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.
Many believe that the best litter box is one with deep sides to keep urine and litter inside the box and that has no cover. Most cats are fine with a standard plastic box. The advantage of a cover is that is provides your cat with privacy and blocks odor, litter and urine from leaving the box. The problem with the cover is that it traps dust and if odor builds up, your cat might avoid the box unless it is cleaned frequently.