Your dog's nutritional needs depend in large part on his breed and size. Some dogs reach physical maturity much more rapidly than others, with large-breed puppies taking the longest to mature. Although the growth of any dog starts to slow at about six months of age, large and giant-breed dogs don't reach full physical maturity until they're two and sometimes even three years old. They should eat a large-breed puppy diet or a diet customized by your veterinarian until they're two years old.

Small and medium-size dogs have special needs as well. Small dogs of any age, especially those that weigh less than 20 pounds, have a higher metabolic rate than large-breed dogs, so they burn energy more quickly. They need a nutrient-dense diet that gives them a lot of nutrition in a small amount of food. That's because their stomachs just aren't that big relative to their needs. Most small and medium-size dogs can start eating a food formulated for adult dogs at nine months to one year of age. At this age, their skeleton will be full size, or almost there. The exceptions to this rule are the small but stocky breeds, such as pugs, that are also prone to skeletal problems if they grow too quickly. They can start eating an adult food as early as five months of age.

Determining Mature Weight

What if you don't know how big your puppy is going to be? Mixed-breed puppies adopted from animal shelters don't come with papers detailing their parents' breed or size. But there's a rule of thumb that will get you in the ballpark. Take the puppy's weight at eight weeks. Multiply by four or five, and the result is an estimate of his adult size. For example, an eight-week-old pup that weighs 15 pounds is likely to weigh 60 to 75 pounds as an adult. This estimate is probably a little on the low side, but it can help you figure out whether you need to feed a food geared for a large or small breed.

Switching Foods

When it's time to switch your dog from puppy food to adult food, or if you simply want to change brands, always do so gradually. Dogs have sensitive stomachs, and a rapid dietary change can bring on vomiting or diarrhea. Begin by adding small amounts of the new food to your dog's regular food over a period of five to ten days. The more different the foods are from one another, the more gradual the change should be.

For instance, if you're switching from Brand A puppy food to Brand B adult food, the change should go more slowly than if you're changing within the same line of food; say, from Brand X puppy food to Brand X adult food. Also change gradually if you're switching from dry food to canned food or vice versa. Allow plenty of time for the dog's gastrointestinal system to fully adapt to the new diet. Whatever you choose to feed your dog, be sure that it offers complete and balanced nutrition for his life stage.

Nutrition for Older Dogs

A healthy older dog can continue to eat the same diet as long as he maintains a good weight and his coat and skin remain in good condition. One of the signs that your dog might need a change in diet is weight gain. Like people, dogs tend to become less active with age, so they need fewer calories to maintain an appropriate weight. Because their aging body is less able to metabolize protein efficiently, they need a food with reduced levels of fat and calories, but high levels of protein. Added fiber can also help reduce the calorie count, while still giving your dog the feeling of a full belly.

Older dogs can also develop a thinner coat or dry itchy skin. Certain nutritional supplements may help, such as essential fatty acids, vitamin E, and zinc. Ask your veterinarian for a recommendation. Checking thyroid levels is important on older dogs with weight gain or coat changes as well.