here is some information for keeping your cat healthy:
http://cats.about.com/cs/catfood/a/humanfood.htm
Milk
Although milk is not toxic to cats, it may have adverse effects. Simply put, adult cats fed a nutritious diet don't need milk, and many cats are lactose-intolerant, which means that the lactose in milk and milk products produces stomach upset, cramps, and gassiness. If your cat loves milk, and begs for it, a small amount of cream may be okay, two or three times a week. (The more fat in the milk, the less lactose.) Another compromise is CatSip, a product made from skim milk with an enzyme added that helps the digestion of lactose. Catsip is available in supermarkets such as Safeway, Albertson's and A&P, as well as pet products chains, such as PetSmart and Petco.
These are the most commonly seen "people foods" that are potentially harmful to cats. The bottom line is to feed your cat nutritious food developed with his needs in mind and choose treats designed for cats instead of table scraps.