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.
I rescued two 8 week old kittens-both brothers. Apparently very healthy, playing, sleeping, using the kitty litter box. One is eating the dry kitten chow and drinking water. The other kitten is refusing to eat or drink. However, the one that is refusing to eat the dry cat food or drink the water, will drink milk.