Nutrition and the Immune System
Wholesome nutrition is the key to maintaining a healthy immune system and resistance to disease. Commercial foods ingested by animals on a regular basis may not be balanced in terms of major nutrients, minerals and vitamins, and some continue to add chemicals 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 immune function, especially in those animals genetically susceptible to immune dysfunction (immune deficiency, autoimmunity, allergies).
Genetic differences between individuals lead to quantitative variations in dietary requirements for energy and nutrient needs, and to maintain health. Also, genetic defects may result in inborn errors of metabolism that affect one or more pathways involving nutrients or their metabolites. While minimal and maximal nutrient requirements have been established for most vitamins and trace mineral elements, optimum amounts for every individual cannot be assumed. Examples of important vitamin and mineral requirements 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.
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 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 pet foods.
Synthetic antioxidants like butylated hydroxyanisole (BHA) and butylated hydroxy-toluene (BHT) have been used as preservatives in human and animal foods for more than 30 years. Today, pet food manufacturers may prefer to use ethoxyquin, because of its excellent anti-oxidant qualities, high stability and reputed safety. But significant controversy surrounds issues related to the safety of ethoxyquin when chronically fed at permitted amounts in dog and cat foods. Chemical 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. Toy breed dogs 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 (vitamins E and C) are used more commonly in animal foods today, in response to consumer and professional queries about the chronic effects of feeding synthetic chemical antioxidants to pets.
Nutrition 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. Iron and zinc also are important minerals in regulating thyroid metabolism. Another link has recently been shown between selenium deficiency and hypothyroidism. 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 the synthetic antioxidants still used in some foods to protect fats from rancidity can impair the bioavailability of vitamin A, vitamin E and selenium, and alter cellular membrane function, metabolism and detoxification. Because animals with autoimmune thyroid disease have generalized metabolic imbalance and often have associated immunological dysfunction, it is advisable to minimize their exposures to unnecessary drugs, chemicals and toxins, and to optimize their nutritional status with healthy balanced diets. Families of dogs susceptible to thyroid and other autoimmune diseases show generalized improvement in health when fed premium cereal-based diets preserved naturally with vitamins E and C rather than with the synthetic chemical antioxidants mentioned above. Fresh vegetables cooked with Italian herbs and garlic, dairy products such as yogurt or low fat cottage cheese, or a variety of meats and whitefish can be added.
Nutritional Management (Commercial, Home-Made and Raw Food Diets)
Many veterinarians treating animals suffering from immunological diseases appreciate that alternative nutritional management is an important step in minimizing their patient's environmental challenges. The results of this approach have been remarkable. 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 aggravate the condition of dogs with behavioral problems (dominant aggression, hyperactivity, and fear). For allergic animals, elimination diets with restricted or novel antigen source are given for 6-12 weeks to evaluate their benefit to the patient. Homemade diets can also be used provided that the formula is properly balanced. All other food supplements, including treats, are withdrawn. Example ingredients that have been used successfully, include whitefish, rabbit, venison, duck, ostrich, emu, buffalo, and turkey mixed with potato, sweet potato and other vegetables (except onions and cruciferous vegetables). Grains are often avoided, at least initially, although novel grains like quinoa, sorghum, barley or flax usually have been well tolerated.
Raw food diets have been gaining in popularity as well. A key feature of these diets is the variety they provide. One of the prototype diets [BARF (bones and raw food)] of Dr. Ian Billinghurst recommends feeding a dog 60% raw meaty bones (chicken backs,wings and necks), with the rest of the diet composed of ground vegetables mixed with ground meat, and supplements such as kelp, vitamin E and vitamin C. Nutritional analyses on some commercially available raw diets suggest that the raw meaty bones commonly used provide 40-70% protein, and the meat/vegetable mixtures range from 20-50% protein. The question has arisen about the potential for such high protein diets to affect renal function when fed continuously, as high protein diets are reported to induce renal hypertrophy, and increase renal blood flow and glomerular filtration rate. While this concern may not pertain to healthy dogs, it could play a role in dogs with previously compromised renal function. At present, there are no data to support or refute this issue.
Maintaining the appropriate ratio of trace minerals, vitamins, fatty acids and other nutritive elements is especially important for patients with acute
and chronic diseases, as their metabolic demands have increased to sustain cell turnover and tissue repair. Typical supplements include: vitamin-mineral
mix, antioxidants (vitamins A,C, D, and E and selenium), digestive enzymes, brewer’s yeast, kelp, honey, coat additives, apple cider vinegar, hydrochloric
acid (used sparingly), yoghurt, Willard Water, liver, eggs, garlic, and plenty of fresh potable water.
Vitamin A and E have been shown to enhance immune function in small animals, as the former can beneficially influence T-helper responses, and the latter is known to improve both cellular and humoral immunity. Dietary carotenoids, especially lutein and beta-carotene, have been reported to modulate both cell-mediated and humoral immunity in dogs but not in cats.
Raw Food Diet Study
In collaboration with Dr. Susan Wynn, we investigated the basic clinical laboratory parameters of 256 healthy adult dogs of varying ages and breed types being fed raw food diets for at least 9 months. The same laboratory (Antech Diagnostics) analyzed the samples from 227 of the dogs. From this group, there were 87 dogs fed the classical BARF diet of Dr. Ian Billinghurst, 46 dogs were fed the Volhard diet of Wendy Volhard, and the remaining 94 dogs were fed other types of custom raw diets.
There were 69 dog breeds represented, including 233 purebreds, 16 crossbreds, 1 mixed breed and 6 of unknown breed type. The predominant breeds represented included: 28 Labrador Retreivers, 21 Golden Retrievers and 21 German Shepherd Dogs, 10 Whippets, 8 Shetland Sheepdogs and 8 Bernese Mountain Dogs, 6 Rottweilers, 6 Border Collies, 6 Doberman Pinschers, and 6 German Pinschers, and 5 Cavalier King Charles Spaniels, 5 Australian Shepherds, 5 Borzoi, and 5 Great Danes. Most of the dogs were neutered males (73) or spayed females (85), whereas there were 31 intact males and 32 intact females. Another 6 dogs were of unknown sex. The mean age of the group was 5.67 ± 3.52 years (mean ± SD); and the mean length of time fed a raw food diet was 2.84 ± 2.54 years. The data from this group of dogs were compared to the same laboratory parameters measured at Antech Diagnostics from 75 healthy adult dogs fed a commercial cereal-based kibbled diet. Preliminary statistical comparisons of results for the raw and cereal-based diets found them to be essentially the same with the following notable exceptions:
- Higher packed cell volume (hematocrit) in all raw diet fed groups (range of 51.0 ± 6.6 – 53.5 ± 5.6 %) versus cereal-based kibble (47.6 ± 6.1 %).
- Higher blood urea nitrogen (BUN) in all raw diet fed groups (range of 18.8 ± 6.9 – 22.0 ± 8.7 mg/dL) versus cereal-based kibble (15.5 ± 4.7 mg/dL).
- Higher serum creatinine in the Volhard raw diet group only (1.20 ± 0.34 mg/dL) versus cereal-based kibble (1.07 ± 0.28 mg/dL).
While a more detailed analysis of other parameters has yet to be completed, initial results indicate that dogs fed raw meats (natural carnivores) have higher red blood cell and blood urea nitrogen levels than dogs fed cereal-based food (obligate omnivores). Thus, the normal reference values for dogs fed raw fod diets should probably be revised.
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