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.
1 At 2 weeks of age, the kittens should be alert and be making attempts to stand up. 2  At 3 weeks, they try to climb outside of their box or pen.  At 4 weeks, all of the kittens should be walking, running and playing.