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.
Is your cat not using the litter box? Cats stop using their litter boxes for a variety of reasons, including issues with the box or litter, dissatisfaction with the placement or number of boxes, changes in the environment inside or outside the house, and undiagnosed medical conditions.