We take in calcium as +2 ions, and use it in the same state when making calcium phosphate for bones. That helps because calcium, like most metals, takes a lot of energy to reduce to its metallic form. Titanium and iron are also naturally found as ions. Getting bulk quantities of the metallic form would be a waste from an evolutionary standpoint, when it could just make a strong mineral-based composite material like bone.

> There's even a lot of titanium - fully 0.6%, much higher than 'rare' elements like carbon! [0]

Almost all our carbon goes back to the air.

This leads to the interesting concept that when you are losing weight, you are actually exhaling most of your weight loss.

Edited: Downcommenter is correct. Sorry, glitched. I was thinking about various things and pulled plants into the mix when the article is only about humans.