This prospect could also go hand in hand with immune suppression vs immune stimulation.
I also believe domination of the wrong microbes (which could be predicated by altered scfa production) could drive this in the wrong direction as well.
Do microbiotas warm their hosts?
Consistent with this concept are the reports that treatment of rabbits10 and rodents11 with antibiotics lowered their body temperature. A similar decrease in body temperature was found in germ-free mice and miniature piglets in comparison to conventionally raised animals.12 Heat output by gut microbiota may also help explain the observation that germ-free mice had 40% less total body fat than conventionally raised mice, even if their caloric intake was 29% higher than that of conventionally raised animals.13