The body DOES have several systems in place that can
slow metabolism somewhat - studies done on moderately hypocaloric individuals on extended diets show a moderate decrease in thyroid hormone production, changes in leptin / ghrelin levels (two hormonal mediators of food intake and metabolism), and slight changes in androgen / estrogen levels.
Angiotensin II receptor type 1 (Agtr1a): Lowering Agtr1a protein levels protects mitochondrial function and modestly extends life in mice, though as for many of these methods of
somewhat slowing aging there are probably many other changes to the operation of
metabolism that are as yet unexplored.