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.
Type of litter: Do not use clumping litter with kittens under 4 months. Litter can get into the eyes and cause infections. Kittens also tend to eat the litter when young. You may wish to start out with a small container for the litter box with sides that are only 2 inches high.