Physiological Functions

Selenium is a component of glutathione peroxidases which are primarily responsible for reducing peroxide free radicals that include lipid peroxide formation in cell membranes. Reduction of peroxides formed by oxidation of membrane phospholipids breaks the auto-oxidative chain reaction that damages cell membranes. Selenium-dependent antioxidant protection of cell membrane phospholipids is synergistic with vitamin E. Selenium also has a role in prostaglandin synthesis by protecting the oxidative state of lipid intermediates formed during cyclooxygenase reactions which determines the balance of the end products and whether proaggregatory, pro-inflammatory or anti-aggregatory, anti-inflammatory responses will dominate.

Factors Affecting Availability

Food sources provide selenium in either the inorganic forms selenite or selenate or in an organic form where it displaces sulfur in methionine or cysteine. Soil selenium content determines the amount of selenium concentrated in plant sources which can vary as much as 200-fold between crops grown in different regions. Since produce and grain products consumed in the US are obtained from various regions, the average selenium intake from plant sources is similar between different geographical areas. Processing of grains decreases the selenium content of the grain products. Meat and poultry are more reliable selenium sources because livestock feed is supplemented with selenium.

Deficiency

Clinical selenium deficiency is rarely observed in humans. Marginal intakes may reduce activities of selenium-dependent peroxidases. Changes in these enzyme activities have been associated with development of Keshan Disease (cardiomyopathy) and Kashin-Beck Disease (chondrodystrophy) in children in selenium-deficient regions of China. Selenium deficiency can also develop in malnourished patients dependent on enteral or parenteral nutrition for long periods of time. Muscle pain, weakness, and tenderness have been reported by these patients. Selenium supplementation can correct this type of deficiency.

Toxicity

Excessive amounts of selenium (> 750 mcg/day) can cause nausea, vomiting, diarrhea, loss of hair and nails, tenderness and swelling of the fingers, fatigue, irritability, skin lesions, tooth damage and nervous system disturbances.

Supplementation that provides > 3 times the DRI is more likely to cause toxicity than what is consumed from dietary sources. Organic selenium supplements (selenomethionine and selnocycsteine) are better absorbed than inorganic forms. Symptoms of selenium toxicity include dermatitis, loose hair, brittle nails, and peripheral neuropathy.

The upper limit of safety for selenium established by the Food and Nutrition Board of the Institute of Medicine is 400 micrograms daily for adults. See table below for more age- and gender specific guidelines.

Selenium Tolerable Upper Intake Levels

Life Stage	Selenium(mcg/day)
Infants
     0-6 months	45
     7-12 months	60
Children
     1-3 years	90
     4-8 years	150
Males, Females
     9-13 years	280
     14-18 years	400
     19-70 years	400
     70 years	400
Pregnancy
     < 18 years	400
     19-50 years	400
Lactation
     < 18 years	400
    19-50 years	400

Dietary Requirements

The Daily Reference Intakes (DRI) for selenium are shown in the table below.

Selenium Requirements Daily Reference Intakes

Life Stage	Selenium(mcg/day)
Infants	
     0 - 6 months	15
     7 - 12 months	20
Children	
     1 - 3 years	20
     4 - 8 years	30
Males	
     9 - 13 years	40
     14 - 18 years	55
     19 - 30 years	55
     31 - 50 years	55
     51 - 70 years	55
     > 70 years	55
Females	
     9 - 13 years	40
     14 - 18 years	55
     19 - 30 years	55
     31 - 50 years	55
     51 - 70 years	55
     > 70 years	55
Pregnancy	
      < 18 years	60
     19 - 30 years	60
     31 - 50 years	60
Lactation	
     < 18 years	70
     19 - 30 years	70
     31 - 50 years	70

Dietary Sources

Selenium is commonly found in Brazil nuts, seafood, kidney, liver, meat, poultry, whole grain pasta, sunflower seeds, oatmeal, soy nuts, other nuts, eggs and low-fat dairy products. The list below provides the selenium content of selected foods.

Selenium Content of Food

Food	         Selenium (mcg)
Brazil nuts, 1/4 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, 1/4 cup        28
Molasses, blackstrap, 2 T. 	25
Sunflower seeds, 1/4 cup	26
Cooked oatmeal, 1 cup	19
Soy nuts, 1/2 cup       	17
Freshwater fish, 3.5 oz	15
Egg, boiled, one        	13
Tofu, 1/2 cup       	11