due to conservation of energy, energy lost during reactions between and into metabolites becomes body heat, which can be measured; also virtually all of it consumes inhaled oxygen, which can also be measured
Body heat can't be accurately measured. You can just measure heat loss via the surface. But there's variations on the cell level. Since heat can be stored inside the body for a longer period, measuring heat loss over a short period of time is not representative.
Why would that be? Eg Organs are warmer than hands etc - there certainly is a gradient. The heat from metabolic processes will not become obvious immediately because it’s redistributed before it’s lost to the environment.
Besides heat loss, there’s also kinetic energy (muscle contractions, peristalsis etc), various pumps (eg sodium/potassium), fluid flow (blood, lymph etc) and weird things like biophotons.