Essential fatty acids (like omega 3 and omega 6 fatty acids) and fat soluble vitamins (Vitamin A, Vitamin E, Vitamin, D, Vitamin K). The plain fat is a nutrient too.
Kittens, generally, can begin to use a litter box at about four weeks of age, and a kitten being raised without its mother or adult cat will take slightly longer to litter train; so, keep this in mind when you begin the task of litter training your kitten. - Place the kitten in the box 1/2 hour after play or meals.
Most people tend to place the litter box in an out-of-the-way spot to minimize odor and prevent cat litter from being tracked throughout the house. But if the litter box ends up in the basement next to an appliance or on a cold cement floor, your cat may be less than pleased. So you may have to compromise.