“People food” is not good nutrition for your pet. Some dogs and cats do like human foods, but animals also eat things that have no appeal to people to people such as grass, faeces, vomit, garbage and carrion. People and pets have different nutrient requirements. Most pet foods are better balanced to meet the needs of dogs and cats than are diets for people. Human nutrition is often of public health concern (even is first world countries) as there is an inadequate intake of calcium, complex starches and fibre and excess intake of fat, saturated fat, cholesterol, slat, sugar and calories.
For pet owners to feed their pet the same diet they eat, could be a nutrition step backward for their pet. Dogs and cats need at least 10 times more calcium than people and they have different protein, vitamin and mineral requirements. It is of course possible to make good quality home prepared foods for your pet. However just feeding raw bones is not enough, nor is feeding table scraps. There is a science to nutrition and home-made diets can be done well or very poorly. Please ask us how to do it well, be aware it does take time and effort. Supermarket pet foods are sold and made based on price. Palatability is important, so these foods are usually high is salt and / or sugar. The manufactures want you to think their food is good because your pet eats it with enthusiasm. ‘Gourmet’ foods are a marketing niche that is all about palatability for finicky eaters but doesn’t actually mean much. ‘Natural’ pet foods are an advertisers dream. There is no legal definition or regulation of the word natural, so it’s anything goes. There is a risk of nutritional deficiencies in these foods.
The common problems in dogs fed supermarket foods are : poor growth performance, skin and ear problems and poor hair coat. Some of these problems are due to deficiency of zinc and / or vitamin A and deficiency of essential fatty acids and / or high levels of minerals: calcium, phosphorus, magnesium, phytates and poor digestibility of the food.
I believe the main different between supermarket and premium food is quality. Premium foods have regular analysis of nutritional adequacy through feeding trials. Some companies spend a lot more money on research and development.
How much salt is in your dog’s diet?
Supermarket Food Salt Content
Proplan (purina) 1.98%
Beneful (pruina) 2.75%
Nature’s gift 2.2%
Premium Food Salt Content
Eukanuba 0.55%
Royal Canin 0.55%
Hills Science 0.23%
Treats
Schmacko’s 6.6%
Liver treats 0.72%
The toxic level of salt is 2% (2g/kg of dry matter) Salt and fat are cheap and taste good: this is why supermarket pet foods are cheaper. Some treats are very high in salt and should be given very rarely.
 

Cats

Cats have different dietary needs than dogs. Cats should not be fed dog food. Cats need a source of taurine and arachidonic acid in their diet. They need more protein than dogs. They can’t digest large amounts of simple carbohydrates. Cats can digest and use high levels of fat (from animal tissue). Plant based foods are not good for cats. Cats need about 4 times more B vitamins than dogs (particularly niacin and pyridoxine). They can’t get enough vitamin D through their skin: they need it in their food.
Cats can’t convert B-carotene to vitamin A so need to get vitamin A from animal tissue in their diet. They are also sensitive to excess vitamin A. They need less water than dogs.
Raw meat is commonly fed to cats and while cats might like it, it has risks. There is no benefit to feeding raw (rather than cooked) meats to cats. The risks are: raw meat may contain harmful bacteria (especially salmonella and E. Coli) and parasites (eg toxoplasma). Raw meat is nutritionally poor and needs vitamin and mineral supplementation. There is also the risk of iodine deficiency and nutritional secondary hyperarathyroidism.
Raw bones are also a risk for above reasons. Also they may cause damage to mouth ( including fractures of teeth), may get lodged in oesophagus or intestines and can cause colitis and constipation.
Many pet meats  are kept looking fresh and red by addition of sulphur dioxide (a preservative) but this can be toxic (it is illegal to add it to human food but not to pet food).
Some supermarket cat foods have been shown to cause diseases in cats – in particular cystitis (including struvite bladder crystals) and diabetes. Some disease can be fixed by a change in diet so a complex and balanced food is best avoid problems.