In my physiology (biological science) class it was explained that, upon eating some food source, some energy is converted into a useful form for the body, and some energy is lost in the form of heat. The lecturer didn't explain what form this heat is in. I thought that 'heat' must be an EM wave, however I read that photons don't play a significant role in conduction. It seems heat must take some form or another. I have trouble accepting a kind of 'free energy' can exist in no real state. If there are only 4 fundamental forces, mustn't this 'heat' in question take the form of, or be explained by, one of them? I don't think nuclear forces, nor gravitation, is responsible for much in the digestion of food in the human body. Can someone please explain what form of energy 'heat' may take?