Salt

Diet, evolution and aging--the pathophysiologic effects of the post-agricultural inversion of the potassium-to-sodium and base-to-chloride ratios in the human diet.

Pros: 
Some people (like me) have hypotension (seems to have a genetic factor), healthy kidneys (of person who eats the right diet, is not vitamin D deficient, exercises) should be able to deal with it
Cons: 
Causes hypertension in some people = strokes and heart attacks, most diets net acid producing and do not have the sort of potassium to sodium ratio our ancestors did, would have been rare in ancestral diets, linked to stomach irritation