IMHO, it is all about lowering oxidative stress. Eating creates oxidative stress in the body because to eat means to create ATP and that means electrons are increased in the mitochondria. More electrons = more oxidative stress in the form of superoxides.
I will say that Calorie Restriction with Optimal Nutrition (or CRON) is the healthiest diet but should be geared towards ones heritage or genetics (which is why I think all of the studies on humans have such variable and confounding outcomes.).