But when it comes to tannins, it’s not all good news. Tannins also act as anti-nutrients, meaning that they block and interfere with the absorption of certain nutrients. In this case, a big one is iron.
That’s why, if you’re iron deficient, your doctor may have advised that you either don’t drink tea or only drink it in between meals, rather than with an iron-rich meal. You can also reduce any potential negative effects by adding milk to your tea. The tannins bind with the milk protein, instead of the proteins in your gut, which prevents them from interfering with iron absorption. Eating vitamin C-rich foods, like bell peppers, potatoes, cantaloupe, and/or oranges right before or after you drink your tea can also neutralize the tannins.
According to a July 2014 report in the International Journal of Nutrition and Food Sciences, tannins can also block the digestion and absorption of proteins by either inhibiting the enzymes that you need to break them down or by making the protein biologically unavailable.
And since tannins interfere with the proper functioning of enzymes, it can mess with your digestion as a whole. On that note, Nour Zibdeh, M.S., RDN, a functional and integrative dietitian and nutritionist, also points out that if you have a sensitivity to tannins, they can constipate you. Overconsumption of tannins can also cause stomach upset and nausea.
However, researchers from a January 2017 study that was published in Current Developments in Nutrition wanted to make it clear that most studies that demonstrate a negative effect of tannins, especially when it comes to iron absorption, use amounts that you wouldn’t take in with average tea consumption.
For example, one 5-ounce cup of tea typically contains around 25 to 80 mg of tannins. That means, even if you drank three cups of tea per day, you would only be taking in 75 to 240 mg of tannins. Most studies exceeded this amount, some using up to 1,000 mg doses of tannins.