The puppy stage passes all too quickly. After about a year (depending on the breed), your dog can completely switch to an adult diet. However, you should avoid upsetting a dog's digestive system by performing this change gradually. Start by mixing in just a little adult dog food into the puppy food, and increase the proportion of adult food a bit more every day.
In addition to size and breed, a lot of a dog's nutritional needs will depend on its activity level. Because of their increased energy needs, active dogs need to eat more calories. But it's not simply a matter of feeding them more of the same food. That's because active dogs also need a higher percentage of protein and fat from those calories. Compared to sedentary dogs, very active and performance dogs may need about twice the percentage of calories from fat [source: Becker].
A good way to determine if your dog is eating a healthy amount is to keep an eye out for signs that the dog is underweight or overweight. Underweight dogs look emaciated, with very visible ribs, vertebrae and pelvic bones. It's healthy to feel at least a layer of fat under its ribs. If you can't feel the ribs at all, or only through a very thick layer of fat, this is a sign that the dog is overweight -- a more common problem for dogs with plenty of access to food. An overweight dog might need to be put on a low-calorie, low-fat and low-protein diet. Because of a dog's tendency to overeat, as well as the need for a nutritionally balanced diet, limit treats and table scraps to no more than 10 percent of a dog's daily intake of food [source: VMRCVM].
Some dog owners assume that because vitamins and minerals are so essential, there's no harm in adding supplements to a dog's diet. But this isn't the case. Unless a veterinarian recommends specific supplements, there's no need to add them to a dog's diet. It might actually cause harm, such as upsetting the dog's delicate balance of calcium and phosphorous. In addition, some potential toxins to avoid are onions and garlic, as well as grapes and raisins. Chocolate is notoriously toxic to dogs as well.