This 2015 Portuguese rodent study showed the underlying mechanism to explain why restricting calories delays aging.
A calorie reduction of 20 to 40% increased production of a normally occurring molecule (neuropeptide Y) in the hypothalamus part of the limbic system. The increased amounts of the molecule stimulated autophagy (the breakdown and recycling of cellular components) in hypothalamic neurons.
“Because both hypothalamic autophagy and neuropeptide Y levels decrease with age..modulation of hypothalamic neuropeptide Y levels may be considered a potential strategy to produce protective effects against hypothalamic impairments associated with age and to delay aging.”
http://www.pnas.org/content/112/13/E1642.full “Neuropeptide Y stimulates autophagy in hypothalamic neurons”