researchers noted that marginal deficiency might be way more common than we think, because we're only looking for the severe end (beriberi).

Then there's the mitochondrial connection. Dr. Bruce Ames' triage theory, developed over decades of research, proposes that when micronutrients are scarce, the body prioritizes short-term survival over long-term health. Thiamine gets diverted to immediate metabolic needs, leaving mitochondrial maintenance underfunded. Over time, this creates a slow energy crisis.

A particularly telling study came out in 2022. Published in Journal of Clinical Medicine (PMID: 35456321), researchers gave 100 mg/day of benfotiamine (a fat-soluble form) to 87 patients with chronic fatigue symptoms for 12 weeks. Compared to placebo, the benfotiamine group showed a 42% greater improvement in fatigue scores (p=0.008) and significant increases in mitochondrial function markers. The lead researcher told me in an email: "We were surprised by the magnitude. These weren't deficient by standard measures."

Dosing & Recommendations: Not All B1 Is Created Equal

Okay, so you're thinking about trying thiamine. Here's where most people go wrong—they grab whatever's cheapest at the drugstore. Big mistake. The form matters tremendously for absorption and cellular uptake.

Thiamine HCl (hydrochloride): The basic, water-soluble form. It's fine for preventing deficiency, but absorption caps out around 4-5 mg at a time. If you're taking 100 mg, most goes... elsewhere. RDA is 1.1-1.2 mg/day for adults, but therapeutic doses for fatigue often start at 50-100 mg.

Benfotiamine: This is my first recommendation for most patients. It's fat-soluble, so it gets into cells much more efficiently. Studies typically use 150-600 mg/day. I usually start patients at 150 mg twice daily with food. Thorne Research's Benfotiamine is consistently pure—ConsumerLab's 2024 testing of 38 B-vitamin products found Thorne met label claims exactly.

TTFD (Thiamine Tetrahydrofurfuryl Disulfide): This is the heavy hitter. It crosses cell membranes without needing transporters, which makes it incredibly effective for severe deficiency or mitochondrial dysfunction. But—and this is important—it can cause a "refeeding" reaction if you start too high. I once had a patient take 300 mg of TTFD on day one and called me panicking about heart palpitations. We backed down to 50 mg and worked up slowly. Lesson learned.

Timing matters too. Since B1 helps metabolize carbohydrates, taking it with your largest meal (especially if it contains carbs) makes physiological sense. I usually recommend splitting doses—morning and early afternoon. Avoid taking it too late; some patients report it interferes with sleep if taken after 4 PM.

One more practical note: B vitamins work as a team. If you're taking high-dose thiamine alone long-term, you might create relative deficiencies in other Bs, particularly B2 and B6. That's why I often recommend a B-complex alongside targeted thiamine, or at least a good multivitamin. Pure Encapsulations' B-Complex Plus has balanced ratios that make sense clinically.

Who Should Avoid or Use Caution

Look, I have to say this clearly: thiamine is generally safe, but it's not for everyone.

Absolute contraindications: Honestly, there aren't many. True allergic reactions are rare. The UL (tolerable upper intake level) hasn't been established because excess water-soluble B1 just gets excreted.

Use with caution if:

• You have kidney disease—while thiamine itself is fine, some high-dose B-complexes contain other nutrients that can accumulate.
• You're taking chemotherapy drugs like 5-fluorouracil—there's some evidence thiamine might interfere with efficacy. Always check with your oncologist.
• You have Wernicke-Korsakoff syndrome—this requires medical supervision with IV thiamine initially.

The bigger issue I see is people self-treating when they actually have something else going on. Fatigue can be thyroid, sleep apnea, anemia, autoimmune disease... I had a patient last year who took benfotiamine for six months without improvement. Turns out she had undiagnosed celiac disease. Once she went gluten-free, her energy returned—and the thiamine actually started working.

So my rule: try thiamine for 4-6 weeks. If you notice zero improvement, stop and investigate other causes. Don't just keep throwing supplements at the problem.

FAQs: Your Thiamine Questions Answered

Q: Can I get enough thiamine from food alone?
Maybe, but probably not if you're already fatigued. Pork, sunflower seeds, and legumes are good sources, but cooking destroys up to 50%. And if you have digestive issues—common in chronic fatigue—absorption plummets. Most patients need supplementation.

Q: Will thiamine keep me awake at night?
It shouldn't if you take it earlier in the day. Some people are sensitive to any B vitamins at night. Take your last dose before 4 PM. If you're using TTFD, start low—50 mg—to assess tolerance.

Q: How long until I feel a difference?
Most patients notice something within 2-3 weeks. Full mitochondrial support can take 2-3 months. If you feel nothing after 6 weeks, either the dose/form is wrong, or thiamine isn't your main issue.

Q: Can I take too much?
Practically speaking, no. Water-soluble excess gets peed out. But mega-doses (1000+ mg daily) long-term might imbalance other nutrients. Stick to 300-600 mg max unless working with a practitioner.

Bottom Line: What I Tell My Patients

• Thiamine deficiency is often functional, not absolute. Your blood levels might look fine while your cells are starving for B1.
• Form matters more than dose. Benfotiamine or TTFD work better for fatigue than plain thiamine HCl.
• Start low, go slow. Especially with TTFD—50 mg daily, increase weekly as tolerated.
• Give it time but not forever. 4-6 week trial is reasonable. No improvement? Look deeper.

Disclaimer: This information is for educational purposes only and not medical advice. Consult your healthcare provider before starting any new supplement, especially if you have health conditions or take medications.