This question sort of comes to mind when hearing how efficient an internal combustion engine is turning chemical energy in mechanical energy (something like 20-40%) with lots of excess heat.  As an analog, how efficient is (or potentially) the human body at turning food into energy?  Please bare with me, I realise there LOTS of different variables (how much the person weighs vs mass, metabolism, diet, etc).  But I would imagine that there shouldn't be much margin of error given that most people maintain the same constant temperature (98 F +/- 1 degree).