Certain cells of the body store surplus food in the form of fat or animal starch. This is released from storage, when there is not enough food in the blood to supply the energy demands of the body. Few of the things we eat can be used directly by the body cells. They must be changed chemically before they can supply the energy required. This called digestion and carried out in the stomach and in the small intestine, from where this digested food must be transported to the cells.
Now the food, in the form of digested sugars, proteins, and starches dissolves in water, passes into the blood system where, as blood, it can circulate through the body in less than one minute. Once the food is in the blood stream it is soon delivered to all the cells of the body by means of the red corpuscles in the blood.
It is when this supply falls low that the energy stores come into action.