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.
The litter box is a big deal in the cats' world and much fighting and bullying in multi-cat households centre on access to the litter box. The general rule is that you should have one more litter box than you do cats. This means, for example, that if you have four cats, you should have fiver litter boxes.