SNUTRITION AND IMMUNOLOGY: IS THERE A RELATIONSHIP?
Michael G. Hayek, PhD
Stefan P. Massimino, MS
Michael A. Ceddia, PhD
Research and Development Division
The Iams Company, Lewisburg, Ohio USA
Presented at the Iams Breeder' Symposium, 2002 - 2003 Edition
Altering the immune system through diet is an ever-growing area in pet food nutrition. Much work in this area has now been conducted, and this research has shown justification for a 'cradle-to-grave' mentality. By this, we mean that nutrition has been shown to play a beneficial role with the immune system in nearly every lifestage of the dog, from birth to death. Dogs can specifically benefit from a stronger immune system. Fewer days missed due to illness, and an overall healthier dog are just two benefits for an adult dog, but there are benefits for both puppy and senior dogs as well. A puppy's immune system is immature and still developing, while senior dogs experience an age-associated decline in their functional immune system. Keeping dogs productive and active into their golden years, or just happy and healthy as retired family pets, is another way that nutrition's effect on the immune system can help these dogs.
OVERVIEW OF THE IMMUNE SYSTEM
The immune system is an intricate network of specialized and interacting organs, tissues, cells, and chemicals. All dogs have various mechanisms to protect against invading disease agents (pathogens), ranging from non-specific barriers to specific defenses.
Immunity can be classified as either innate or acquired (Figure 1). Puppies are born with innate immunity, which consists of non-specific barriers, and cellular and chemical defense mechanisms. Non-specific physical barriers, such as skin and mucous membranes, protect against the initial entry of pathogens such as bacteria, viruses, and parasites. However, once those barriers are overcome, a functional immune system is required to mount a specific response to clear the infection and protect the dog.
Cellular and chemical defenses rely heavily on detection of the difference between invading microorganisms (called pathogens) and what is considered "self" or part of the individual's body. When these pathogens are detected, enzymes that digest bacterial cell walls are activated and cells that recognize these invading microorganisms and destroy them are deployed. This response is specific to the invading organism and does not require priming (no lag time), but is slow and usually not sufficient to clear the pathogen once it has become established. Rather, it serves to contain the infection until the next level of defense, known as acquired immunity, develops
Acquired immunity is a much more complex system that can rapidly develop a specific response against invading pathogens. It can be divided into either cell-mediated or humoral immunity. Cell-mediated immunity includes the interaction of macrophages, B cells and T cells. These cells work together to generate an immune response by recognizing pathogens. Through cell to cell interactions and release of soluble immune mediators, production of additional T and B cells occurs. These cells are then responsible for sustaining the immune response, destroying the invading pathogen and infected cells, and terminating the immune response once the infection has been cleared. Some cells, known as memory cells, survive so that in the event of another attack by the same pathogen, the immune system is able to respond much more rapidly and vigorously.
Humoral immunity is also commonly referred to as the antibody-mediated immune response. When an invading pathogen has been recognized, pathogen-specific B cells proliferate and are transformed into antibody-secreting cells. Antibodies are blood-borne immune proteins that are able to bind specifically with infected cells, as well as free microorganisms, which leads to their destruction. As with T cells, memory B cells remain after the infection to produce specific antibodies if the same pathogen is detected.
NUTRITION AND IMMUNE FUNCTION
Interactions between nutrition and immunity have been well-documented.1 Diets deficient in protein, energy, minerals, vitamins, and essential fatty acids have long been known to impair immunity. More recently, supplementation with nutrients above and beyond minimum required levels has been reported to be successful in improving health and immune function in a wide range of species, including dogs.
Specific nutrients that have attracted special interest for these purposes are antioxidants and fatty acids. Antioxidants are thought to benefit immune function by their effects on free radicals. Free radicals are chemically reactive compounds that are produced daily in the body as a result of aerobic (oxygen-requiring) metabolism and normal immune system functioning. Therefore, free radical production is not only normal, but required as a consequence of having to breathe oxygen. However, if free radical accumulation is not controlled, it can damage healthy cells. The membranes that surround the various cells of the body are primary targets for free radical damage.
Immune cells are especially susceptible to this free radical damage because their cell membranes contain high levels of polyunsaturated fatty acids, which are more easily damaged. The body has several systems in place to combat these free radicals, including antioxidant enzyme systems and various endogenous factors.
Another alternative to these internal antioxidants are the various diet-derived antioxidants including vitamin E, beta-carotene, and lutein
Vitamin E. Vitamin E is a term used to encompass a group of potent, chemically similar antioxidants. One form of vitamin E, alpha-tocopherol, is most abundant in the body, has the highest biological activity, and reverses vitamin E-deficiency symptoms. In cells, vitamin E contributes to cell membrane stability, regulates cell membrane fluidity, and protects cellular components from oxidative damage.2
Immune cells possess a higher vitamin E level than other cells, and as previously mentioned, these cells coincidentally contain higher levels of polyunsaturated fatty acids making them more susceptible to oxidative damage. This might be one way in which immune cells try naturally to protect themselves from damage from free radicals.
Supplementation with vitamin E has been reported to increase lymphocyte proliferation and antibody production in several species.3 Interleukin-2 production (a pro-inflammatory soluble immune mediator) and the delayed-type hypersensitivity response (an excellent determination of the cellular immune response; DTH) have also been reported to be elevated with vitamin E supplementation in older rodents and senior humans. Lastly, prostaglandin (PG) E2 production (an immune-suppressing compound) was significantly decreased in rodents after vitamin E supplementation.
Beta-Carotene. Beta-carotene belongs to a family of antioxidants called carotenoids. Carotenoids are naturally occurring plant pigments that have been suggested to play important roles in modulating immunity and health of animals. Studies have shown that beta-carotene supplementation is able to affect both specific as well as nonspecific cellular defenses.4,5 Iams-sponsored studies have revealed that beta-carotene is effectively absorbed in dogs and can affect the canine immune system.6-8 Studies in dogs have shown that beta-carotene supplementation results in increased antibody levels, an increased DTH response, modified immune cell numbers, and increased T and B cell proliferation responses. Feeding beta-carotene has also been shown to improve various measures of immune function in senior, as well as young adult dogs.9
Lutein. Lutein is another naturally occurring carotenoid antioxidant found abundantly in plants and microorganisms. Unlike beta-carotene, lutein cannot function as a precusor for vitamin A synthesis (it cannot be used to make vitamin A in the body). However, like beta-carotene, lutein functions as an antioxidant protecting cell membranes from oxidative damage.
In dogs, lutein can be absorbed from the diet and taken up by lymphocytes.10 Lutein supplementation in dogs has resulted in increased cell-mediated immune responses, such as the delayed-type hypersensitivity and lymphocyte proliferation after only 6 weeks. In addition, humoral immunity as measured by elevated antibody levels, was also increased in dogs with lutein supplementation.11
Taken together, the above studies show that the anti-oxidant nutrients vitamin E, beta-carotene, and lutein have a positive influence on the immune system. Also noted in these studies is that these nutrients interact with different parts of the immune system. Therefore, incorporating a combination of these nutrients will have a stronger effect on the overall immune system than one nutrient alone.
Dietary Fat. Dietary fat has also been reported to modulate immune function. Historically it was thought that high-fat diets were responsible for suppressing the immune response.12 However, work conducted within the last 10 to 15 years has shown that the type of fat in the diet plays an even larger role in modulating immunity. Omega-3 fatty acids exert their influence on the immune response by their ability to be incorporated into the cell membrane and act as substrates for eicosanoid metabolism. This results in the production of eicosanoids with lower inflammatory potential than those eicosanoids produced by the omega-6 fatty acid series.13 Indeed, the 2-series prostaglandins, 2-series thromboxanes, and 4-series leukotrienes that are generated from the omega-6 fatty acid arachidonic acid, have been characterized as proinflammatory, proaggregatory, and thrombotic. This is in contrast to the 3-series prostaglandins, 3-series thromboxanes, and 5-series leukotrienes that are generated from the omega-3 fatty acid eicosapentanoic acid, which has been characterized as anti-inflammatory, anti-thrombotic, and vasodilatory. The ultimate goal of nutritional immunology is to regulate all these modifiable components of the immune system through nutrition, to bring about the desired response.
IMMUNITY IN THE PUPPY
When puppies are born, they emerge from a sterile environment (the uterus) to become exposed to a host of microorganisms, all of which are potentially pathogenic. Unfortunately, the immune system is not fully functional and developed for some time after birth. As a result, newborn puppies are especially vulnerable to infection in the first few weeks of life and require immune assistance in order to survive. This assistance is provided by the bitch, by transfer of immune cells and components through the colostrum and milk which immediately confers some level of immune protection for the newborn. This transfer of immunity from dam to newborn is very important for the newborn's survival.
The immune system then requires time to develop to its fully functional capacity (Figure 2). Both the distribution of immune cell types and their responses have been reported to change as puppies grow and develop. T cell populations are significantly smaller and their proliferation response to stimulation is less in puppies, compared with that in adult dogs. Only by 16 weeks of age, puppies have been reported to possess lymphocyte populations similar to that of healthy adult dogs.
Unfortunately, puppy losses do occur during growth and development and mainly during specific times, including in utero, at birth, immediately after birth, and immediately after weaning. Losses during this postweaning period are typically the result of disease brought on by a compromised immune system. Therefore, a stronger immune system as early as possible can help puppies grow and develop into healthy adult dogs.
A recent Iams-sponsored study14 showed that puppies weaned (6 weeks of age) on a diet supplemented with the antioxidants vitamin E, beta-carotene, and lutein had higher levels of T cell activation (Figure 3) at 14 and 22 weeks of age when compared to their age-matched controls (puppies weaned on a diet containing standard vitamin E levels and no added lutein or beta-carotene). This effect was also seen for B cell activation (Figure 4). Puppies fed the antioxidant-supplemented diet also were seen to produce higher antibody levels to specific vaccines such as distemper, parvovirus, and parainfluenza (Figure 5).
To summarize, puppies can benefit from a boost in immune function since they possess a lower level of immune response when compared with adult dogs. During this vulnerable period, puppies are at a higher risk for developing disease. Previous research in adult dogs, as well as other species, shows that nutritional supplementation can influence immune function. This study showed that in puppies, dietary supplementation with antioxidants can improve both cell-mediated (T and B cell response) and humoral immune function (antibody production) which enhances the responses necessary to protect puppies against infectious disease.
EXERCISE AND IMMUNITY
Once puppies have successfully grown into young adult dogs they may undergo a variety of stressful situations. These may include stresses of travel, competing in the show ring or participating in exercise during sporting events. The effect of exercise on immunity is a stress situation that has recently received research attention. Although exercise in the long term is beneficial (lower percent body fat, higher percent lean body mass, improved cardiovascular system), acute levels of exercise produce short, but intense bursts of oxidative products such as free radicals. Increased levels of free radicals have been theorized to suppress various parameters of immune function. Indeed, many studies in the field of exercise immunology have reported fluctuations with immune cell numbers and function.
Natural killer cells are part of the innate immune system, and as such act as the first barrier of defense against pathogens that breach the body's physical barriers. These cells are involved in the early response to viral infection and tumor growth. Natural killer cell cytotoxic activity increases acutely and proportionately with exercise intensity, and then returns to resting levels soon after brief to moderate exercise.15,16 However, it continues to decline and remains below resting levels for up to 6 hours following intense and prolonged exercise.17
Neutrophils, which are also known as polymorphonuclear leukocytes, represent 50 to 60% of the total circulating leukocytes and also constitute part of the first line of defense against infectious agents. Once an inflammatory response is initiated, neutrophils are the first cells to be recruited to sites of infection or injury. Their targets include bacteria, fungi, protozoa, viruses, virally infected cells, and tumor cells. Studies have suggested that although acute exercise stimulates neutrophil function, prolonged periods of intense exercise are associated with downregulation of neutrophil function.18
Macrophages are a first line of defense against pathogens and malignancies by nature of their phagocytic, cytotoxic, and intercellular killing capacities. Ceddia and Woods demonstrated that exhaustive exercise suppressed macrophage function for up to 24 hours post-exercise.19 This suppression in macrophage function was due to the inability of macrophages to degrade athogens.20
Lymphocytes are also influenced by exercise. Lymphocyte stimulation has been reported to be particularly sensitive to exercise-induced changes. Brief, moderate exercise has little effect (it may actually slightly stimulate lymphocyte activation), but intense or prolonged exercise suppresses the proliferative response for up to 3 hours.17 The effect of intensive exercise on oxidative stress was examined in sled dogs. Several studies have examined the levels of oxidative stress markers released in the blood of sled dogs during a three day exercise bout (15-20 mile race per day for three days).21-23 During this exercise period the authors noted increases in serum uric acid, isoprostane levels,22 serum 7, 8-dihydro-8-oxo-2'deoxyguanosine, and an increase in the lag time of in vitro oxidation of lipoprotein particles.23 These results indicate an increase in free radical production due to the exercise regime.
Due to the increase in oxidative stress noted in the sled dog, it was of interest to determine if there is an effect on the immune system similar to that reported in other species.7 In this study, 62 trained sled dogs were randomized to either a sedentary (n=22) or exercised group (n=21) or an exercised group receiving supplemental antioxidants (n=19). All dogs were fed a commercially available diet containing 35% protein, 30.8% fat, 23.1% carbohydrates, and an omega-6 to omega-3 fatty acid ratio of 5.9:1. Antioxidant supplementation consisted of 1 biscuit per day containing 21.6 mg beta-carotene and 18.4 mg lutein as well as 400 IU of alpha-tocopherol in the form of a softgel capsule.
Similar to observations in other species, several immune indices were altered due to the 3-day exercise session. The proportion of blood neutrophils were increased while the proportion of lymphocytes, eosinophils, and monocytes were decreased. Also a decrease in lymphocyte activity and alterations in the proportions of T cells and B cells were noted. Lastly, exercise resulted in an increase in the blood levels of acute phase proteins indicating that the exercise resulted in a generalized inflammatory response. Supplementation with antioxidants resulted in a normalization of the acute phase proteins as well as the proportions of certain T cells and B cells. These data demonstrate that supplementation with antioxidants result in alleviating some of the effects of exercise on the immune response.
AGING AND IMMUNITY
The dysregulation in immune function is a well-documented consequence of aging. This can lead to an increased incidence of morbidity (illness) and mortality (death). Cell-mediated immunity is clearly the component of the immune system most adversely affected with advancing age, primarily T cells. Age-related T cell immunity dysfunction has been implicated as the cause of many chronic degenerative diseases in elderly humans, including arthritis, cancer, autoimmune diseases, and increased susceptibility to infectious diseases.
There are many theories that have been put forth to try and explain the mechanism(s) responsible for this decline, but no one theory can fully account for all the changes observed. The free radical theory of aging is particularly interesting. This theory is based on the premise that a single common process, modifiable by genetics and environmental factors, is responsible for the aging and death of all living things. Proposed by Harmon in 1956,24 this theory suggests that aging is caused by free radical reactions and accumulation of reactive oxygen by-products.
As explained previously in this chapter, free radical production and accumulation can have several damaging effects on various cells, including those of the immune system. Therefore, much research with aging animals has been done looking at dietary antioxidants as a means of reducing free radical reactions and accumulation.
Senior dogs have been reported to show a decreased immune system response compared to younger dogs (Figure 6). Older dogs also differ in the make up of their immune system compared to younger dogs. Based on these observations, the aging process results in a dysregulation of the immune response in dogs too, as is similar with other species. Studies recently conducted by Iams have reported benefits from feeding senior dogs a diet supplemented with beta-carotene (Figure 6).
In conclusion, there is ample evidence to warrant nutritional support of the immune system through all walks of life. There are not only issues with immune function through growth and development in the puppy and through decline in the senior dog, there are also issues that arise during certain conditions such as exercise. Studies have reported that nutrition, especially antioxidant supplementation, can help in all these scenarios. However, it is important to realize some of the dynamics behind antioxidant supplementation.
Studies have shown a dose-response with antioxidants like vitamin E and beta-carotene. At very high and very low levels, these antioxidants lose their effectiveness. There appears to be an optimal level for these compounds as far as immune function is concerned. Eukanuba™ dog foods are formulated with important antioxidants, such as vitamin E.
In summary, antioxidants such as vitamin E, beta-carotene, and lutein can improve several markers of immune function, thereby lowering the risk for infectious disease and ultimately helping the dog to remain productive and stay healthy from puppyhood into senior years.
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