I got this from http://www.felinefuture.com
"Physiologically, the cat is adapted to gorge food, followed by periods of fasting. This eating habit is necessary to provide sufficient time between meals for the body to de-tox, because a nearly all meat diet produces many toxic metabolic by-products, which are naturally filtered from the blood by the kidneys, and excreted in the urine. If the cat were to eat continuously, high toxic levels would be maintained in the blood stream and accumulative effect the cat's health. Meal feeding in 8-12 hour intervals assures that toxins in the blood peak only for short periods after ingestion and during digestion, after which they are nearly completely filtered from the body.
It is very beneficial for healthy adult cats, to withhold all food for a 48hour period on occasions. This extended fast will enable the body to clear some accumulated toxins from fat tissue, and completely empty the bowl. Healthy cats don't fast voluntarily, but are frequently faced with an involuntary fast in a natural setting through unavailability of prey, or repeated unsuccessful hunts. They will, however, frequently refuse food when unwell as a possible means to speed recovery. Cats are unharmed by the absence of food for several days, and in extreme times of famine may loose up to 50% of their body mass and still survive. It is, however, critical that the cat has unrestricted access to fresh water during fasting."
The first paragraph is actually only giving reasons why we shouldn't free feed, but sort of relates to the fasting part.