Selenium
 

Selenium Introduction

Selenium is an essential trace mineral and critical component to the many biochemical processes taking place within human physiology. In the mid 1900’s, selenium was thought to be a toxic mineral. Current research has disproved this notion, as selenium is only found in trace amounts in our systems. Selenium has garnered significant notoriety among the medical community within the past two decades and researchers continually stress the overall importance of this mineral. Selenium affords the body protection, not only by means of antioxidant defense, but also by playing a significant role in the detoxification processes correlated within the human body’s enzymatic activities.

Selenium is the key component of the enzyme glutathione peroxidase, or as it more commonly referred to as, GPX. Glutathione peroxidase works synergistically with vitamin E in the reduction of external peroxide free radicals. Free radicals are caused by oxidative stressors, which in turn, lead to the damaging of various cell membrane structures. They are defined as normal by-products of oxygen metabolism that can lead to the damaging of internal cellular structures, and because of this, may eventually lead the development of chronic disease and illness. [1]

Protection from free radicals is just one of many positive factors credited to GPX. GPX assists in achieving homeostasis in the body through proper growth and development, and by aiding in the defense and maintenance of our immune systems. Another point of interest is the seloenzyme, thiredoxin reductase. It has been extensively researched and is more specifically known for its properties concerning oxidative metabolism. [2] This is important in its ability to act as a detoxifier to those found not only in the environment, but also in persons receiving radiation, chemotherapy, and other toxic medicines. This enzyme is especially important in ridding the body of toxic metals such as cadmium, lead, and mercury, as well.

Selenium is also a fundamental component of other mammalian enzymes, particularly phospholipid hydroperoxide glutathione peroxidase (PHGPX); acting as a protective device to peroxides that are already bound to membrane surfaces. [3] More research is necessary, but these findings may help us to understand enzymatic protective abilities, and the diverse and various inhibitions of peroxidative chain branching; both of which are of great importance in reducing the occurrence of chronic illnesses associated with the cardiovascular system.

There are numerous forms of selenium. The food sources we consume either contain inorganic or organic forms of this nutrient. Selenite, or selenate is found in inorganic food sources, where as organic food sources transfer sulfur in either the forms of methionine or cysteine. It seems that selenite is absorbed at a rate of 80 percent or greater, whereas selenomethionine (a source of selenate) has a 90 percent assimilation rate. [4, 5]

Selenium Food Sources

As stated, selenium is very efficiently absorbed by humans, in either inorganic or organic forms. The soil content of selenium where the specific food was harvested is the main determinant in a source’s overall bioavailability of this mineral. Different geographical regions produce different amounts of selenium content.

Vegetable and fruit sources vary tremendously in the amount of selenium they provide. Meat sources are the most reliable forms for obtaining daily demands, as the majority of livestock are supplied with selenium-enriched grains. The graph below is a generalization of selenium content found in a variety of popular consumer foods:

FoodAmount per servingMicrograms (mcg)
Brazil nuts ¼ cup 1036
Oysters 3.5 oz. 115
Chicken liver 3.5 oz 71
Raw oyster 3.5 oz. 70
Steamed clams 3.5 oz 64
Beef liver 3.5 oz. 57
Sardines 3.5 oz. 46
Crab 3.5 oz. 40
Whole wheat pasta 1 cup 36
White pasta 1 cup 30
Wheat germ ¼ cup 28
Molasses, blackstrap 2 Tbs. 25
Sunflower seeds ¼ cup 26
Oatmeal, cooked 1 cup 19
Soy nuts ½ cup 17
Freshwater fish (various) 3.5 oz. 15
Egg, boiled 1 med. 13
Tofu ½ cup 11

Selenium content is easily lost through the preparation of foods, especially when they are processed or refined. Whole food sources, when unprocessed, provide a great amount of obtainable selenium for human digestion.

Selenium Uses

Lower serum levels of selenium are responsible for a myriad of the most popular and well-known chronic and acute conditions of the human body. Selenium has been clinically researched to treat conditions resulting from constant exposure to free radicals. These conditions include, but are not limited to, cigarette smokers, obese persons, and alcoholics. Its effectiveness remains positive in not only these individuals, but also for healthy individuals seeking prevention from possible disease conditions.

Cardiovascular health may be among the top concerns regarding the proper supplementation of this trace mineral. [6] Heart disease remains the number one killer of adults in America. The dietary supplementation of selenium, as well as other potent antioxidants (i.e. vitamin C, vitamin E, and beta-carotene), has been shown to lower total LDL cholesterol levels, while increasing, or maintaining, HDL serum levels in the blood. [7] This may be extremely important as researches speculate that a deficiency of selenium may actually worsen cardiovascular conditions like atherosclerosis, by aiding in the plaque build up on artery walls.

Clinical research indicates that improper selenium intake, or a lack there of, may be responsible for various types of cancers. The soil content of selenium found in the many geographical regions of the world has been extensively studied; especially in relation to the selenium content in foods sources harvested from these soils, and its related impact to the onset of specific forms of cancers. In fact, one particular study showed that supplemental selenium lowered mortality rates and regularity of colon cancer in a particular region of the world where the selenium content of the soil was extremely poor. [8]

The National Institute of Health (NIH) is currently studying over 32,000 male participants in a clinical application designed to evaluate selenium supplementation, and synergistic abilities with other antioxidants, to thwart cancer of the prostate. [9, 10] Results have yet to be determined regarding the effectiveness of antioxidant and selenium supplementation in reducing the risk factors associated with the development of this disease. Selenium in the involvement with other antioxidants, micronutrients, and essential fatty acids, may prove helpful in the prevention and treatment of other types of cancer as well. [11] Those cancers of interest using this supplementation method include; cervical, skin, breast, and colorectal cancers. [12, 13, 14, 15]

Selenium in the form of selenoproteins, are of great importance in the metabolism of our thyroid, and parathyroid glands. [16] They assist in the production and conversion of the thyroid hormones T3 (triiodothyronine) and T4 (thyroxine). The selenium enzyme, iodothyronine deiodinase, is responsible for the conversion of T4 (a thyroxine prohormone) to T3, the most efficient and bioavailable form of the prohormones. [17] A deficiency of selenium may lead to hypothyroidism, or an under active thyroid gland, and reduce its ability to produce these imperative hormones. This can result in lethargy, unwanted weight fluctuations, and/or extreme weight gain.

Immunological benefit may also result from selenium intake. Numerous studies show that selenium is critical in our defense against various illnesses, and on our overall ability to fight against these infections. [18] Deficiencies in selenium may cause an increased incidence for the development of cold or flu. Nutritional supplementation with added zinc has proved equally beneficial in the prevention and increased resistance to specific infections and illnesses. [19]

Persons suffering from immunological disorders such as HIV, or AIDS may derive benefit from selenium when added to existing treatment protocols as well. [20] Weight loss is of great concern in these individuals. In a double-blind clinical study, selenium was used in conjunction with other nutrients (glutamine, beta-carotene, N-acetylcysteine, and vitamins C and E) for no less than a twelve week period. A dramatic increase in weight was seen in those consuming the supplementation as opposed to those receiving placebo. [21] Data suggests that the notable increases in body mass are paralleled to an increase in immunological functioning.

The dietary supplementation of selenium may aid in the fertility of both men and women as they continue to age. Personal assistance with this condition remains, largely, unmentioned because of the embarrassment or negativity one is faced with during preconception. In men certain selenoproteins provide for the formation of sperm. Deficiencies of selenium have also been noted to have a profound impact on sperm motility. [22]. However, more research is necessary regarding the impact of selenium intake upon sperm count in men.

In women, selenium may be directly responsible for the prevention of complications during the trimesters of pregnancy.[23] These problems include the ability to carry a full term pregnancy, and the incidence of a miscarriage during pregnancy.

Supplementation of selenium may also assist in disorders of the eyes, especially in cataracts. [24] Positive selenium levels may also provide for the prevention and formation of conditions such as; Inflammatory Bowel/Gastrointestinal Diseases, diseases and disorders of the liver and pancreas, and various skin conditions (i.e. acne, psoriasis, eczema). [25, 26, 27] Rheumatoid Arthritis and Diabetes mellitus are conditions that have been extensively researched as well, but the findings are inconclusive. [28] More research is necessary to discover if selenium supplementation is the direct cause of improvement in these particular diseases.

Selenium Dosages

It is widely recommended by medical practitioners that persons supplement their daily dietary intake of selenium with vitamin E, A, and beta-carotene. These four nutrients work synergistically with one another, each possessing unique abilities to prevent free radical damage to cellular membranes.

Recommended dietary allowances (RDA) for selenium in adults and children are expressed in milligrams and are as follows:

Age Group (years)Males and FemalesPregnancyLactation
1-3 years 20 mcg
4-8 years 30 mcg
9-13years 40 mcg
14-18 years 55 mcg 60 mcg 60 mcg
19 years + 55 mcg 60 mcg 70 mcg

[29]

Adequate intake for healthy infants who are fed by breast milk/formula::

InfantsFemales (mcg/day)
0-6 months 15
7-12 months 20

Tolerable upper intake levels for infants, children, adolescents, and adults:

Age GroupMales and Females (mcg/day)
0-6 months 45
7-12 months 60
1-3 years 90
4-8 years 150
9-13 years 280
14-18 years 400
19 years + 400

[30]

Selenium Toxicities and Deficiencies

Selenium Deficiencies

Like other trace minerals, a selenium deficiency is extremely rare in the United States. However, the same cannot be said for the many rural villages of China. The selenium content of the soil in various Chinese countrysides is very poor. The human consumption of food sources grown in these regions are inadequate, and fail to meet the amount of selenium needed to meet an individual’s daily requirements. Two major disease conditions occur as a result of this mineral deficiency - Keshan disease, and Kaschin-beck disease.

Keshan disease is characterized by fatigue, cardiac arrhythmia, loss of appetite, cardiomegaly (enlargement of the heart), impairments in heart functioning, and in extreme cases and if the deficiency is left untreated, congestive heart failure. [31]

Kaschin-beck disease is seen via the necrosis of joints. The degenerization of the arms and leg joints and structural changes in the fingers and long bone are often combined with growth retardation and stunting in this condition. [32]

Person’s receiving paternal nutrition with the lack of adequate selenium levels in solution are also at an elevated risk for the development of selenium deficiency.

Selenium Toxicities

Extreme dosages of selenium intake can result in selenosis. [33] Symptoms may include; nausea, vomiting, diarrhea, loss of hair and nails, tenderness of the extremities, irritability, various skin lesions, digestive dysfunction, central nervous system disorders, and a “garlicky” breath odor. [34] It should also be noted that all abnormalities stemming from an overdose in the body can be reversed when regular (or decreased) selenium intake resumes.

References

1. Combs GF, Jr and Gray WP. Chemopreventive agents: Selenium. Pharmacol Ther. 1998;79:179-92.

2. Howie, A.F., Arthur, J.R., Nicol, T., Walker, S.W., Beech, S.G. & Beckett, G.J. 1998. Identification of a 57-kilodalton selenoprotein in human thyrocytes as thioredoxin reductase. J. Clin. Endocrino.I Metab., 83: 2052-2058.

3. Ursini, F., et al. Biochim. Biophys. Acta 839:62-70 (1985).

4. WHO. 1987. Selenium, Geneva, World Health Organization. (Environmental Health Criteria, 58).

5. Patterson, B.H., Zech, L.A., Swanson C.A. & Levander, O.A. 1993. Kinetic modeling of selenium in humans using stable isotope tracers. J. Trace Elem. Electrolyte Health Dis. 7: 117-120.

6. Kendler BS. Recent nutritional approaches to the prevention and therapy of cardiovascular disease. Prog Cardiovasc Nurs. 1997;12(3):3-23.

7. Cheung MC, Zhao XQ, Chait A, Albers JJ, Brown BG. Antioxidant supplements block the response of HDL to simvastatin-niacin therapy in patients with coronary heart disease and low HDL. Aterioscler Thromb Vasc Biol. 2001;21(8):1320-1326.

8. Yang GQ, Xia YM. Studies of human dietary requirements and safe range of intakes of selenium [in China] and their application in the prevention of related endemic diseases. Biomed Environ Sci. 1995; 8:187-201.

9. Brawley OW, Panes H. Prostate cancer prevention trials in the USA. Eur J Cancer. 2000;36(10):1312-1315.

10. Fleshner NE, Kucuk O. Antioxidant dietary supplements: Rationale and current status as chemopreventative agents for prostate cancer. Urol. 2001; 57(4 Suppl 1): 90-94.

11. Combs GF Jr, Clark LC, Turnbull BW. Reduction of cancer risk with an oral supplement of selenium. Biomed Environ Sci. 1997;10(2-3):227-234.

12. Batieha AM, Armenian HK, Norkus EP, Morris JS, Spate VE, Cornstock GW. Serum micronutrients and the subsequent risk of cervical cancer in a population based nested case control study. Cancer Epidemiol Biomarkers Prev. 1993;2(4):335-339.

13. Clark LC, Combs GF Jr, Turnbull BW, et al. Effects of selenium supplementation for cancer prevention in patients with carcinoma of the skin. JAMA. 1996;276:1957-1963.

14. Garland M, Willet WC, Manson, JE, Hunter DJ. Antioxidant micronutrients and breast cancer. J Am Coll Nutr. 1993;12(4):400-411.

15. Psathakis D, Wedemeyer N, Oevermann E, Krug F, Siegers CP, Bruch HP. Blood selenium and glutathione peroxidase status in patients with colorectal cancer. Dis Colon Rectum. 1998;33:1057-1061.

16. Corrilain, B., Contempre, B., Longombe, A.O., Goyens, P., Gervy-Decoster, C., Lamy, F., Vanderpas, J.B. & Dumont, J.E. 1993. Selenium and the thyroid: how the relationship was established. Am J Clin Nutr. Supplement 57: 244S-248S.

17. Berry, Maria J., et al,. Nature 349:438-40 (January 31, 1991)

18. Beck MA, Nelson HK, Shi Q, Van Dael P, Schiffrin EJ, Blum S, Barclay D, Levander OA. Selenium deficiency increases the pathology of and influenza virus infection. FASEB J. 2001;15(8):1481-1483.

19. Girodon F. Impact of trace elements and vitamin supplementation on immunity and infections in institutionalized elderly patients. Arch Int Med. 1999;159:748-754.

20. Dworkin BM. Selenium deficiency in HIV infection and the acquired immunodeficiency syndrome (AIDS). Chem Biol Interact. 1994;91:181-186.

21. Shabert JK, Winslow C, Lacey JM, Wilmore DW. Glutamine antioxidant supplementation increases body cell mass in [HIV] AIDS patients with weight loss: a randomized, double-blind controlled trial. Nutrition. 1999;11:860-864.

22. Sinclair S. Male infertility: nutrional and environmental considerations. Alt Med Rev. 2000;5(1): 28-38.

23. Gabbe SG, ed. Obstetrics-Normal and Problem Pregnancies. 3rd ed. New York, NY: Churchill Livingston; 1996.