By W. Jean Dodds, DVM



Wholesome nutrition is a key component for maintaining a healthy immune system and resistance to disease.  Foodstuffs ingested by animals on a regular basis may be imbalanced in terms of major nutrients, minerals and vitamins, and often contain chemicals added to the final product to enhance its stability and shelf-life.  Nutritional deficiencies or imbalances, as well as exposures to various chemicals, drugs and toxins, present a continual immunological challenge which can suppress overall immune function, especially in those animals genetically susceptible to immune dysfunction (immune deficiency, autoimmunity, allergies).  For example, Beagles are susceptible to distemper, demodectic mange, and autoimmune thyroid disease; Boxers to cancers; Doberman Pinschers and Rottweilers to parvovirus gastroenteritis with vomiting and diarrhea; and Shar Peis to IgA deficiency.  However not all individuals at risk will become affected.  One hypothesis to explain why some animals become affected with specific diseases while others remain normal, despite their common susceptibility, is called the "Threshold Model."  In this situation, genetically susceptible individuals develop disease following the additive effects of inducing agents such as drugs, exposure to toxic or noxious substances, hormonal imbalances and dietary influences.

Genetic differences between individuals lead to quantitative variations in dietary requirements for energy, nutrients and overall health.  Also genetic defects may result in inborn errors of metabolism that affect one or more pathways involving nutrients or their metabolites.  Many inborn errors of metabolism are fatal, whereas others may show significant clinical improvement with nutritional management.  Minimal and maximal nutrient requirements that can be important in this regard include vitamin C, vitamin E and selenium, vitamin A, copper and vitamin B-12.  Similarly, a wide variation occurs in the energy needs of dogs depending on their breed, age, sex and size.  Breeders quickly learn to adjust the caloric intake of their animals depending on the optimal requirements of each individual.

Many environmental factors cause or trigger immune dysfunction leading either to immune deficiency or immune stimulation (reactive response, autoimmunity).  One of the most common disorders of increasing prevalence today is autoimmune thyroid disease.  Affected individuals have generalized metabolic imbalance and often have associated immunological dysfuncton.  An important facet of managing these cases is minimizing exposure to unnecessary drugs, toxins and chemicals, and optimizing nutritional status with healthy balanced diets.  Because of the genetic predisposition to autoimmune disorders, the same recommendations apply to family members.  Individuals susceptible to these disorders are at increased risk for adverse effects from immunological challenges following exposure to viruses, vaccines, and other infectious agents; a variety of chemicals, drugs and toxins and hormonal imbalances.  Related to these events is the susceptibility to and development of cancer, a disruption of cell growth control.

Nutritional Factors and the Immune System

As alluded to above, an adequate nutritional state is important in managing a variety of inherited and other metabolic diseases as well as for a healthy immune system.  Examples where nutritional management is important in inherited disorders include:  adding ingredients to the diet to make it more alkaline for Miniature Schnauzers with calcium oxalate bladder or kidney stones;  use of the vitamin A derivative, etretinate, in Cocker Spaniels and other breeds with idiopathic seborrhea of the skin;  management with drugs and/or diet of diseases such as diabetes mellitus and the copper-storage disease prevalent in breeds like the Bedlington Terrier, West Highland White Terrier, and Doberman Pinscher; wheat-sensitive enteropathy in Irish Setters; and treatment of vitamin B-12 deficiency in Giant Schnauzers.  Other nutritional influences include the vitamin K dependent coagulation defect elicited in Devon Rex cats following vaccination; hip dysplasia in puppies fed excessive calories; osteochondritis dissecans (OCD) in dogs fed high levels of calcium; and hypercholesterolemia in inbred sled dogs fed high fat diets.

Nutritional factors that play an important role in immune function include zinc, selenium and vitamin E, vitamin B-6 (pyridoxine), and linoleic acid.  Deficiency of these compounds impairs both circulating (humoral) as well as cell-mediated immunity.  The requirement for essential nutrients increases during periods of rapid growth or reproduction and also may increase in geriatric individuals, because immune function and the bioavailability of these nutrients generally wanes with aging.  As with any nutrient, however, excessive supplementation can lead to significant clinical problems, many of which are similar to the respective deficiency states of these ingredients.  Supplementation with vitamins and minerals should not be viewed as a substitute for feeding premium quality fresh and /or commercial dog foods.

Nutritional Factors and Thyroid Metabolism

Nutritional influences can have a profound effect on thyroid metabolism.  The classical example is the iodine deficiency that occurs in individuals eating cereal grain crops grown on iodine-deficient soil.  This will impair thyroid metabolism because iodine is essential for formation of thyroid hormones.  Another important link has recently been shown between selenium deficiency and hypothyroidism.  Again, cereal grain crops grown on selenium-deficient soil will contain relatively low levels of selenium.  While commercial pet food manufacturers compensate for variations in basal ingredients by adding vitamin and mineral supplements, it is difficult to determine optimum levels for so many different breeds of animals having varying genetic backgrounds and metabolic needs.

The selenium-thyroid connection has significant clinical relevance, because blood, but not tissue, levels of thyroid hormones rise in selenium deficiency.  Thus, selenium-deficient individuals showing clinical signs of hypothyroidism could be overlooked on the basis that blood levels of thyroid hormones appear normal.  The selenium issue is further complicated because synthetic antioxidants used to protect fats from rancidity can impair the bioavailability of vitamin A, vitamin E and selenium, and alter cellular membrane function and metabolism.  As manufacturers of many premium cereal-based pet foods began adding the synthetic antioxidant, ethoxyquin, in the late 1980's, its effects, along with those of other synthetic preservatives (BHA or BHT), discussed below, are likely to be detrimental over the long term.  The total cumulative antioxidant load needs to be considered as well, because the common use of BHA to preserve animal fat sources is additive to the ethoxyquin incorporated into the finished product.  An important question is what effect these induced changes in vitamin A, vitamin E and selenium quotients have wrought over the last 7-8 years with respect to the health and performance of companion animals.  The way to avoid this potential problem is to use foods preserved with natural antioxidants such as vitamin E (tocopherol) and vitamin C (ascorbic acid) or prepare entirely home-cooked fresh natural ingredients.

Synthetic and Natural Antioxidants

Synthetic antioxidants like BHA and BHT have been used as preservatives in human and animal foods for more than 30 years.  Many pet food manufacturers prefer to use ethoxyquin today however, because of its excellent antioxidant qualities, high stability and reputed safety.  But significant ongoing controversy surrounds issues related to its safety when chronically fed at permitted amounts in dog and cat foods.  The same antioxidants have been linked to inducing or promoting a wide variety of cancers, although the published literature is both disturbing and contradictory in this regard.  These safety questions pertain mostly to genetically susceptible breeds of inbred or closely linebred dogs.  Toy breeds may be particularly at risk because they ingest proportionately more food and preservative for their size in order to sustain their metabolic needs.

Naturally occurring antioxidants (vitamin E and C) are also used in pet foods, and have become more popular in response to consumer and professional queries about the chronic effects of feeding synthetic chemical antioxidants to pets.  While they are somewhat less effective and more expensive than the synthetic antioxidants, proponents of natural antioxidants believe their safety outweighs these drawbacks.  It should be appreciated, however, that pet foods devoid of synthetic antioxidants added at the time of processing often contain ingredients (such as animal tallow or other fats and oils) that are preserved with antioxidants.  Thus, claims made about the use of "all natural" antioxidant preservatives should also apply to preservatives used in the raw materials.

The net effect of these concerns has resulted in a major change in the pet food industry.  Manufacturers of premium pet foods and most newly introduced foods have begun offering products preserved with natural antioxidants.  This is a triumph for the views of the pet-owning public which prefers to use natural ingredients whenever possible.

Nutritional Management

Many veterinarians treating animals suffering from immunologic diseases appreciate that alternative nutritional management is an important step in minimizing their patients' environmental challenges.  The results of this approach have been remarkable.  Standard commercial diets containing synthetic chemical preservatives are replaced with naturally preserved foods.  The replacement food must be of good quality and preferably of relatively low protein content (20-22%).  Increasing carbohydrate and reducing protein content, while maintaining high quality protein, has been shown to be beneficial for many affected animals and is also believed to have a positive effect on behavior.  Diet and behavior appear to be linked because certain highly nutritious foods may contribute to deterioration in the condition of dogs with behavioral problems (dominant aggression, hyperactivity, and fear).

For allergic animals, elimination diets are given for 6-8 weeks in order to evaluate their benefit to the patient.  Homemade diets can be used instead of naturally preserved commercial diets, provided that the formula is properly balanced.  All other food supplements, including treats, are withdrawn, with the exception of such ingredients as fresh or stewed vegetables; low-fat cottage cheese or plain non-fat yogurt; boiled or scrambled eggs; chicken, turkey, fish, rabbit, venison or lamb stewed after removing the skin and fat; potatoes, steamed brown rice and pasta.  A mixture of fresh vegetables, excluding onions and the cabbages, can be stewed together with basic kibbled cereal.  Animals with known or suspected intolerance to dairy products, eggs or other ingredients should not be given these foods.  A variety of individual diets can be developed by owners in consultation with their veterinarian and a nutritionist.  While animals treated in this way sometimes experience complete remission of symptoms, most show a 75% or greater improvement of their existing condition.  This holistic approach to diet is combined with the minimal amount of drug therapy required to extend the clinical remission and enhance the well-being and longevity of the patient.  It is easy to implement, employs a common sense methodology and is inexpensive.  The clients and patients like it, and best of all, it works!

Selected References

*Berry MJ, Larsen PR 1992.  The role of selenium in thyroid hormone action.  End Rev 13 (2): 207-219.

*Burkholder WJ, Swecker WS Jr. 1990. Nutritional influences on immunity. Sem Vet Med Surg (Sm Anim) 5 (3): 154-166.

*Cargill J. 1993.  Feed that dog.  Parts I-III.  Dog World 78 (7-9): 24-29, 10-16, 14-22.

*Cargill J, Thorpe-Vargas S 1993/1994.  Parts IV-VI.  Dog World 78 (10-12): 36-42, 28-31, 36-41; 79 (1-2): 18-22, 36-42

*Dodds WJ 1991. Nutritional approach can help enhance immune competence.  DVM Magazine 22 (4); 15-18.

*Dodds WJ. 1992 Autoimmune thyroid disease.  Dog World 77(5): 44-48

*Dodds WJ 1992. thyroid can alter behavior.  Dog World 77 (10): 40-42.

*Dodds WJ, Donoghue S. 1994. Interactions of Clinical Nutrition with Genetics.  In:The Waltham Book of Clinical Nutrition of the Dog and Cat, Chapter 8.  Pergamon Press Ltd., Oxford, England, pp. 105-117.