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.
If you're having a hard time persuading your cat to use the litter box, it may be time to draw a line in the sand. Most cats are happy to use a litter box because their instinct is to bury their urine and feces.