Not true, unlike many other mammals humans can survive a long time without food largely because the liver regulates metabolism in response to calorie intake. The thyroid hormone T3 is the main regulator aka “gas pedal” of cellular metabolism, and conversion of inactive T4 into T3 is done mostly by the liver. Conversion is halted when fasting or severely restricting calories… much to the frustration of dieters. This is why mainstream advice is to not lose weight too fast.
Indeed, and there are a few other mammals for which there are mechanisms to reduce ATP generation, reduce VO2, etc., and some which do it much better than humans do. Emperor Penguins, for example.