What happens when you take black tea with thiamine?
Black tea is one of the dietary sources of antithiamine factors (ATFs) - naturally occurring compounds that chemically inactivate vitamin B1. The relevant ATFs in tea are polyphenols, particularly tannins (theaflavins, thearubigins) and chlorogenic acid. These molecules have ortho-positioned hydroxyl groups that can oxidise thiamine into thiochrome-negative, biologically inactive products. The reaction happens in the gastrointestinal lumen when thiamine and tea polyphenols are present at the same time, before the vitamin has been absorbed.
This is a different mechanism from thiaminases, which are enzymes (found in raw fish, ferns, and some shellfish) that cleave the thiamine molecule itself. Tea polyphenols are non-enzymatic ATFs that oxidatively destroy thiamine. The destruction is partially blocked by ascorbic acid (vitamin C), which acts as a competing reducing agent and protects thiamine. Practical studies in regions of Thailand where strong tea drinking and chewing of fermented tea leaves are widespread have linked the habit to lower thiamine status and a higher prevalence of subclinical beriberi-like symptoms, even when reported dietary thiamine intake looks adequate on paper.
Why is this important?
Thiamine is essential for carbohydrate metabolism, nerve conduction, and cardiac function. Deficiency causes beriberi (wet beriberi: high-output heart failure; dry beriberi: peripheral neuropathy) and, in alcohol-related deficiency, Wernicke-Korsakoff syndrome. In high-income countries frank thiamine deficiency is uncommon, but subclinical low B1 status is more frequent than people realise, especially in older adults, heavy alcohol drinkers, people who eat highly refined carbohydrate diets, patients on long-term loop diuretics (furosemide), and people who have had bariatric surgery.
For most healthy adults with a balanced diet, drinking several cups of black tea a day is unlikely to push them into deficiency. The concern is mainly for two groups. First, populations with marginal thiamine intake who drink large volumes of strong tea (or chew tea leaves) can be tipped into clinically relevant deficiency. Second, people taking a thiamine supplement specifically to treat a deficiency state (alcohol-use-disorder rehabilitation, refeeding syndrome, post-bariatric, peripheral neuropathy) want every milligram of B1 to count - inactivating part of the dose with tea is counter-productive.
Compared with the much stronger tea-iron and tea-folate interactions, the tea-thiamine effect is relatively modest, which is why the severity is rated low. It is real, though, and worth knowing about for clinicians managing nutritional deficiencies.
What should you do?
If you take a daily B-complex or standalone thiamine supplement, swallow it with water (or with orange juice) and leave a gap of at least 1-2 hours before drinking a strong cup of black tea. Many people simply take their morning multivitamin with breakfast and have tea later in the morning, which works well.
Adding a source of vitamin C protects thiamine from the antithiamine reaction. A 100-200 mg vitamin C tablet or a glass of orange juice taken with a thiamine supplement effectively neutralises the polyphenol effect even if tea is consumed soon afterwards. Eating fresh fruit and vegetables alongside meals does the same job naturally.
If you are being treated for an active thiamine deficiency (Wernicke-Korsakoff, refeeding syndrome, severe alcohol-use-disorder), your medical team will usually use intravenous or intramuscular thiamine, which completely bypasses the gut and is unaffected by tea. Oral supplementation during the maintenance phase is best timed away from tea, coffee, and betel chewing.
Which specific products are affected?
The interaction applies to oral thiamine supplements (thiamine hydrochloride, thiamine mononitrate, benfotiamine - a fat-soluble derivative used for diabetic neuropathy, and sulbutiamine), B-complex multivitamins, and prenatal vitamins containing B1. Dietary thiamine from whole grains, legumes, pork, and fortified cereals is also susceptible to the antithiamine effect of strong tea consumed at the same meal.
On the tea side, fully fermented black teas have the highest concentration of oxidised polyphenols and the strongest antithiamine effect. Pu-erh, fermented tea leaves chewed as a habit (miang in Thailand), and very long-brewed strong black teas are most problematic. Green and white teas contain ATFs too but at lower levels per cup. Coffee and betel nut also contain ATFs and add to the cumulative effect for people who consume both.
The bottom line
Black tea contains antithiamine factors that can oxidise and inactivate vitamin B1 in the gut. For most well-nourished adults the effect is minor, but for heavy tea drinkers, populations with marginal diets, and people being treated for thiamine deficiency, it matters. Take thiamine and B-complex supplements with water and vitamin C, separate from tea by 1-2 hours, and use parenteral thiamine for active deficiency states. Routine moderate black tea consumption alongside a balanced diet is not a problem.