Because tea leaves contain a high content of acid. This substance will cause the Protein content in the food we consume to be hardened thus difficult to digest however Japanese Green tea is known as a drink which has many benefits for your health.
The effect of tea on iron absorption was studied in human volunteers. Absorption from solutions of FeCl3 and FeSO4, bread, a meal of rice with potato and onion soup, and uncooked haemoglobin was inhibited whether ascorbic acid was present or not. No inhibition was noted if the haemoglobin was cooked.
The effect on the absorption of non-haem iron was ascribed to the
formation of insoluble iron tannate complexes. Drinking tannin-containing beverages such as tea with meals may contribute to the pathogenesis of iron deficiency if the diet consists largely of vegetable foodstuffs.
According to this study published in the American Journal of Clinical Nutrition (and referred to in this medical news article), consuming tea with meals has a significant effect on iron absorption.
This study shows that tea consumed simultaneously with an iron-containing porridge meal leads to decreased nonheme iron absorption and that a 1-h time interval between a meal and tea consumption attenuates the inhibitory effect, resulting in increased nonheme iron absorption. These findings are not only important in relation to the management of iron deficiency but should also inform dietary advice, especially that given to those at risk of deficiency.
However, the same study reports that if tea is consumed an hour or more after the meal, the inhibitory effect of tea on iron absorption is attenuated. It is therefore recommended to have at least an hour gap between meals and tea