Humans, like most species, need a range of substances to survive, from vitamins (https://en.wikipedia.org/wiki/Vitamin) to proteins (which are made of amino-acides, which we needs to make our own proteins), via metals (like magnesium and zinc), fat and sugars.
It seems that chocolate does contain some iron, calcium, magnesium and potassium as well as vitamin E and B vitamins. But eating chocolate only wouldn’t cover all your body’s needs, – and your brain’s as a consequence.
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