Turn off weight gain?

i was doing a little bit of thinking... and so, considering before modern times, man needed to be able to gain fat to store for the winter, but on the flip side, other times he needed to be able to process food fast for energy for hunting and such. so technically wouldn't there be a sort of "weight gain/metabolize" bodily "switch"? or have we lost that adaptability a long time ago? (or i could be completely wrong on the whole idea) if you have any ideas, explanations, or corrections, please let me know XD