Adverse reactions to ingredients in the diet may well be one of the commonest yet least recognised causes of ill-health in pets and humans. In theory, any dietary ingredient can cause an intolerance. In practice, protein (e.g. beef, milk) or carbohydrate (e.g. wheat, lactose) sources are the most likely causes although it is possible that chemical additives such as food colourings and preservatives could cause a reaction. Understanding and recognising dietary intolerance is not helped by pet food legislation which allows pet food manufacturers to declare ingredients such as 'animal derivatives and cereals' rather than naming the actual ingredients. This means that one cannot know precisely what one is feeding and the ingredients can be changed at will.
"it is possible that chemical additives such as food colourings and preservatives could cause a reaction"
In general, adverse reactions to food can be divided into those which act through the immune system (allergy) and those which do not (intolerance). Although the mechanisms of the two are different, the symptoms are indistinguishable.
Signs of intolerance (symptoms) vary widely and are not well understood or documented. A reaction can be instantaneous or delayed, even for several days. It can be mild and ill-defined causing non-specific signs of ill-health such as lethargy, loss of appetite, abdominal pain, stiffness, unthriftiness, unpleasant body odours, bad breath, discharge from orifices and so on. At the other extreme an adverse reaction can be severe and unmistakeable. An anaphylactic reaction can cause shock and death. Some people with an allergy to nuts are affected in this way.
In theory, any organ or system can be affected and this can give rise to a wide range of symptoms. In dogs dietary intolerance most commonly affects the skin or digestive system causing disease/disorders related to these organs. This might be eczema, itchy skin, dermatitis, otitis (ear inflammation), vomiting, diarrhoea, colitis, gastroenteritis, pancreatitis, hepatitis, abdominal discomfort etc. Other organ systems can also be affected e.g. immune system, musculoskeletal system, nervous system, endocrine (hormonal) system. Any symptom or disease affecting any of these organ systems could be caused by a dietary intolerance. This would include arthritis, convulsions (epilepsy), abnormal behaviour, allergic (pollen, house dust mite) and inflammatory reactions (pancreatitis, hepatitis), susceptibility to infection, Cushing’s, Addison’s, under- and over-active thyroid etc. Long-term unrecognised dietary intolerance may be the underlying cause of degenerative diseases such as heart or kidney failure and cancer.
It is probably true that dietary intolerance is much more prevalent than pet owners and veterinary surgeons realise. There are diagnostic tests but these are only possible for the minority which involve the immune system. In any case they are inaccurate and even misleading in that they produce both false-positive and false-negative results. Any symptom of ill-health which persists despite treatment or which recurs after treatment should arouse suspicion of food intolerance.
A feeding (elimination) trial is the only reliable method of diagnosis. This is the rationale of the Burns Health Management Programme. This involves eliminating the existing food from the diet and replacing it with a food which is new to the animal. Home-made food allows more control over ingredients but a commercial food like Burns may be more suitable for long-term use. If symptoms improve when the suspect food is eliminated then it can be presumed that a dietary intolerance was responsible. Confirmation would require the re-introduction of the suspect food to see if the symptoms recur but, of course most pet-owners will be understandably reluctant to take that step.
Disorders of the digestive system which are due to dietary intolerance often disappear within a few days of eliminating the food which is responsible but most disorders will take 3-4 weeks to respond to removal of the offending food.
Most cases of dietary intolerance (those where the immune system is not involved) are dose-sensitive. This means that the amount of food can determine whether or not signs of intolerance will disappear. This is why it is important that whatever food is given, it should be fed sparingly. It is likely that many people suffer from undiagnosed dietary intolerance with similar consequences except that the respiratory system is more commonly affected than the skin.