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Because it can severely reduce iron absorption, especially if you have iron-deficiency anemia.
Multiple studies have found that coffee and other caffeinated beverages reduce iron absorption.
One study found that drinking coffee while eating a burger blocked iron absorption by 39 percent. And drinking tea while eating a burger blocked 64 percent of iron absorption. The stronger the coffee or tea, the less iron is absorbed. [1]
Another study found that drinking a cup of instant coffee alongside bread hinders iron absorption by 60-90%. [2]
What hinders iron absorption?
First up is Caffeine.
One study found that caffeine itself binds to about 6 percent of the iron in a meal, preventing it from being absorbed. [3]
Caffeine doesn’t seem to have that much of an effect on its own, but it really isn’t the only factor that hinders iron absorption in this type of beverage.
The chlorogenic acid and tannins in coffee and tea can also hinder iron absorption by combining with iron to form compounds.
In one study, drinking a drink containing 20-50 mg of chlorogenic acid reduced iron absorption from a meal by 50-70%, while increasing the dose to 100-400 mg, which reduces iron absorption by 60-90%. [4]
Another study found that consuming 5 mg of tannins inhibited iron absorption by 20%, while 25 mg reduced iron absorption by 67 percent, and 100 mg reduced iron absorption by 88 percent. [5]
What?
Beverages such as coffee or tea will only block some iron absorption when you consume iron, but will not “squeeze” out the iron that has been stored in your body, so as long as
strong>Don’t drink while eating.
For example, studies have found that drinking coffee one hour before meals has no effect on iron absorption. [6]
You can also drink it between meals, or wait at least an hour after a meal before drinking coffee or tea.
Thinking about it this way, it makes sense that Daiyu’s father taught her to drink tea after a meal.
References:
[1] Morck TA, Lynch SR, Cook JD. Inhibition of food iron absorption by coffee. Am J Clin Nutr. 1983 Mar;37(3):416-20. doi: 10.1093 /ajcn/37.3.416. PMID: 6402915.
[2] Hurrell RF, Reddy M, Cook JD. Inhibition of non-haem iron absorption in man by polyphenolic-containing beverages. Br J Nutr. 1999 Apr;81(4):289 -95. PMID: 10999016.
[3] evgi Kolayl , Mira Ocak, Murat Kü ük, R za Abbaso lu,Does caffeine bind to metal ions?,Food Chemistry,Volume 84, Issue 3,2004,Pages 383-388,ISSN 0308-8146
[4] Hurrell RF, Reddy M, Cook JD. Inhibition of non-haem iron absorption in man by polyphenolic-containing beverages. Br J Nutr. 1999 Apr;81(4):289-95. PMID: 10999016.
[5] Brune M, Rossander L, Hallberg L. Iron absorption and phenolic compounds: importance of different phenolic structures. Eur J Clin Nutr. 1989 Aug;43(8):547-57. PMID: 2598894.
[6] Matsui Y, Tanaka Y, Iwahashi H. A comparative study of the inhibitory effects by caffeic acid, catechins and their related compounds on the generation of radicals in the reaction mixture of linoleic acid with iron ions. J Clin Biochem Nutr. 2017 May;60(3):162-168. doi: 10.3164/jcbn.16-54. Epub 2017 Mar 25. PMID: 28584397; PMCID: PMC5453020.