A.L. Moxon Honorary Lectures

Special Circular 167-99

Selenium Nutrition in Swine: The Emerging Value of Organic Selenium

Donald C. Mahan
Department of Animal Sciences
The Ohio State University
Columbus, OH 43210.

Introduction

Whenever a rumor starts, regardless of its truthfulness, it is often difficult to stop it from escalating, whereupon it easily gets out of control. This often precipitates a reputation that is difficult to overcome. In relation to selenium, that is exactly what has happened. Where its toxicity was initially shown to result in the death and malformation of animals (Moxon, 1937; Meyer and Buran, 1995), later its inclusion in the diet was discovered to be critical for animal survival and health. Although Se was initially identified as a poison, and still is perceived that way by much of the public, it was not until 1957 when Se was clearly shown to be an essential nutrient (Schwartz and Foltz, 1957). As a side note, but perhaps important from a historical and accuracy point of view, the Ph.D. work of Moxon reported an animal growth response to low dietary Se levels, but this research never received its recognized importance. From these discoveries in history, Se would subsequently be found to prevent major diseases in animals that had plagued the industry for years (Muth et al., 1958; Oldfield et al., 1960). The discoveries that Se was a component of glutathione peroxidase (Rotruck et al., 1973) and the more recent involvement of the seleno-enzyme iodothyronine 5'-deiodinase (Behne et al., 1990) in thyroid function, provides solid and important evidence regarding biological function of SE in the animal.

Organic Se --- Nature's Source for Animals and Humans

Selenium, as found in most non-accumulator grains, is in an organic form. Schwartz and Foltz (1957) discovered by feeding brewers yeast that a liver abnormality could be corrected in rats. Brewers yeast was later found to contain a vital component (i.e., Factor 3) which initially established Se as an essential nutrient. Consequently, its discovery as a dietary essential was established from its organic origin. Olson et al. (1970) later demonstrated that selenomethionine was the principal form of Se in wheat. Thus, this seleno amino acid is considered to be the major Se constituent in most non-accumulator plants. Because it had been a common practice prior to 1970 to house animals outdoors, particularly reproducing animals, and to formulate diets using animal and forage products unknowingly with an indigenous source of organic Se, the deficiency occurrence of this element was not readily apparent until livestock confinement was practiced. Upon the discovery that the liver and white muscle problems encountered with livestock were a Se deficiency, the provision of injectable or dietary Se became a necessity. It was the inorganic form of the element, notably sodium selenite, that became the commonly used form of supplemental Se. The inorganic form (sodium selenite or sodium selenate) was subsequently approved by the United States Food and Drug Administration (FDA, 1974). Although initially approved at .10 ppm, it was later increased to a supplemental level of .30 ppm (FDA, 1982). Although the higher level was expected to prevent the occurrence of Se deficiencies and to achieve maximum glutathione peroxidase activity in livestock, deficiency occurrences still persist today, albeit at lower frequencies.

A regional study by Ku et al. (1972) clearly demonstrated wide differences in tissue Se contents when swine were fed diets formulated with grains grown in different regions of the United States (Figure 1). This demonstrated not only regional differences existed between Se storage reservoirs in animals, but also helped to explain why deficiency occurrences differed by regions within the United States. Subsequent research by Meyer et al. (1974) demonstrated that .30 ppm inorganic Se maximized glutathione peroxidase activity in weaned pigs, but the work of Ku et al. (1973) also demonstrated that when sodium selenite was supplemented to a diet that already contained a high Se content from natural organic Se, very little of the added inorganic Se was retained (Table 1). This suggested that the utilization of Se sources by the body differed. With the recognition that Se's transfer through the placenta was minimal (Mahan and Kim, 1996), and that sows had lower milk Se contents as they aged (Mahan, 1991; 1994), the importance of body Se reservoirs in the prevention of the deficiency onset and the possibility that this element may have a role in animal longevity in the herd became evident.

Figure 1. Relationship of Natural Dietary Se on Loin Se Content

Various plant, fish and animal products can contain relatively high concentrations of Se. Only recently a strain of yeast was discovered that metabolically replaces S with Se, thus incorporating high concentrations of Se into the yeast cell. Consequently, the importance and subsequent role of organic Se in animal and human nutrition escalated. Later confirmation that the Se was in the protein component of the yeast cell wall demonstrated that Se was deposited in high concentrations (> 1,000 ppm), and that it had an organic seleno amino acid profile (Kelly and Power, 1995). This suggested that this Se source may be helpful in increasing body tissue Se reservoirs, in preventing the deficiency occurrence in livestock and would be added at low dietary inclusion rates. The incorporation of the element into the organic structure of selenoproteins is now showing additional evidence that the role of organic Se in several biological mechanisms may surpass that of the inorganic form. Organic Se may thus be one of the important nutritional keys to improved human and animal health in the 21st Century. Because the amount of research conducted with organic Se from the Se-enriched yeast or other organic sources is not of the magnitude that the inorganic form of the element enjoys, many questions have yet to be asked and answered before we completely understand its role in animal nutrition.

Organic and Inorganic Se --- Its Metabolism in Pigs

Sodium selenite is a water soluble compound, while the organic Se compounds are components of an organic mixture of amino acid analogs (40 to 50% selenomethionine). Consequently, the digesti-bility and absorption mechanisms differ between Se forms. In a recent study evaluating digestibility and inorganic or organic Se in growing pigs, it was demonstrated that the urinary excretion was much higher when the inorganic form was fed (Table 2; Mahan and Parrett, 1996). Subsequent research demonstrated that much of the absorbed organic Se was retained in muscle tissue, thus reducing the amount excreted (Mahan and Parrett, 1996). Overall, there was approximately 20% less total Se excreted when the organic form of the element was fed to growing pigs (Figure 2).

Figure 2. Total Excretion of Se From Organic and Inorganic Sources

Sodium selenite does not apparently have a regulatory mechanism for the absorption of Se, except that exposure to certain elements (e.g., S, Pb) in the intestinal tract may result in its chelation, which can reduce its absorption. On the other hand, selenomethionine appears to be actively absorbed in the same manner as does methionine. Dietary levels of either form or the Se status of the animal do not apparently affect its rate of absorption. Upon absorption, selenite Se is rapidly incorporated into the red blood cell, possibly converted to another chemical form, and then extruded back into the plasma. Apparently, a difference exists between species to the degree that Se is held by the erythrocyte, with ruminants having a longer retention of Se in the red blood cell than nonruminants.

Upon absorption, Se is retained by tissues in various priorities. Approximately 40% of total body Se, when from inorganic Se, is used for production and is present as one of the glutathione peroxidase enzymes. This enzyme contains Se and plays an active antioxidant role in the cytosol of the cell where it prevents the accumulation of superoxide molecules and the formation of toxic free radicals. There are several types of glutathione peroxidase (GSH-Px) enzymes produced in the body and each performs a function at the different cellular locations. The one that has been most commonly used to establish overall body status of Se in the pig is the plasma or serum GSH-Px. Research previously has demonstrated that with weanling swine the activity of this enzyme plateaued at approximately .30 ppm Se (Meyer et al., 1974). An experiment evaluating the efficacy of the inorganic and organic forms in grower-finisher pigs on serum GSH-Px activity demonstrated that the dietary level of Se necessary to achieve maximum serum activity was lower than that for weanling pigs (Figure 3). The data also demonstrate that higher dietary Se levels do not further stimulate the production of the enzyme, and that the activity of the enzyme increases with pig age. When organic and inorganic Se were compared, the inorganic Se generally resulted in a somewhat higher GSH-Px activity when fed at lower dietary Se level than does organic Se (Mahan and Parrett, 1996). This suggests that the bioavailability of inorganic Se is perhaps higher for the production of this enzyme than the organic Se source. In addition, the longer the animal is fed a low Se diet, the greater the time lag in achieving similar GSH-Px activities in the serum.

Figure 3. Effect of Dietary Se Levels on Serum GSH-Px Activity (G/F Pigs)

The other body tissues contain variable quantities of Se, depending on the dietary source fed to the animal. Liver tissue has one of the highest priorities for Se deposition, followed by glandular tissue. Liver tissue is considered to contain the most labile form of Se, with Se deficient states demonstrating a rapid decline in liver Se content. Muscle Se content was highly dependant upon Se source. The amount of Se retained in muscle increased as the dietary organic Se level increased but was of a lower magnitude when sodium selenite was the dietary Se source (Figure 4). Because organic Se is largely selenomethionine, or secondly, selenocysteine, the Se from the organic origin was incorporated into the muscle proteins probably as one of these amino acids, whereas the Se from inorganic Se was retained in muscle in lower quantities as a selenoprotein "w" of unknown function. With the increasing concentration of Se into muscle proteins from organic Se, its importance in meat quality and human nutrition becomes of interest.

Figure 4. Effect of Se Source and Levels on Loin Muscle Se Content (Grower Pigs)

Organic Se for Pig Reproduction

Previous research demonstrated that very little vitamin E was transferred through the placenta of the gestating sow. Consequently, pigs are born with low liver tissue reserves of both ?-tocopherol and Se (Mahan, 1991; 1994). Loudenslager et al. (1986) presented evidence that the progeny of sows fed diets low in vitamin E and Se had an antioxidant stress upon them at birth because of their low tissue reserves of both nutrients, and that an iron injection exacerbated the antioxidant stress in these young pigs. Subsequent research has demonstrated that sow milk Se and ?-tocopherol declined with advancing parity (Figure 5; Mahan, 1991; 1994), suggesting the adult sows become more depleted of both nutrients with age, and that their progeny are more prone to the deficiency onset. Consequently, body reservoirs of both nutrients appear to be of importance for sow longevity and for the prevention of the deficiency onset in her progeny. Sows fed diets low in Se and vitamin E have shown higher incidences of mastitis, metritis, agalactia (MMA), which is lowered with supplemental vitamin E and Se (Whitehair and Miller, 1986; Mahan, 1991; 1994).

Figure 5. Selenium Content of Sow Milk - 21 Day Postpartum

The tissue Se reserves of the developing gilt and reproducing sow can be of importance in preventing the deficiency onset in the sow and particularly in her progeny. The Se status of the developing fetus and weanling pig is influenced largely by the Se status of the female pig. The gestation data presented in Figure 6 demonstrated that the transfer of organic Se to the young pig was higher than when inorganic Se was fed. Both forms of Se are transferred through the placenta, as reflected by the tissue Se content of the neonatal pig, but the amount transferred was higher when the organic Se source was fed to the pregnant sow. Consequently, neonatal pigs were at a higher Se status when the organic Se was fed. In addition, the colostrum and milk contents of the lactating sow were higher when organic Se was fed. This subsequently resulted in higher serum, liver and loin Se contents in the young pig at weaning (Figure 7). These data, however, clearly demonstrate that the organic form of the element was superior for both placental and mammary transfer during gestation and lactation, and that the Se status of the pigs at weaning was improved when the organic Se source was fed to the female.

Figure 6. Effect of Inorganic or Organic Se on Neonatal Liver Se

Figure 7. Effect of Inorganic or Organic Se Fed to Sows on Weaned Pig Liver Se

A recent study evaluating the effects of feeding inorganic and organic Se to gilts during the grow-finish period demonstrated that providing the diet with organic Se supplementation throughout the growth phase resulted in higher liver stores of the element (Table 3). The liver is the most labile tissue in the body for Se stores, and a larger reservoir of Se in this tissue at the start of the reproductive cycle will better assure that the gilt will have a longer period of Se adequacy.

To further evaluate the efficacy of Se forms on its transfer to sow milk, an experiment was conducted where various levels of both Se sources were provided from late gestation through lactation. All sows had been fed .3 ppm Se as sodium selenite from breeding to 109 days of gestation, whereupon they were fed one of six treatment lactation diets (Mahan, 1999). The results presented in Figure 8 demonstrated that, within 4 days of feeding the organic Se form to sows, the Se content of the milk increased linearly (P < .01) as the dietary level increased. When the inorganic form of the element was provided, there was a small increase in milk Se, whereas the magnitude of the increase when the organic source was provided was much greater. The effect of feeding both sources at .15 ppm Se resulted in a milk Se content that was almost identical to the milk Se content when sows had been fed .15 ppm Se in the organic form. This suggested that the organic Se form was more effectively transferred across the mammary tissue than the inorganic form. Consequently, organic Se provided the nursing pig with a higher Se supply than what would have been consumed had the sow been fed sodium selenite. Although it is common in situations where the deficiency is encountered to increase the dietary inorganic Se level above .3 ppm, these results suggested there is little benefit to such an adjustment.

Figure 8. Effect of Organic or Inorganic on Milk Se Contents (Ave 7 and 14 days)

Grower-Finisher Swine

Two reports are helpful in identifying organic and inorganic Se's role in meat quality. Munoz et al. (1996) fed grower-finisher pigs an antioxidant supplement, which contained a combination of organic Se, vitamin C and vitamin E. Their results presented in Figure 9 demonstrated that the water-holding capacity of pork loin was affected by Se source. Drip loss increased post-slaughter and was lower when pigs were fed the antioxidant supplement. A recent study by Mahan et al. (1999) demonstrated that there was a higher drip loss from pork muscle when inorganic selenite was fed, whereas when organic Se was provided, drip loss was similar to the pigs being fed the non Se fortified basal diet (Figure 10). The data demonstrated that pigs fed organic Se did not affect drip loss in pork loin, whereas when inorganic Se was fed, there was an apparent detrimental effect on water loss. Evaluating the effect of Se source and Se level on loin color demonstrated an increasingly pale color in pork loins when inorganic Se was provided. No effect was evident when the organic Se source was fed (Figure 11).

Figure 9. Antioxidant Supplementation on Drip Loss from Swine Loin Muscle

Figure 10. Effect of Se Source on Pork Loin Drip Loss

Figure 11. Effect of Se Source on Pork Loin Color (Hunter L Value)

Animal Product Consumption and Effect on Human Health

The amount of Se consumed by humans varies greatly among different regions of the world with the emphasis moving toward an interest in vegetarian diets. Much of the world's indigenous source of Se for humans is from Se-deficient grains or their by-products. Consequently, the possibility of Se deficiencies in humans will undoubtedly increase. The data available demonstrate that an average consumption of Se in most of the world averages < 200 µg per day. Perhaps one of the better ways to supply an adequate quantity of Se to the human is through meat, milk and eggs. Because pork is consumed in large quantities throughout much of the world, it, along with other animal products, would be an excellent way of providing Se in the humans diet.

The quantity of Se needed by humans has not been accurately determined. Perhaps as important is the amount and form of Se that will enhance human health. Recent studies demonstrated that organic Se may have an important role in preventing the rate of cancer development in humans. A 10-year study reported by Clark et al. (1997) suggested that when 200 µg of organic Se was consumed daily, the incidence of several types of cancers was reduced by approximately 50%. The two cancers that were more noticeably reduced were colorectal and prostate cancer (Figure 12).

Figure 12. Incidence of Cancer in Placebo and Se Treated Subjects

Summary

Although inorganic Se has played a valuable role in supplementing the diets of pigs for the reduction of the Se/vitamin E deficiency that was prevalent during the 1960's and 1970's, the deficiency still persists even when unusually high (> 100 IU/kg) vitamin E, and in some cases, high (> .3 ppm) Se levels are added to pig diets. With the advent of organic Se, the pig's ability to retain higher percentage of Se with a higher retention in all body tissues suggests that the organic form may have a more important future role in swine production than inorganic Se. Little is known about the advantages of organic Se, but it enhances placental Se transfer and increases mammary transfer of Se. Both aspects result in improving the Se status of the young pig during the critical stages when the deficiency onset is most prevalent. Evidence exists that inorganic Se may be detrimental to pork quality, by yet unknown mechanisms. Organic Se fed to humans has been shown to reduce several forms of cancer.

References

Behne, D., A. Kyriakopoulos, H. Meinhold and J. Kohrle. 1990. Identification of type I iodothyronine 5?-deiodinase as a selenoenzyme. Biophys. Res. Comm. 173:1143.

Clark, L. C., G. F. Combs, Jr., B. W. Turnbull, E. Slate, D. Chalker, J. Chow, L. Davis, R. Glover, G. Graham, E. Gross, A. Kongrad, J. Lesher, K. Park, B. Sanders, C. Smith and J. Taylor. 1997. Effects of Se supplementation for cancer prevention in patients with carcinoma of the skin: a randomized clinical trial. J. Am. Med. Assoc. (Cited by Combs, 1997).

FDA. 1974. Food additives permitted in feed and drinking water of animals: Selenium. Final rule. Fed Reg. 39:1355.

FDA. 1982. Food additives permitted in feed and drinking water of animals: Selenium. Final rule. Fed. Reg. 47:26814.

Kelly, M. P. and R. F. Power. 1995. Fractionation and identification of the major selenium compounds in selenized yeast. J. Dairy Sci 78(Suppl. 1):237 (Abstr.).

Ku, P. K., W. T. Ely, A. W. Groce and D. E. Ullrey. 1972. Natural dietary selenium, ?-tocopherol and effect on tissue selenium. J. Anim. Sci. 34:208.

Ku, P. K., E. R. Miller, R. C. Wahlstrom, A. W. Groce, J. P. Hitchcock and D. E. Ullrey. 1973. Selenium supplementation of naturally high selenium diets for swine. J. Anim. Sci. 37:501.

Loudenslager, M. J., P. K. Ku, P. A. Whetter, D. E. Ullrey, C. K. Whitehair, H. D. Stowe and E. R. Miller. 1986. Importance of diet of dam and colostrum to the biological antioxidant status and parenteral iron tolerance of the pig. J. Anim. Sci. 63:1905.

Mahan, D. C. 1991. Assessment of the influence of dietary vitamin E on sows and offspring in three parities: reproductive performance, tissue tocopherol, and effects on progeny. J. Anim. Sci. 69:2904.

Mahan, D. C. 1994. Effects of dietary vitamin E on sow reproductive performance over a five parity period. J. Anim. Sci. 72:2870.

Mahan, D. C. 1999. Effect of organic and inorganic selenium sources and levels on sow colostrum and milk composition J. Anim. Sci. (submitted)

Mahan, D.C. and Y.Y. Kim. 1996. Effect of inorganic or organic selenium at two dietary levels on reproductive performance and tissue selenium concentrations in first parity gilts and their progeny. J. Anim. Sci. 74:2711.

Mahan, D.C. and N.A. Parrett. 1996. Evaluating the efficacy of Se-enriched yeast and inorganic selenite on tissue Se retention and serum glutathione peroxidase activity in grower and finisher swine. J. Anim. Sci. 74:2967.

Mahan, D. C., T. R. Cline and B. Richert. 1999. Effects of dietary levels of selenium-enriched yeast and sodium selenite selenium sources fed to grower-finisher pigs on performance, tissue selenium, serum glutathione peroxidase activity, carcass characteristics and loin quality. J. Anim. Sci. (submitted)

Meyer, R. D. and R. G. Buran. 1995. The geochemistry and biogeochemistry of selenium in relation to its deficiency and toxicity in animals. In: Selenium in the Environment: Essential Nutrient, Potential Toxicant. Proc. National Symposium. Univ. CA., p. 38.

Meyer, W. R., D. C. Mahan and A. L. Moxon. 1981. Value of dietary selenium and vitamin E for weanling swine as measured by performance and tissue selenium and glutathione peroxidase activities. J. Anim. Sci. 52:302.

Moxon, A. L. 1937. Alkali disease, or selenium poisoning. South Dakota Agriculture Experiment Station Bulletin 311. South Dakota State University, Brookings, 91 p.

Munoz, A., M. D. Garindo and M.V. Granados. 1997. Effect of selenium yeast and vitamins C and E on pork meat exudation (cited by T.P. Lyons In: Biotechnology in the Feed Industry, 14th Annual Symposium, p. 1.

Muth, O. H., J. E. Oldfield, L. F. Remmert and J. R. Schubert. 1958. Effects of selenium and vitamin E on white muscle disease. Science 128:1090.

Oldfield, J. E., J. R. Schubert and O. H. Muth. 1960. Selenium and vitamin E as related to growth and white muscle disease in lambs. Proc. Soc. Exp. Biol. and Med. 103:799.

Olson, O. E., E. J. Novacek, E. I. Whitehead and I. S. Palmer. 1970. Investigation on selenium in wheat. Photochemistry 9:1181.

Rotruck, J. T., A. L. Pope, H. E. Ganther, A. B. Swansown, D. G. Hafeman and W. G. Hoekstra. 1973. Selenium: Biochemical role as a component of glutathione peroxidase. Science 179:588.

Schwartz, K. and C. M. Foltz. 1957. Selenium as an integral part of Factor 3 against dietary necrotic liver degeneration. J. Am. Chem. Soc. 79:3292.

Whitehair, C. K. and E. R. Miller. 1985. Vitamin E and selenium in swine production In: Selenium Responsive Diseases in Food Animals. p. 11. West States Vet. Conf.

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