Several elements, like mercury, thallium, and lead, seem to do nothing good for us and are positively detrimental if consumed excessively.* Others are also unnecessary but far more benign, of which the most notable is gold. That is why gold can be used as a filling for teeth: it doesn’t do you any harm. Of the rest, some twenty-two elements are known or thought to be of central importance to life, according to Essentials of Medical Geology. We are certain about sixteen of them; the other six we merely think are vital. Nutrition is a remarkably inexact science. Consider magnesium, which is
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