EU law defines a complete pet food as "any food which, by reason of its composition, is sufficient for a daily ration" and defines a daily ration as "The average total quantity of a specific pet food that is needed daily by a pet of a given species, age category and life style or activity to satisfy all its energy and nutrient requirements".
In theory, this means you can rest assured that any food you find labelled as 'complete' has everything your dog needs to stay fit and healthy. Since most manufacturers recommend feeding only their foods, the principal of dietary 'completeness' is absolutely central to pet health and yet it is exactly this premise that is drawing increasing amounts of criticism from pet food campaigners all over the world.
The main critisism is levelled at the recommended nutrient requirements themselves. In Europe, the nutritional guidelines are set by the European Pet Food Industry Federation (FEDIAF) who's other job it is to represent the interests of European pet food manufacturers. Their recommended nutritional levels are based on "scientific studies (including NRC 2006) and unpublished data from the industry". Investigations by the Association for Truth in Pet Food reveal that the NRC (National Research Council in the US) itself has extremely strong ties with just about all of the large pet food corporations which means that, essentially, the established pet food industry is setting its own rules. With such a clear conflict of interest, many have suggested that their advice might be more about protecting their interests and the status-quo rather than pursuing nutritional excellence.
For example, could it simply be coincidence that FEDIAF's recommended protein level for adult dogs of 18% dry matter just happens to be consistent with the low-meat, high-carb diets that their biggest members churn out?
Then there's the question of nutrient quality. It is now becoming more and more clear that the source and grade of ingredients and the nutrients they contain (proteins from fresh meats vs proteins form vegetable derivatives, for example) are absolutely paramount to assessing food quality and yet the recommended nutrient levels for complete foods do not even approach the subject. Again, this plays directly into the hands of the manufacturers of low grade pet foods.
So basically, 'complete pet food' as it is currently defined is a myth. Nevertheless, by choosing a good food and by spicing up the diet with a bit of variety, you can be sure that no areas of the nutritional spectrum are being neglected. Mixing or alternating between two or more high quality dog foods is always a good idea as is bolstering the diet with healthy extras like vegetables, fruits and meats. Take a look at our guide on feeding human foods for more info.