Understanding and recognising dietary
intoleranceAdverse 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.
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.
puppyIt 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 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 which is low fat, low protein
and easily digestible 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
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
Common pet health problems
John Burns Pet Health Management