1 teaspoon to 1 Tablespoon of nutritional yeast mixed into cat food along with diatomaceous earth (food grade) small amounts sprinkled and rubbed into the cat coat, cloth furniture and carpeting, and along the bottom of walls. This along with combing the cat is a natural way to combat fleas.
The diatomaceous earth (De Pro) is available from ProTerra, out of Utah. It is a fine dust I believe from a geological layer of seashells. Avoid breathing the fine dust – it is does not work out for insects.