I figure that metabolisms vary at least 10% as much as minds

That may be understating it. Have you come across the book "Biochemical Individuality"? First published in 1956, it catalogues the at times surprisingly large amount of variation among healthy individuals of even such gross physiology as the relative sizes and positioning of the organs. It appears to have been rediscovered in the 90s, but primarily by the less scientific end of the nutrition advice industry. I do not know if there has yet been any reliable work done on the implications of individual uniqueness for, well, just about the whole of medicine, nutrition, and the life and social sciences in general.