Role of Selenium in Pets Health and Nutrition: A Review

INTRODUCTION

Selenium, like the other trace minerals is necessary to sustain life and is essential for basic physiological functions in both animals and humans. While the daily requirement for this mineral is obviously less, its importance and impact on the health and well being of livestock and humans are well documented in research. It was discovered in 1817 by Swedish chemist (Mugesh and Singh, 2000). It was considered as a poison until identified as a micronutrient for bacteria, mammals and birds (Schwarz and Foltz, 1957). It has been found to be present in at least 15 different mammalian selenoproteins and up to seven microbial selenoenzymes so far (Sunde, 1997).

The animal body is under constant attack from free radicals, formed as a natural consequence of the body’s normal metabolic activity and as part of the immune system’s strategy for destroying invading microorganisms. It has been calculated that about 2x10¹⁰ molecules of Reactive Oxygen Species (ROS) are generated per cell per day (Chance et al., 1979). These reactive species are involved in the initiation, propagation and maintenance of both acute and chronic inflammatory processes (Halliwell et al., 1982; Mugesh and Singh, 2000). Ultimately this leads to aging, Alzheimer disease, inflammation and certain types of cancer. There is a need for defensive action against these reactive species. An antioxidant may be defined as any substance that when present at low concentrations, compared with those of the oxidizable substrate, significantly delays or inhibits oxidation of that substrate (Gutteridge, 1994)_. Antioxidants play an important role since they protect the cells from oxidative cell damage. Scientific evidence suggests that antioxidants reduce the risk for chronic diseases including cancer and heart disease. The main characteristic of an antioxidant is its ability to trap free radicals. Antioxidants which scavenge the active oxygen species are found in variety of food stuff. Selenium has been known to be intimately involved in the activity of enzymes such as glutathione peroxidase (Gpx) and thioredoxin reductase and protect the biomolecules against reactive oxygen species and free radical damage (Nordberg and Arner, 2001).

Dietary sources are the good sources of selenium for the body and supplements are needed for the consumers living in the areas that are deficient of selenium. Selenium deficiencies have been reported to suppress the immune response in various species (Sheffy and Schultz, 1979). Selenium deficiency reduces T-cell dependent antibody responses, which further gets magnified with vitamin E deficiency. Several metabolic disorders have been reported due to the dietary deficiencies of selenium and vitamin E in several species like chickens (Schwarz et al., 1957), turkeys (Scott et al., 1967), rats (Schwarz and Foltz, 1957), calves, lambs with several health disorders. Current selenium supplements rely on inorganic forms such as sodium selenite (Na₂SeO₃) or sodium selenate (Na₂SeO₄). Organoselenium compounds have been found to be an alternative to inorganic selenium compounds with greater bioavailability (Jacob et al., 2004). More importantly, organic selenium is usually found to be less toxic than inorganic forms (Narajji et al., 2007; Arenholt-Bendsleve et al., 1988). The margin between its protective role and adverse effect is very low and depends on the form of selenium being used.

Role of selenium in dogs: Research studies to date have indicated that selenium does have beneficial physiological effects on mammals. For example, it is known that selenium, when ingested, reduces the rate of oxidative damage caused by chemicals, by entering the membranes of the body's cells and protecting the contents of the cells from reacting with oxygen in a manner that damages the cells. Selenium deficiency may be associated with a myopathy in dogs (Manketlow, 1963). The diet of these dogs was principally mutton from an area of New Zealand where selenium-responsive diseases of sheep were noted. A fatal, myocardial necrosis was seen in young pups and a skeletal myodegeneration in an adult dog. Two bitches that had lost litters during previous perinatal periods were dosed with selenium during pregnancy and subsequently whelped normal litters.

Synergistic effect of selenium with vitamin E was observed when administered together to Beagles. It was demonstrated that Beagles which were initially 5 to 8 weeks old, developed clinical signs of vitamin E-selenium deficiency after 40 to 60 days of consuming an unsupplemented semisynthetic diet (Van Vleet, 1975). Generalized muscular weakness progressed from unsteadiness to prostration and coma. In a case study it was observed that multiple severe deaths due to acute myocardial degeneration occurred in a commercial kennel. Further losses were prevented when vitamin E and selenium supplementation was instituted (Green and Lemckert, 1977).

Selenium has been found to reduce incidences of cancer in both dogs and human. Selenium supplementation has shown to decrease DNA damage and increase epithelial cell apoptosis within the aging canine prostate (Waters et al., 2003). Research suggests that selenium also helps with improving long-term joint health and can reduce risks of Kashin-Beck disease (Sudre and Mathieu, 2001; Levander and Beck, 1997; Beck et al., 2003) which involves the articular cartilage between joints degenerating, thyroid disease and cancers. Selenium is also thought to help prevent hip dysplasia (Hamilton, 1999).

Selenium can increase the health of the skin, potentially reducing dandruff and dry skin. It plays an important role in hair growth. Selenium can also improve the hair coat quality, making it more soft and shiny. As a result of a healthier coat, there is also the possibility of less shedding and hair loss. In a study conducted by Yu et al. (2006) it was demonstrated that both low and high selenium in diet reduced hair growth in adult dogs.

Pet animals obtain dietary selenium from cereals and grains, or from the tissues of other animals, depending on dietary habit. The forms of dietary selenium from both plant and animal sources include a range of inorganic and organic selenium compounds (Whanger, 2002). The primary form of selenium in plants is selenomethionine, together with smaller amounts of selenocysteine and selenite. The forms of selenium found in animals include selenoproteins (formed from biologically active selenocysteine, e.g., glutathione peroxidase, selenoprotein P), Se-containing proteins (formed from non-specific incorporation of selenomethionine or selenocysteine) as well as nonprotein and inorganic selenium (selenite, selenate) and methylated selenium (forms that are excreted) (Lobinski et al., 2000).

Biochemical role of selenium in mammals is clearly established. It forms the active center for selenoenzymes. Glutathione peroxidase is one such enzyme which contains selenol (-SeH) group in the active center. Glutathione peroxidase catalyzes the oxidation of reduced glutathione and allows for the reduction of hydrogen peroxide to water, preventing lipid peroxidation and cellular damage (Rotruck et al., 1973). The GPx catalytic site includes a Selenocysteine residue in which the selenium undergoes a redox cycle involving the selenol (ESeH) as the active form that reduces hydrogen peroxides and organic peroxides. Selenocysteine is known as 21st amino acid (Rayman, 2005). It is located in the N-terminal end of helix α₁ (Epp et al., 1983). The selenol is oxidized to selenenic acid (ESeOH), which reacts with reduced glutathione (GSH) to form a selenenyl sulphide adduct (ESeSG). A second glutathione then regenerates the active form of the enzyme by attacking the ESeSG to form the oxidized glutathione (GSSG). Thus, in the overall process, two equivalents of glutathione are oxidized to the disulphide and water, while the hydroperoxide is reduced to the corresponding alcohol as shown in Fig. 1 (Roy et al., 2005).


Fig. 1:	Schematic diagram of proposed catalytic mechanism for reduction of hydroperoxides by Gpx

Table 1:	Selenium levels in different food items

*DV: Daily value. DVs are reference numbers developed by the Food and Drug Administration (FDA) to help consumers determine if a food contains a lot or a little of a specific nutrient. The DV for selenium is 70 μg. Most food labels do not list a food's selenium content. The percent DV (%DV) listed on the table indicates the percentage of the DV provided in one serving. A food providing 5% of the DV or less is a low source while a food that provides 10-19% of the DV is a good source. A food that provides 20% or more of the DV is high in that nutrient. It is important to remember that foods that provide lower percentages of the DV also contribute to a healthful diet. For foods not listed in this table, please refer to the U.S. Department of Agriculture's Nutrient Database Web site: http://www.nal.usda.gov/fnic/cgi-bin/nut_search.pl

Finally glutathione disulfide gets reduced by NADPH which is catalyzed by glutathione reductase. This results in the formation of glutathione.

Nutritional forms of selenium: The Brazil nut (Bertholletia excelsa) which is a South American tree in the family Lecythidaceae have relatively high selenium concentrations (Table 1). Foods of low protein content, including most fruits and vegetables, provide little selenium (Simcock et al., 2005).

Selenium and pet food diets: Fish, meat, poultry, whole grains and dairy products are typical sources of this nutrient. AAFCO and the FDA have approved a selenium supplement to animal diets, most commonly in the form of sodium selenite for pet foods. It has been observed that inorganic selenium is less available when the animal is under stress. In addition, there is a significant loss of selenium after periods of stress, making selenium levels in the blood less stable. Depending on the nature of ingredients used in pet food formulations, the selenium levels vary in all diets. Research (Fan and Kizer, 1990; Olson, 1986) proved that inorganic selenium sources can be toxic in high doses; affecting an animal’s blood, liver and muscles. Inorganic selenium cannot be fully metabolized or stored in the body. Consequently, selenium deficiencies still arise in animals that are supplemented with inorganic selenium (Lopez et al., 1969).

SELENIUM METABOLISM IN MONOGASTRIC SPECIES

The wild dog would consume selenium obtained from animal tissue, however as the wild dog would not usually eat a whole carcass at one time, the amount and form of selenium ingested depends on the part of the animal consumed and its metabolic pathway as indicated in Fig. 2. For example the liver is a primary site of selenoprotein synthesis and contains selenoprotein P, glutathione peroxidase and other functional selenoenzymes. In contrast, the gastrointestinal tract may contain plant material that the prey animal has eaten and therefore may contain inorganic selenite or selenate, as well as organic selenomethionine.

Since, cat generally consumes a diet of whole prey such as rats or mice, the forms of selenium ingested would be in the form of functional selenoproteins such as glutathione peroxidase and selenoprotein P, the selenoamino acids selenomethionine and selenocysteine, as well as non-functional stored selenium that has been incorporated into body proteins such as skeletal muscle, hair and nails. The AAFCO has defined he minimum selenium requirement in cats and dogs (Table 2).

Hence, animal tissues play a primary role as source of dietary selenium for both cats and dogs, i.e., the organic forms of selenomethionine and selenocysteine. The pets which are fed with commercial diets today are somewhat different from what their ancestors would have eaten. Most of these diets contain a high proportion of plant material and for the cat as a true carnivore this may not be particularly suitable. Selenium concentrations in pet foods is highly variable.

Most dry and canned dog foods today use an inorganic type of selenium, sodium selenite or sodium selenate. In addition to selenium present in the pet food ingredients, additional selenium sources are added in commercial diet formulations. Selenite (Forceville and Meaux, 2007) was found to have a pro-oxidant effect and therefore the use of selenate was preferred.


Fig. 2:	Overview of selenium metabolism in monogastric species

Table 2:	Selenium nutrient profiles as published in 2008^a by AAFCO for dog and cat food

However because inorganic selenium cannot be stored in the body, organic forms of selenium are being increasingly used as they are safer and more efficiently used in the body.

CONCLUSIONS

The role of trace mineral selenium in animals particularly pets is discussed and reviewed. Selenium deficiency is found to be one of the key factor behind many diseases like Kashin beck disease, cardiomyopathy, etc., Selenium levels and its forms in the diets play an important role in managing the trace mineral level in the body of the pet animal. However, toxicity associated with the high dose of selenium makes it vulnerable to pets and other animals due to the narrow difference between its required dose and the toxic dose. Current selenium supplements are mainly dependent on inorganic sources like sodium selenite which are found to be less bioavailable and also toxic. However, relative uses of selenium and its forms would be dependent on its nature of application and end use requirement. Keeping safety of the pet animals and environment as main focus areas, organoselenium compounds would be a good and alternate prospective choices for research scientists working in pet animal nutrition.