People with low levels of selenium have a high risk of asthma.^{1 , 2 , 3 , 4 , 5} Asthma involves free-radical damage⁶ that selenium might protect against. In a small double-blind trial, supplementation with 100 mcg of sodium selenite (a form of selenium) per day for 14 weeks resulted in clinical improvement in six of eleven patients, compared with only one of ten in the placebo group.⁷ Most doctors recommend 200 mcg per day for adults (and proportionately less for children)—a much higher, though still safe, level.

In some studies, people who consumed more selenium in their diet had a lower risk of heart disease.^{8 , 9} In one double-blind report, people who had already had one heart attack were given 100 mcg of selenium per day or placebo for six months.¹⁰ At the end of the trial, there were four deaths from heart disease in the placebo group but none in the selenium group; however, the number of people was too small for this difference to be statistically significant. Some doctors recommend that people with atherosclerosis supplement with 100–200 mcg of selenium per day.

Selenium has been reported to have diverse anticancer actions.^{12 , 13} Selenium inhibits cancer in animals.¹⁴ Low soil levels of selenium, probably associated with low dietary intake, have been associated with increased cancer incidence in humans.¹⁵ Blood levels of selenium have been reported to be low in patients with a variety of cancers,^{16 , 17 , 18 , 19 , 20 , 21 , 22 , 23} including colon cancer.²⁴ In preliminary reports, people with the lowest blood levels of selenium had between 3.8 and 5.8 times the risk of dying from cancer compared with those who had the highest selenium levels.^{25 , 26}

The strongest evidence supporting the anticancer effects of selenium supplementation comes from a double-blind trial of 1,312 Americans with a history of skin cancer who were treated with 200 mcg of yeast-based selenium per day or placebo for 4.5 years, then followed for an additional two years.²⁷ Although no decrease in skin cancers occurred, a dramatic 50% reduction in overall cancer deaths and a 37% reduction in total cancer incidence were observed. A statistically significant 58% decrease in cancers of the colon and rectum was reported.

Little is known about the effects of selenium in the treatment of people with existing cancer. Selenium supplementation was reported to improve immune function in colon cancer patients,²⁸ but no long-term follow-up was done to evaluate whether these patients ultimately lived longer or fared better.

In the double-blind study cited above,²⁹ the large reduction in cancer deaths found in such a short period of time (6.5 years) suggests that these researchers may have been successfully, though unknowingly, treating some people with undiagnosed cancer. However, this speculation has yet to be proven.

Less than optimal intake of selenium may have adverse effects on psychological function, even in the absence of signs of frank selenium deficiency. In a preliminary trial of healthy young men, consumption of a high-selenium diet (226.5 mcg selenium per day) was associated with improved mood (i.e., decreased confusion, depression, anxiety, and uncertainty), compared to consumption of a low-selenium diet (62.6 mcg selenium per day.)³⁰ In a double-blind trial, people who had a low selenium intake experienced greater improvement in depression symptoms after selenium supplementation (100 mcg per day) than did people with adequate selenium intake, suggesting that low-level selenium deficiency may contribute to depression.³¹

A deficiency in the selenium-containing antioxidant enzyme known as glutathione peroxidase has been reported in DH.^{32 , 33} Preliminary³⁴ and double-blind³⁵ trials suggest that supplementation with 10 IU of vitamin E and 200 mcg of selenium per day for six to eight weeks corrected this deficiency but did not lead to symptom improvement in the double-blind trial.

In a preliminary study, individuals with lymphedema of the arm or head-and-neck region were treated with approximately 230 mcg of selenium per day, in the form of sodium selenite, for four to six weeks. A quality-of-life assessment showed an improvement of 59%, and the circumference of the edematous arm was reduced in 10 of 12 cases.³⁶

Selenium deficiency is an independent factor associated with high mortality among HIV-positive people.⁴⁴ HIV-positive people who took selenium supplements experienced fewer infections, better intestinal function, improved appetite, and improved heart function (which had been impaired by the disease) than those who did not take the supplements.⁴⁵ The usual amount of selenium taken was 400 mcg per day.

Selenium deficiency has been found more often in people with HIV-related cardiomyopathy (heart abnormalities) than in those with HIV and normal heart function.⁴⁶ People with HIV-related cardiomyopathy may benefit from selenium supplementation. In a small preliminary trial, people with AIDS and cardiomyopathy, 80% of whom were found to be deficient in selenium, were given 800 mcg of selenium per day for 15 days, followed by 400 mcg per day for eight days. Improvements in heart function were noted after selenium supplementation.⁴⁷ People wishing to supplement with more than 200 mcg of selenium per day should be monitored by a doctor.

Most,^{48 , 49} but not all,⁵⁰ double-blind studies have shown that elderly people have better immune function and reduced infection rates when taking a multiple vitamin-mineral formula. In one double-blind trial, supplements of 100 mcg per day of selenium and 20 mg per day of zinc, with or without additional vitamin C, vitamin E, and beta-carotene, reduced infections in elderly people, though vitamins without minerals had no effect.⁵¹ Burn victims have also experienced fewer infections after receiving trace mineral supplements in double-blind research.⁵² These studies suggest that trace minerals may be the most important micronutrients for enhancing immunity and preventing infections in the elderly.

Most,^{53 , 54} but not all,⁵⁵ double-blind studies have shown that elderly people have better immune function and reduced infection rates when taking a multiple vitamin-mineral formula. In one double-blind trial, supplements of 100 mcg per day of selenium and 20 mg per day of zinc, with or without additional vitamin C, vitamin E, and beta-carotene, reduced infections in elderly people, though vitamins without minerals had no effect.⁵⁶ Burn victims have also experienced fewer infections after receiving trace mineral supplements in double-blind research.⁵⁷ These studies suggest that trace minerals may be the most important micronutrients for enhancing immunity and preventing infections in the elderly.

Premature infants with very low birth weight have an increased susceptibility to infections. In a double-blind trial, premature infants were given either selenium supplements (5 to 7 mcg per 2.2 pounds of body weight) or placebo. Those receiving the selenium supplements had fewer hospital-acquired infections.⁵⁸

Selenium has been reported to have diverse anticancer actions.^{59, 60} Selenium inhibits cancer growth in animals.⁶¹ Low soil levels of selenium (probably associated with low dietary intake), have been associated with increased cancer incidence in humans.⁶² Blood levels of selenium have been reported to be low in patients with many cancers,^{63, 64, 65, 66, 67, 68, 69, 70} including lung cancer.⁷¹ In preliminary reports, people with the lowest blood levels of selenium had between 3.8 and 5.8 times the risk of dying from cancer compared with those who had the highest selenium levels.^{72, 73}

The strongest evidence supporting the anticancer effects of selenium supplementation comes from a double-blind trial of 1,312 Americans with a history of skin cancer who were treated with 200 mcg of yeast-based selenium per day or a placebo for 4.5 years and then followed for an additional two years.⁷⁴ Although no decrease in skin cancers occurred, a 50% reduction in overall cancer deaths and a 37% reduction in total cancer incidence was observed. A 46% decrease in lung cancer incidence and a 53% drop in deaths from lung cancer also occurred. These findings were all statistically significant.

In a double-blind study of infertile men with reduced sperm motility, supplementation with selenium (100 mcg per day for three months) significantly increased sperm motility, but had no effect on sperm count. Eleven percent of 46 men receiving selenium achieved paternity, compared with none of 18 men receiving a placebo.⁷⁵

Based on the personal experience of a doctor who reported his findings,⁷⁶ some physicians recommend vitamin E (400 IU per day) and selenium (50 mcg three times per day). One well-known, nutritionally oriented doctor reports anecdotally that he has had considerable success with this regimen and often sees results in two to six weeks.⁷⁷

People with PKU may be deficient in several nutrients, due to the restricted diet which is low in protein and animal fat. Deficiencies of long-chain polyunsaturated fatty acids (LC-PUFAs),^{78 , 79 , 80} selenium,^{81 , 82 , 83 , 84} vitamin B12,⁸⁵ and vitamin K may develop on this diet.⁸⁶

Selenium is important for normal antioxidant function. Research suggests that selenium deficiency and decreased antioxidant activity may contribute to the brain and nerve disorders associated with PKU.⁸⁷ In two preliminary studies involving selenium-deficient people with PKU, supplementation with selenium in the form of sodium selenite corrected the deficiency,⁸⁸ whereas supplementation with selenium in the form of selenomethionine did not.⁸⁹

Selenium has been reported to have diverse anticancer actions.^{90, 91} Selenium inhibits cancer in animals.⁹² Low soil levels of selenium (probably associated with low dietary intake), have been associated with increased cancer incidence in humans.⁹³ Blood levels of selenium have been reported to be low in patients with prostate cancer.⁹⁴ In preliminary reports, people with the lowest blood levels of selenium had between 3.8 and 5.8 times the risk of dying from cancer compared with those who had the highest selenium levels.^{95, 96}

The strongest evidence supporting the anticancer effects of selenium supplementation comes from a double-blind trial of 1,312 Americans with a history of skin cancer who were treated with 200 mcg of yeast-based selenium per day or placebo for 4.5 years and then followed for an additional two years.⁹⁷ Although no decrease in skin cancers occurred, a dramatic 50% reduction in overall cancer deaths and a 37% reduction in total cancer incidence were observed. A statistically significant 63% decrease in prostate cancer incidence was reported.⁹⁸ However, in a follow-up double-blind trial that included 35,533 healthy men, supplementing with 200 mcg per day of selenium for an average of 5.5 years had no effect on the incidence of prostate cancer.⁹⁹ In another trial, 5,141 men were randomly assigned to receive a placebo or a daily supplement containing 100 mcg of selenium, 120 mg of vitamin C, 30 IU of vitamin E, 6 mg of beta-carotene, and 20 mg of zinc 20 for eight years. Among men with a normal PSA level at the start of the study, there was a statistically significant 48% reduction in the incidence of prostate cancer. Among men with an initially elevated PSA level, the supplemented group had an increased incidence of prostate cancer that was not statistically significant.¹⁰⁰ In a double-blind study of men who already had non-metastatic prostate cancer, supplementation with selenium 200 or 800 mcg per day for up to 5 years had no significant effect on the progression rate of the cancer.¹⁰¹

People with RA have been found to have lower selenium levels than healthy people.^{102 , 103} One¹⁰⁴ of two double-blind trials using at least 200 mcg of selenium per day for three to six months found that selenium supplementation led to a significant reduction in pain and joint inflammation in RA patients, but the other reported no beneficial effect.¹⁰⁵ More controlled trials are needed to determine whether selenium reduces symptoms in people with RA.

Low levels of selenium¹⁰⁶ and low dietary intake of vitamin C^{107 , 108} have been observed in women with cervical dysplasia.

There is some evidence that combinations of antioxidants such as vitamin E, vitamin C, and selenium may help improve symptoms of asthma throught to be caused by air pollution.¹⁰⁹ In one double-blind study, 46 Dutch bicyclists were randomly assigned to receive a placebo or 100 mg of vitamin E and 500 mg of vitamin C daily for 15 weeks.¹¹⁰ Lung function was measured before and after each training session on 380 different occasions, and ambient ozone concentrations were measured during each training session. After analysis, researchers concluded that bicyclists with the vitamins C and E blunted the adverse effects of ozone on measures of lung function. In another double-blind study, 17 adults (18 to 39 years old) were randomly assigned to receive either 400 IU per day of vitamin E and 500 mg per day of vitamin C or a placebo for five weeks.¹¹¹ Tests showing improved measures of lung function led researchers to conclude that supplementation with vitamins C and E inhibited the decline in pulmonary function induced in asthmatics by exposure to air pollutants. Also using a double-blind design, another study of 158 children with asthma living in Mexico City were randomly assigned to receive, a daily supplement containing 50 mg of vitamin E and 250 mg of vitamin C or a placebo.¹¹² Tests results suggested that supplementing with vitamins C and E may reduce the adverse effect of ozone exposure on lung function of children with moderate to severe asthma.

Gross deficiency of dietary selenium may cause many heart problems, including arrhythmia. Based on this finding, one author has theorized that correction of low selenium status may improve many arrhythmias, even in the absence of overt deficiency symptoms.¹¹³ Controlled research is needed to evaluate this possibility.

Selenium deficiency has occasionally been reported as a cause of cardiomyopathy.^{114, 115} Selenium deficiency is the probable cause of Keshan’s disease, a form of cardiomyopathy found in China^{116, 117} but only rarely reported in the United States.¹¹⁸ Studies comparing populations in parts of the world other than mainland China have not supported a link between selenium deficiency and DCM,^{119, 120} except in Taiwan.¹²¹ Moreover, no clinical trials outside of China have explored the effects of supplementation with selenium for people with DCM, nor is there reason to believe that selenium supplementation would help most people outside of China and Taiwan suffering from cardiomyopathy.

Selenium is a mineral known to have antioxidant properties and to be involved in healthy immune system activity. Recent animal and human research suggests that selenium deficiency increases the risk of viral infection and that supplementation prevents viral infection.^{122, 123, 124, 125, 126} In a controlled trial, children with a specific viral infection (respiratory syncytial virus) who received a single supplement of 1 mg (1,000 mcg) of sodium selenite (a form of selenium) recovered more quickly than children who did not receive selenium.¹²⁷ While it is possible that childhood exanthemous viral infections might similarly be more severe in selenium-deficient children and helped through supplementation, none of the current research involves these specific viruses.

Blood levels of the antioxidant minerals selenium and zinc were normal in one study of people with Down’s syndrome,¹²⁸ but others have found selenium^{129 , 130} and zinc^{131 , 132 , 133} levels to be low. In some studies more than 60% of patients with Down’s syndrome had low zinc levels.^{134 , 135} A preliminary study of selenium supplementation in children with Down’s syndrome found that the antioxidant activity in the body improved; however, the implications of this finding on the long-term health of these people is unclear.¹³⁶ Zinc is critical for proper immune function, and in one preliminary study the majority of patients with Down’s syndrome examined had low zinc levels and low immune cell activity. Supplementation with zinc resulted in improved immune cell activity.¹³⁷ In preliminary intervention trials, improved immune cell activity was associated with reduced rates of infection in Down’s syndrome patients given supplemental zinc in the amount of 1 mg per 2.2 pounds of body weight per day.^{138 , 139} A controlled trial, however, did not find zinc, at 25 mg daily for children under 10 years of age and 50 mg for older children, to have these benefits.¹⁴⁰ Zinc has other roles in the body; preliminary data have indicated that zinc supplementation, at 1 mg per 2.2 pounds of body weight per day, improved thyroid function in Down’s syndrome patients,^{141 , 142 , 143} and increased growth rate in children with Down’s syndrome.¹⁴⁴

Nutritional supplements recommended by some doctors for prevention and treatment of periodontitis include vitamin C (people with periodontitis are often found to be deficient),¹⁴⁵ vitamin E, selenium, zinc, coenzyme Q10, and folic acid.¹⁴⁶ Folic acid has also been shown to reduce the severity of gingivitis when taken as a mouthwash.¹⁴⁷

A potent antioxidant combination may protect the liver from damage in people with hepatitis C, possibly decreasing the necessity for a liver transplant. In a preliminary trial,¹⁴⁸ three people with liver cirrhosis and esophageal varices (dilated veins in the esophagus that can rupture and cause fatal bleeding) caused by hepatitis C received a combination of Alpha lipoic acid (300 mg twice daily), silymarin (from milk thistle; 300 mg three times daily), and selenium (selenomethionine; 200 mcg twice daily). After five to eight months of therapy that included other “supportive supplements,” such as vitamin C and B vitamins, all three people had significant improvements in their liver function and overall health. Larger clinical trials are needed to confirm these promising preliminary results.

Selenium plays a role in thyroid hormone metabolism. Severe selenium deficiency has been implicated as a possible cause of goiter.¹⁴⁹ Two months of selenium supplementation in people who were deficient in both selenium and iodine was shown to induce a dramatic fall of the already impaired thyroid function in clinically hypothyroid subjects.¹⁵⁰ Researchers have suggested that people who are deficient in both selenium and iodine should not take selenium supplements without first receiving iodine or thyroid hormone supplementation.¹⁵¹ There is no research demonstrating that selenium supplementation helps people with hypothyroidism who are not selenium-deficient.

Selenium levels have been found to be low in people with liver cirrhosis¹⁵² and the need for antioxidants has been found to be increased.¹⁵³ A small, preliminary trial suggested that 100 mcg per day of selenium may improve liver function in people with alcoholic cirrhosis.¹⁵⁴ Larger, double-blind trials of selenium in people with liver cirrhosis are needed.

Sunlight triggers oxidative damage in the eye, which in turn can cause macular degeneration.¹⁵⁵ Animals given antioxidants—which protect against oxidative damage—have a lower risk of this vision problem.¹⁵⁶ People with high blood levels of antioxidants also have a lower risk.¹⁵⁷ Those with the highest levels (top 20th percentile) of the antioxidants selenium, vitamin C, and vitamin E may have a 70% lower risk of developing macular degeneration, compared with people with the lowest levels of these nutrients (bottom 20th percentile).¹⁵⁸ People who eat fruits and vegetables high in beta-carotene, another antioxidant, are also at low risk.¹⁵⁹ Some doctors recommend antioxidant supplements to reduce the risk of macular degeneration; reasonable adult levels include 200 mcg of selenium, 1,000 mg of vitamin C, 400 IU of vitamin E, and 25,000 IU of natural beta-carotene per day. However, a preliminary study found no association between age-related macular degeneration and intake of antioxidants, either from the diet, from supplements, or from both combined.¹⁶⁰ Moreover, in a double-blind study of male cigarette smokers, supplementing with vitamin E (50 IU per day), synthetic beta-carotene (about 33,000 IU per day), or both did not reduce the incidence of age-related macular degeneration.¹⁶¹

Selenium is a mineral nutrient with an important role in immune function and infection prevention,^{162, 163, 164} and selenium deficiency has been reported in patients after intestinal surgery.¹⁶⁵ A controlled trial of critically ill patients, including some with recent major surgery, found that those receiving daily intravenous selenium injections for three weeks showed less biochemical signs of body stress compared with unsupplemented patients. The amount used in this trial was 500 mcg twice daily for the first week, 500 mcg once daily for the second week, and 100 mcg three times daily for the third week.¹⁶⁶