Isn't heat essentially a by-product of muscles "burning" the glucose? I'm not sure we actually need more heat besides what's already being generated as a by-product.
This is, after all, the distinguishing characteristic of mammals. Reptiles have no shortage of muscle mass (just look at aligators or crocs!) but still can’t raise their body temperature to compensate for cooler weather or environments.
This isn't a distinguishing characteristic of mammals though - all birds and a few fish can regulate their body temperature. The distinguishing feature of mammals is the mammary gland - the production of milk for young.
https://www.newscientist.com/article/mg21228411-700-killer-f...
Go out in frigid weather with a T-shirt on. You'll soon shiver - your body is recruiting muscles to generate heat. But also, your "brown fat" cells will start pumping out heat as well.