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.
It is a cat's basic instinct (even big cats) to eliminate away from their food and sleeping area and to bury the waste in order to hide their position from other animals and protect their nests and young. Most kittens start using the litter-box at the age of 3-4 weeks on their own.