In the wild, cats and dogs would eat foods which naturally kept teeth clean and gums healthy. It can sound rather unpleasant to us now, but the diet of a wild dog or cat would have included all the tough and unpleasant parts of a raw animal carcase. These things tend to exercise the teeth and gums and do not leave significant residue in the mouth afterwards.
Modern diets are amazingly good in may ways. The balance of the different nutrients, especially the protein, vitamin and mineral levels, can be tailored to exactly fit with what is required. This is probably at least as good as any animal receives in the wild. Indeed our pets live more than twice as long as their wild ancestors could hope to. However, most modern diets seem to be poor for teeth and gums. They are easier to eat, which means they provide less exercise for the jaw, teeth and gums. They also leave traces of food coating the teeth. This is obviously true of soft foods, but the same applies to biscuits and kibbles. When these are chewed by the back teeth, they tend to break into small pieces, including some fine crumbs. These mix with saliva to form a paste which can coat the teeth for a few hours, providing a culture medium for harmful bacteria. In my veterinary clinic I often point out how the teeth, especially in cats, look like "snow capped mountains". The sharp pointed tips of the teeth, which are pressed into the biscuit, are clean and white. Further down the side of the tooth, especially towards the gum (which is the important area) the teeth are a dirty brown colour where tartar has built up. Gum disease becomes inevitable. (And a lot of these affected cats eat only modern dried foods.)
So our ideal diet has to not only provide all the right nutrients, but also provide chewing exercise and suppress the formation of plaque and tartar. A number of manufacturers now produce diets which try to achieve these aims. The market leaders in these types of diets are the Hills Oral Care range and the Royal Canin Dental diets. Hills also produce their patented Hills Prescription Diet t/d which might well be the best of them all. It is not well suited to young, growing animals because of its nutrient profile, but for adult cats and dogs it is ideal. All of these dental diets can make a difference to oral health and are well worth considering.
A more traditional approach to feeding dogs and cats is to include bones in the diet. This has been out of fashion for a while, though is regaining popularity with some people. Bones can certainly cause all sorts of problems. Cooked bones tend to be worst because they can become hard and brittle. The dog or cat is tempted to swallow large pieces which can become stuck after being swallowed. Even if they pass through the gut OK they can cause painful constipation. Of course everyone has heard of the "rule" that cats should not be given chicken bones. Dogs can also become aggressively protective of a bone. There is however a different side to the argument:
A dog or cat which is used to being given bones from the earliest age will usually deal with them OK. For dogs the bones should be raw, for cats they can be lightly cooked chicken bones, such as the last two sections of a wing (The chickens that we eat these days are such young birds, the bones are not fully calcified and are unlikely ever to splinter into hard, sharp pieces.) A diet which includes bones on a regular basis probably provides many benefits in oral and digestive health. Dogs and cats were after all, originally designed by nature to eat these things. A full discussion of the pros and cons is beyond the scope of this article, so a discussion with your own vet would be the best advice.
It is also worth remembering that daily brushing with a toothbrush provides great benefits if you can do it. Starting off in adult dogs is very difficult, in cats it is usually quite impossible! However if you start when they are young and keep it up regularly it will certainly help enormously.
Providing chews for dogs, especially the hide type, is also very helpful. They should be large enough to last for a while, rather than disappear in an instant. The more time spent chewing, the better the effect.