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.
Three-week-old kittens are too young to be away from their mom, but in some cases the mama may abandon her young and you'll have to take over. At three weeks, your fuzzy pal may be too little to use the litter box on his own. You'll have to train him. Kittens learn to eliminate from their mom.