Vitamin B1 (thiamine) deficiency happens when your body doesn’t get enough thiamine, can’t absorb it properly, loses too much of it, or burns through it faster than normal. Chronic alcohol use is the most common cause in developed countries, but surgery, certain medications, pregnancy, and even diabetes can quietly drain your thiamine levels. Because the body stores only a small reserve, deficiency can develop in as little as 18 to 20 days when intake drops.
How Your Body Uses Thiamine
Thiamine plays a central role in converting carbohydrates into energy. Every time your cells break down glucose, they rely on the active form of thiamine to keep that process running. Without it, your brain and heart, two of the most energy-hungry organs, are the first to suffer. Adults need about 1.1 to 1.2 mg per day, and pregnant or breastfeeding women need 1.4 mg. Those numbers are easy to hit with a varied diet, but several common situations can tip the balance.
Chronic Alcohol Use
Heavy, long-term drinking is the single biggest driver of thiamine deficiency in Western countries, and it attacks from multiple angles at once. Alcohol directly damages the cells lining the small intestine, reducing the number of transport proteins responsible for pulling thiamine from food into your bloodstream. In animal studies, chronic alcohol exposure significantly reduced levels of the main thiamine transporter in the gut wall, and human intestinal cells exposed to alcohol showed the same pattern: fewer transporters, less thiamine getting through.
On top of that, people who drink heavily tend to eat poorly, so less thiamine arrives in the gut to begin with. Alcohol also increases urinary losses. The combination of low intake, poor absorption, and increased excretion makes chronic alcohol use uniquely effective at depleting thiamine stores. Left untreated, this can progress to Wernicke-Korsakoff syndrome, a serious brain condition involving confusion, vision problems, memory loss, and difficulty walking.
Bariatric and Gastrointestinal Surgery
Thiamine is absorbed primarily in the upper portion of the small intestine. Gastric bypass and other bariatric procedures reroute or reduce the section of gut where this absorption happens, creating a long-term risk. In a review of published cases, 94% of patients who developed Wernicke’s encephalopathy after gastric bypass were hospitalized within six months of their surgery, and 90% of them had been experiencing frequent, recurrent vomiting lasting a median of 21 days. Persistent vomiting compounds the problem by flushing out whatever thiamine the body does manage to take in.
This risk isn’t limited to the early post-operative window. Anyone with a shortened or altered digestive tract, whether from weight-loss surgery or other abdominal procedures, needs ongoing monitoring of their thiamine status.
Diets High in Refined Grains
In parts of Asia and other regions where polished white rice is a dietary staple, thiamine deficiency remains a significant public health issue. Milling strips away the outer layers of the grain, which is where most of the thiamine lives. A diet built around refined grains without enough meat, legumes, or fortified foods to compensate can leave you chronically short.
Certain foods also contain enzymes called thiaminases that actively break down thiamine in the gut. These enzymes are found in some raw freshwater fish, shellfish, and certain plants. Cooking destroys thiaminases, so the risk is specific to diets that include large amounts of raw fish or fermented preparations.
Diabetes and Kidney-Related Losses
One of the less obvious causes of thiamine deficiency is diabetes. A study published in Diabetologia found that plasma thiamine levels were about 75% lower in both type 1 and type 2 diabetic patients compared to healthy volunteers. The reason: their kidneys were dumping thiamine into the urine at dramatically higher rates. Thiamine clearance through the kidneys was 24 times higher in type 1 patients and 16 times higher in type 2 patients. So even with a normal diet, people with diabetes can lose thiamine faster than they replace it.
Dialysis poses a similar problem. Each session filters water-soluble vitamins out of the blood, and thiamine is water-soluble. Diuretic medications used for heart failure or high blood pressure also increase urinary thiamine losses over time.
Pregnancy and Severe Vomiting
Pregnancy raises thiamine requirements because the developing baby draws on the mother’s supply. The recommended daily amount increases from 1.1 mg to 1.4 mg during pregnancy and breastfeeding. For most women, this modest increase is easily covered by diet. But hyperemesis gravidarum, the severe form of morning sickness involving persistent vomiting, can make it impossible to keep food down long enough to absorb nutrients. Women hospitalized for hyperemesis are at real risk of thiamine depletion, especially if they receive intravenous glucose (which burns through thiamine during metabolism) without supplemental thiamine alongside it.
Refeeding After Starvation or Malnutrition
When someone who has been severely malnourished starts eating again, their body suddenly shifts back to burning carbohydrates for fuel. That metabolic restart requires a surge of thiamine. If the person’s stores are already depleted from weeks or months of poor nutrition, refeeding can trigger a dangerous drop in thiamine levels. This is called refeeding syndrome, and it’s a recognized risk in hospitals, eating disorder treatment programs, and situations involving prolonged fasting.
Other Conditions That Increase Demand
Hyperthyroidism speeds up your overall metabolism, which means your body chews through thiamine faster than usual. Chronic diarrhea from any cause, whether inflammatory bowel disease, infections, or medication side effects, reduces the time food spends in the part of the gut where thiamine is absorbed. Malabsorption syndromes like celiac disease can impair nutrient uptake across the board, including thiamine.
Parenteral nutrition, the IV feeding used for patients who can’t eat by mouth, is another known risk factor. If the IV formula isn’t supplemented with thiamine, patients can become deficient quickly given how small the body’s reserves are.
Early Signs to Recognize
Mild thiamine deficiency often starts with vague symptoms: fatigue, irritability, poor concentration, and muscle weakness. As the deficiency worsens, it can split into two patterns. “Dry” beriberi primarily affects the nervous system, causing numbness and tingling in the hands and feet, difficulty walking, and muscle wasting. “Wet” beriberi targets the cardiovascular system, leading to a rapid heart rate, swelling in the legs, and shortness of breath from fluid buildup.
The most dangerous complication is Wernicke’s encephalopathy, which involves sudden confusion, abnormal eye movements, and an unsteady gait. Without prompt treatment, it can progress to Korsakoff syndrome, a form of permanent memory damage. Because thiamine stores can be depleted in under three weeks, symptoms can appear faster than many people expect.