Feeding him a nutritious diet low in chemical additives and preservatives is the first step in helping your dog build and maintain a healthy immune system. Adding fresh foods and supplements can keep your dog healthier and prevent some common conditions like ear and urinary tract infections. Good nutrition will keep your dog's coat shiny and benefit his overall health.
Dogs evolved from wolves and are still classified as carnivores. However, unlike cats, who must have meat to survive, dogs can live on a varied diet. Some dogs do fine on a vegetarian diet, although this is not recommended by most experts. Historically and worldwide, dogs have been fed human leftovers and cooked cereals with meat added. Now that commercial diets are available, most dogs eat a kibbled diet high in cooked grains and chemical additives and low in meat and fresh foods. Many theorize this accounts for the rise in allergies and chronic illnesses like diabetes and cancer. In response, some pet owners have started feeding their dog raw or home-cooked diets, and some pet food manufacturers are producing premium kibbles that are naturally preserved and have fewer unnecessary chemical additives.
There are many ways to feed a dog a healthier diet. The link below has extensive information on different feeding choices. Many people feed their dog a raw or "BARF" (biologically appropriate raw food) diet consisting mainly of raw meats and bones, with a little cooked or pulped vegetables on the side. Others swear by a home-cooked diet, which takes a little more time but with practice can be put together efficiently and inexpensively. There are now many excellent kibbled and canned foods available, using minimal grains, high-quality whole meats and natural preservatives like mixed tocopherols and antioxidants.
Dogs digest cooked grains inefficiently, which is why they produce gas and large, smelly stools when fed low quality kibble. They have a short gastrointestinal tract designed to efficiently digest high-protein foods like meats. Their teeth are designed for gripping and tearing food, unlike vegetarians and omnivores with large flat molars for chewing more slowly. Many people find that their dog's health improves when they stop feeding him low-quality, grain-based proteins like brewers rice or corn and wheat gluten. Look for higher protein grains in the ingredients list, like millet, oats, pearl barley or rice. Many foods are now grain free and higher in protein, using sweet potatoes as a carbohydrate source. Although some of these foods are more expensive, much less needs to be fed because the dog utilizes the proteins more efficiently.
It is a common misconception that dogs need carbohydrates (grains) in the diet. They do not, although they can utilize them. Those who have fed generations of dogs a raw diet with few if any carbohydrates will attest to healthy, long-lived dogs with fewer chronic problems. However, the raw diet is not feasible for many owners, and choosing better carbohydrates as outlined in the step above will go a long way toward building a strong immune system.
It has been speculated by some veterinary nutritionists that a diet based on low-quality grains may cause certain cancers. Others have found that such diets promote yeast overgrowth in the ears, which can cause frequent ear infections. Many of the preservatives and colorants used in lower quality foods are known carcinogens. In particular, avoid food and treats containing BHT, BHA, ethoxyquin and red dye 40. Some studies indicate that low-quality, grain-based diets can cause diabetes in dogs, just as a high-carbohydrate, low-quality diet promotes diabetes in people.
A monotonous diet can compromise health, give a dog an unnaturally finicky "sensitive tummy" and may promote allergies. Feeding him a variety of foods and protein sources is better and can prevent food allergies. Adding omega three fatty acids in the form of fish oil (the ones sold for human consumption are fine) will help with skin and coat health and may help dogs with arthritis because it is a natural anti-inflammatory.