Chemical breakdown of the biomolecules from food through metabolic reactions releases chemical energy, which is transformed into thermodynamic work. There are three main types of work within an organism: mechanical, chemical and osmotic. A cell performs osmotic work by separating solutes across a membrane, creating differences of concentration between two compartments. As many of these solutes are charged, a difference of electric potential is built between both sides of the membrane (the membrane potential, which is some kind of potential energy measured in volts), and while ions move through membranes, there are electric currents in living things.

For example, a single neuron may have a membrane potential near to 100 mV, being negative inside in rest conditions. This value of electric potential is raised mainly from differences of concentration of K+ and Na+ inside and outisde of the plasma membrane, established by the exchange of these cations by a special type of enzyme (the Na+/K+ pump). However, when the neuron is activated by specific stimuli the potential changes by the movement of ions through channels and local electrical currents are recorded. The membrane potential level is very dynamic and sensitive to external and internal conditions of the body.

Not only neurons have electrical activity, but heart and muscle are electrically very active also. In fact every cell, human or not, has a membrane potential while being alive and it may change as a response of external conditions, among them, having food or not, and don't forget that foods like water and meat, are plenty of salts of Na+, K+, Cl-, etc.

There is no single step of transformation of heat into any other kind of energy in known living beings and the mere process of “food burnig” by metabolism is not followed by electrical variations in appreciable level.

Heat release is a by-product of the metabolic transformations inside our bodies, and heat transfer through blood vessels is the cause of the regular level of body temperature.

Finally, to have a battery from a body or tissue, you have to make sure that the differences of concentration of ions are maintained a time long enough to be useful. A nice example is plugging two wires of different metals to a potato, this will give you a modest electric potential that will be boosted if you bath the potato into an electrolyte solution with high conductivity.