A bit pedantic but burning is a chemical reaction, and redox reactions are redox reactions no matter where they're happening. Also, substances like ethanol and

A bit pedantic but burning is a chemical reaction, and redox reactions are redox reactions no matter where they're happening. Also, substances like ethanol and acetic acid are not digested, they pretty much just go straight into the bloodstream. There is a limit on how much e.g. ethanol the liver can process per hour, which does put a cap on how many of ethanol's calories are released as ATP + body heat. Acetic acid is a normal temporary metabolite though and so its consumption is much more complete (it gets used by cells all over).