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.
She relented and let one of the kittens go to its new home at seven weeks of age. The kitten was returned at ten weeks, weighing less than it had when it had gone to its new home three weeks before. The owners complained that the kitten had the sniffles and chronic diarrhea and wasn't using the litterbox.