Key points
- Coffee polyphenols bind non-heme iron — 40-60% reduction within an hour.
- It cuts levothyroxine absorption by up to 30%.
- Milk in coffee adds a separate calcium effect on iron and thyroid medication.
Iron: the biggest effect
Chlorogenic acid and related polyphenols in coffee form insoluble complexes with non-heme iron in the intestinal lumen. A cup of coffee with an iron supplement can reduce absorption by 40-60% compared with water.
The effect is dose- and timing-dependent. Coffee drunk an hour after iron has almost no impact; coffee drunk with iron or immediately after does. Decaffeinated coffee shows the same effect, which confirms the polyphenols rather than caffeine are responsible.
Levothyroxine: a 30% penalty
Coffee reduces levothyroxine absorption when consumed within an hour of the dose. Studies measuring TSH found that patients who took levothyroxine with coffee needed higher doses to reach the same endpoint.
If you drink coffee in the morning, take levothyroxine first with plain water and start the coffee at least an hour later. Milk or cream adds a calcium effect on top of the coffee effect.
Zinc and probiotics
Coffee polyphenols bind zinc and modestly reduce its absorption, which matters most when you are correcting a deficiency. Separate by an hour.
Hot coffee and probiotics are a bad combination for a different reason: heat reduces viable organism counts, and caffeine accelerates transit, reducing the time the organisms have to colonise. Take probiotics with cool water on an empty stomach instead.