With over two million sweat glands on the feet, the average person sweats up to ¼ cup a day in their shoe. This sweat changes the ph level of the skin and creates an environment for bugs to grow. It is this bacteria that gives off an unpleasant odor.
The reason that antiperspirants are so effective in dealing with foot odor is that they provide a protein layer that effectively seals the sweat glands. The body has a self regulating system so if we stop sweating from the feet we simply sweat elsewhere.