It has long been known that the body typically maintains a very careful balance of metals. But various metals have interactions with, and “enemies” of other metals, which they prefer to bind with as a molecule no longer useable by the body.
Copper, for example, is essential to convert the body’s iron into hemoglobin. It is also essential for the metabolism of Vitamin C. In addition to foods, it is also found in cigarettes and birth control pills.
Excess copper is quite toxic, but removing the excess from the body is tricky, and can cause many side effects:
http://www.drlwilson.com/articles/copper%20elimination.htm
And it is pretty typical for metals, some of which, like sodium, calcium, and potassium, can be very dangerous to imbalance.
I’d think that curcumin is safe enough given its common usage. But...maybe it does serve to delete such as needed iron. I don’t know.