What am I ?

I am one of the most vital substances in the human body
I am a key part of the structure for every cell in your body
I am vital for all hormone production and reproduction
I am vital for digestion – the body uses me to synthesise bile acids
So without me, you could not digest fat or absorb the vital fat soluble vitamins
I am vital for bones and all the roles performed by vitamin D
And the brain contains about 25% of the me – making me completely vital to mind and memory functions and our entire operation as a human being.