Weight / Fat loss

No, but on the wiki page that eyebrows just linked it says that tannins can inhibit iron absorption

Nutritional deficiency. This can result due to a lack of dietary iron or consumption of foods that inhibit iron absorption, including calcium, phytates and tannins. Black tea steeped for long has high tannins
a deficiency of which can cause mouth ulcers (although I'm not sure if that's entirely true since I read it on a random place on the internet)