[Raphmedia]

Overcooking and oversteeping your tea releases extra unwanted tannin. This is what makes the bitter taste. This can be hard on an empty stomach and for people prone to nausea.

High presence of tannin lowers the absorption of iron:

https://www.cambridge.org/core/journals/british-journal-of-n...

https://academic.oup.com/ajcn/article/106/6/1413/4823172

It also increase the oxalates:

https://www.urologyofva.net/articles/category/healthy-living...

All of that being said, many people aim for teas high in tannin for the taste. There are also studies that will show the potential health benefits of tannin. As with many things, moderation is probably the answer.