Vitamin B1
Vitamin B1 Guide
Vitamin B1, also known as thiamine, is a water-soluble vitamin your body cannot make on its own — every milligram has to come from food, fortification, or supplements. It‘s an essential cofactor for turning carbohydrates into usable energy, and it keeps nerve and heart tissue functioning normally. Because your body can only store a small reserve of thiamine at a time, and because genetics can meaningfully change how efficiently you absorb and use it, understanding your personal thiamine picture is more useful than assuming a standard multivitamin dose covers it.
Impact of Gene Variant
Thiamine doesn‘t cross cell membranes on its own — it needs dedicated transporter proteins to get from your gut into your bloodstream and then into individual cells. Two genes do most of that work:
- SLC19A2 encodes thiamine transporter 1 (THTR1), which helps move thiamine across the intestinal wall and recover it in the kidneys.
- SLC19A3 encodes thiamine transporter 2 (THTR2), which handles cellular uptake more broadly and also transports vitamin B6 (pyridoxine).
Rare, severe mutations in these genes cause serious inherited conditions — thiamine-responsive megaloblastic anemia (from SLC19A2 mutations) and biotin-thiamine-responsive basal ganglia disease (from SLC19A3 mutations). Those are uncommon. What‘s more relevant for most people is the growing body of research on common variants in these same genes, which appear to have a smaller but still measurable effect on how efficiently thiamine is transported — particularly under metabolic stress. A 2025 study on SLC19A3 polymorphisms found that certain variants changed how cells responded to thiamine depletion and to metformin, the first-line diabetes medication, suggesting that people with these variants may have different thiamine needs depending on their metabolic health and medications.
That metformin connection matters beyond this one study. Metformin is known to inhibit thiamine transporters, and people on long-term metformin therapy are already recognized as being at higher risk of mild thiamine insufficiency — genetic variation in transporter efficiency may compound that risk for some individuals. Other medications, including certain antidepressants and chemotherapy drugs, have also been shown to interact with SLC19A3.
Ways to Recognize Problems
Deficiency (low thiamine) symptoms tend to be vague at first, which is part of why thiamine deficiency is under-recognized even though it‘s easy to treat once identified:
- Fatigue and low energy — one of the earliest signs, since thiamine is central to converting food into ATP, the energy currency of your cells.
- Loss of appetite and unintentional weight loss — thiamine appears to play a role in appetite regulation, so a drop in intake can show up before other symptoms.
- Neurological symptoms — tingling or numbness in the hands and feet (peripheral neuropathy), poor coordination, irritability, short-term memory issues, and confusion.
- Cardiovascular symptoms — an unusually fast heart rate, an enlarged heart, or swelling in the legs.
Left untreated, severe and prolonged deficiency progresses to beriberi, which comes in two classic forms: "wet" beriberi (heart failure, swelling, shortness of breath) and "dry" beriberi (nerve damage, muscle wasting, difficulty walking). A related condition, Wernicke-Korsakoff syndrome, is a thiamine-deficiency emergency most often seen with chronic heavy alcohol use, since alcohol both reduces thiamine absorption and increases how much the body needs.
Deficiency is uncommon in people with regular access to a varied diet, but risk climbs with: chronic alcohol use, bariatric surgery or any condition that impairs absorption (Crohn‘s disease, ulcerative colitis, persistent diarrhea), long-term metformin use, dialysis, pregnancy and breastfeeding (higher requirement), and diets built heavily around polished white rice or highly refined carbohydrates.
Excess: Because thiamine is water-soluble, your body simply excretes what it doesn‘t use, so there is no established toxicity from high intake through food or standard supplementation — this is one of the few nutrients where "how much is too much" isn‘t really a practical concern for most people.
Food Sources
Thiamine is found across a fairly wide range of everyday foods, which is one reason true deficiency is rare in people eating a varied diet:
- Whole grains and fortified cereals or flour
- Lean pork, fish, and eggs
- Legumes — beans, lentils, green peas
- Nuts and seeds (sunflower seeds, flax, sesame, chia)
- Nutritional yeast
- Vegetables like asparagus, acorn squash, and green peas
Because milling removes the outer layer of grain where thiamine is concentrated, switching from whole grains to heavily refined white rice or white flour without fortification is a known way to inadvertently lower intake — this is the historical cause of endemic beriberi in populations that shifted to polished rice as a dietary staple.
Improving Your Levels
- Build meals around whole grains rather than refined ones — brown rice, whole wheat, and fortified cereals meaningfully outperform their refined counterparts.
- Pair thiamine-rich foods with regular meals rather than relying on occasional high doses, since your body can‘t store much of it at once.
- Flag it with your doctor if you‘re on long-term metformin, have a diagnosed absorption condition, or drink alcohol heavily — these are the situations where dietary intake alone may not be enough and supplementation is often specifically recommended.
- Limit excessive raw fish and tea/coffee intake as a sole habit — certain compounds in raw fish and tannins in tea can break down thiamine, though this is a minor factor next to overall diet quality.
Interesting Facts
- Thiamine may have been the first vitamin ever identified, with deficiency symptoms described in Chinese medical texts as far back as 2600 BCE.
- The name "beriberi" is thought to come from a Sinhalese phrase meaning "I cannot, I cannot" — a reference to the profound weakness the condition causes.
- Thiamine‘s central role in energy metabolism means demand rises with anything that increases metabolic rate — fever, pregnancy, intense physical training, and hyperthyroidism can all modestly raise requirements.
What Recent Research Is Adding
Most of the newest research isn‘t challenging the basics of thiamine biology — it‘s refining who is most likely to be under-supplied. Structural biology work published in Nature Communications mapped exactly how SLC19A3 recognizes thiamine versus drugs like metformin at the molecular level, helping explain why certain medications interfere with thiamine transport more than others. Separately, clinical researchers have been pushing to raise awareness that thiamine deficiency is more common — and more treatable — than it‘s typically given credit for, particularly in hospitalized and older adult populations where it‘s easy to miss.
For someone without a diagnosed absorption issue or one of the recognized risk factors above, thiamine deficiency remains genuinely uncommon. Genetic variation in transporter efficiency is best understood as one factor that can shift someone from "adequate on a typical diet" to "worth paying closer attention to," rather than something that on its own causes a deficiency.
References:
- National Institutes of Health, Office of Dietary Supplements. Thiamin — Health Professional Fact Sheet. Available at: ods.od.nih.gov
- Cleveland Clinic. Thiamine (Vitamin B1) Deficiency. Available at: my.clevelandclinic.org
- Merck Manual, Consumer Version. Thiamine Deficiency. Available at: merckmanuals.com
- Genetic Variants of the Human Thiamine Transporter (SLC19A3, THTR2) — Potential Relevance in Metabolic Diseases. PMC, 2025. Available at: pmc.ncbi.nlm.nih.gov
- Structural basis of thiamine transport and drug recognition by SLC19A3. Nature Communications, 2024. Available at: nature.com
- Schostak T, San Millan I, Jani A, Johnson RJ. Thiamine deficiency: a commonly unrecognised but easily treatable condition. Postgraduate Medical Journal, 2023. Available at: pubmed.ncbi.nlm.nih.gov
