Diet and exercise/lifestyle go a long way, but individual variation is still large -- perfect example: my wife and i have identical diets and eating habits, and

Diet and exercise/lifestyle go a long way, but individual variation is still large -- perfect example: my wife and i have identical diets and eating habits, and my vitals are all "normal" -- yet she is b12 and vitamin D deficient, because her lower intestine doesn't absorb those into her body as effectively as mine does.