Newborn kittens and puppies receive total nutrition from mother's milk for about the first four weeks of life. After that, food is gradually added, and after a few more weeks they are fully weaned. During the first weeks of life, body weight may double or triple and this rapid growth will continue (although at a gradually decreasing rate) until maturity. Large amounts of energy and nutrients are required in balanced quantities to support this spectacular growth.

Kittens: Kittens are best-fed mom's milk; it's perfect for their needs.  However, circumstances may require that the kittens be fed a "milk replacer." If the queen is ill, has an extremely large litter, doesn't produce enough milk or wanders off or dies, it is necessary to feed the kittens a commercial milk replacer. A properly formulated milk replacer can come very close to matching the growth of kittens nursed by the queen. 

Generally, orphaned or hand-fed kittens will be offered moistened kitten food at about three weeks of age. The "moisture" should be a commercial milk replacer and be gradually reduced over time until the kittens are eating dry kitten food at about five or six weeks of age.

Initially, the food will be more of a plaything than food, but the youngsters will soon catch on as they watch mom eat her food. By the time the kittens are five to six weeks old, they should be nibbling on their dry food consistently. This process of gradually introducing their kitten food is important in training the kittens to eat when they are weaned. It also helps the queen by providing a separate source of nutrition for the rapidly growing kittens.

After weaning, kittens are usually fed free choice - dry or nutrient-dense canned food - with fresh water available at all times.

Most queens will suckle their kittens until 7-8 weeks of age. By this time, 80 - 90% of the kitten's total nutrient intake should be from kitten food. Kittens need large amounts of energy equaling about two to three times that of an adult cat on a kilogram of body weight basis. Kittens also need about 30% of total energy from protein. Therefore, kitten food must meet all the nutritional needs, including high amounts of energy and protein, from weaning until maturity at about one year.

Puppies: As with kittens, puppies occasionally need a replacement for the bitch's milk. Milk replacer for puppies is used similarly to milk replacer for kittens as described above but should have pup-specific instructions on the container.

Puppies generally begin eating puppy food three or four weeks after birth (whelping) and are completely weaned by seven or eight weeks. They require up to twice the energy intake of adults per kilogram of body weight and need to have 25% to 30% of total energy provided by protein depending upon their breed.

Prior to weaning, as with kittens, puppies should have puppy food available. These meals should begin when the pups are three to four weeks old and be small quantities at first. Puppies often play in their food when it is first introduced, but they will quickly learn its value. By the time the pups are ready to wean at six to eight weeks old, they should be eating their dry food consistently. This is important training for the pups. It also helps the bitch by providing a separate source of nutrition for the rapidly growing puppies.

Small breeds of dogs reach mature body weight in nine to twelve months, while giant breeds may not be mature until 24 months of age.

Small breed puppies are those whose adult size will be 20 pounds or less. These pups can often be fed free choice from weaning. With the constant availability of food, most will develop good eating habits and not become overweight. Owners with other pets or concerns about overeating should feed their puppies by the portion control method.

Most medium breed puppies (adult size between 20 and 50 pounds) and all large or giant breed pups (adult size over 50 pounds) are best fed with the portion control method.

The Challenge of Feeding Puppies: If puppies are allowed to over-eat, they may consume too many calories and too much calcium, grow too rapidly and develop bone growth problems. In breeds that are prone to these diseases, such as many large and giant breeds, overfeeding can lead to an increased frequency of hypertrophic osteodystrophy (HOD), osteochondrosis (OCD) and hip dysplasia. The formation of the young growing bone is disrupted and the resulting malformation, lameness and pain may cause serious clinical disease.

The clinical signs seen with these bone growth diseases include bowing of the front legs. Sometimes, these signs are misdiagnosed as weak bones due to calcium deficiency (rickets). Rickets is a very uncommon disease so it is important to accurately diagnose these bone diseases by x-rays.

Adding more calcium to the diets of dogs with HOD, OCD or hip dysplasia will actually worsen the diseases and may result in life-long damage to the bones.

Therefore, with large and giant breed puppies, it is important to aim for a slower rate of growth. Do not over-feed or try to push the growth rate too fast. Controlled feeding of a balanced diet specifically made for large and giant breed puppies facilitates optimal skeletal development. Remember, the adult size of a dog is determined genetically, not by how fast it grows.