Selenium
What is it?
Selenium was identified as an essential trace mineral for humans in the 1970s. The average adult body contains about 20 mg of selenium and most of this is concentrated in the kidneys, liver, heart, spleen and testes.
What it does in the body
Antioxidant activity
As part of the enzyme glutathione peroxidase, selenium acts as an antioxidant. It is extremely powerful and protects red blood cells and cell membranes from free radical damage. Selenium works closely with vitamin E and may enhance its function. Glutathione peroxidase seems to be able to protect against the damaging effects of ultraviolet light.
Immune system
Selenium is important in maintaining resistance to disease. It may enhance the production and effectiveness of white blood cells and protect them from the free radicals they generate in the process of fighting infection. It also appears to increase antibody production, and strengthen the body's surveillance of abnormal cell growth and cancer.
Hormones
A selenium-dependent enzyme is involved in the metabolism of thyroid hormones. Studies have shown that thyroid hormones in elderly people are influenced by selenium status.1
Other functions
Selenium is involved in maintaining normal liver function, protein synthesis and protecting against toxic minerals such as arsenic, cadmium, mercury and lead. It plays a role in promoting male sexual reproductive capacity and maintaining healthy eyes, hair and skin. It may be involved in the metabolism of prostaglandins which control inflammation.
Absorption and metabolism
Organic selenium, such as that found in yeast, is more efficiently absorbed than inorganic salts.
Deficiencies
Severe selenium deficiency has only been seen in those living off foods grown in selenium-deficient soil. Levels of selenium in soil vary between countries and between different regions in the same country. There are low levels in Europe, parts of the USA, New Zealand and parts of China. There are high levels in Japan, Thailand, Philippines and Puerto Rico.
Severe selenium deficiency leads to the heart disorder, Keshan disease, a potentially fatal cardiomyopathy that affects children in low selenium areas of rural China. Kashin Beck disease, another deficiency disease seen in rural China, resembles arthritis.
Marginal selenium deficiency can occur in alcoholics and those living on refined and processed foods, and may increase the risk of a variety of diseases. Blood selenium levels may also be low in those who are critically ill, AIDS patients, fibrocystic breast disease sufferers, those with Down syndrome, and liver disease patients.
Cancer
Epidemiological studies have shown that those who live in areas of low selenium soil are more prone to cancer than those living in areas where the soil is high in selenium. Blood samples taken from large groups of people show that they are more likely to develop cancer if they have low blood levels of selenium and glutathione peroxidase. Low serum, dietary and soil selenium levels are particularly associated with lung and gastrointestinal tract cancers.
Colorectal cancer
In a 1997 study of the relationship between selenium and colon cancer, researchers at the University of North Carolina determined selenium levels in patients referred for colonoscopy. The results showed that those with the lowest selenium levels had almost four times the risk of colon cancer when compared to those with the highest levels.2
A 1998 German study assessed the selenium and glutathione peroxidase levels in 106 colorectal cancer patients and compared these to a gender-matched and age-matched control group. When average selenium levels in the cancer patients were compared with those in the control group, no significant differences were found. However, a significant reduction of serum glutathione peroxidase activity was seen in cancer patients. Those patients with low selenium levels had lower survival times and rates than the patients with higher selenium levels. The lowest selenium level was found for patients with advanced tumor disease. It is unclear from the results of this study whether low selenium levels are a cause or effect of cancer.3
Lung cancer
In a study that started in 1986 and was published in 1993, Dutch researchers examined the links between longterm selenium status and lung cancer among 120 852 Dutch men and women aged 55-69 years. The results showed that the lung cancer risk in those with the highest intake of selenium was half that of those in the lowest intake group. The protective effect of selenium was concentrated in subjects with a relatively low dietary intake of beta carotene or vitamin C.4
Cardiovascular disease
Severe selenium deficiency leads to weakened and damaged heart muscle. People living in low selenium areas have lower plasma selenium levels and an increase in the risk of coronary disease, atherosclerosis, platelet aggregation and levels of compounds such as prostaglandins and leukotrienes which play a role in inflammation and platelet aggregation. (See page 560 for more information.) Selenium seems to be able to affect prostaglandin and leukotriene synthesis.
As part of glutathione peroxidase, selenium takes part in the reduction of hydrogen peroxides and lipid peroxides. The concentration of these peroxides, in turn, affects platelet aggregation. Blood platelets of selenium-deficient people show increased aggregation, which selenium administration inhibits. Thus long-term supplementation with low doses of selenium could have a beneficial effect on the prevention of both thrombosis and coronary heart disease in people who are selenium-deficient.5
Dutch researchers studying the association between selenium status and the risk of heart attack, compared plasma, red blood cell, and toenail selenium levels and the activity of red blood cell glutathione peroxidase among 84 heart attack patients and 84 healthy people. They found lower selenium levels in all the heart attack patients. Because the toenail selenium level reflects blood levels up to one year before sampling, the results suggest that low selenium levels were present before the heart attacks and, may have played a role in causing them.6
However, results from the Physicians' Health Study published in 1995 do not suggest a link between selenium levels and heart attack risk. Researchers analyzed blood selenium levels in 251 subjects who had heart attacks and an equal number of healthy people, matched by age and smoking status. The results did not show significant differences.7
HIV/AIDS
As part of the antioxidant enzyme glutathione peroxidase, selenium is necessary to help prevent oxidative damage and to help the immune system function effectively. Levels of this enzyme have been shown to be low in some HIV-positive patients, particularly in those with more advanced stage of the disease, and low selenium levels appear to be associated with low CD4+ lymphocyte counts and with higher death rates in AIDS patients. Deaths from AIDS are higher in areas where soil selenium is low.
Birth defects
Selenium deficiency in women may result in infertility, miscarriages, neural tube defects and retention of the placenta.8
Asthma
As an antioxidant, selenium may be able to protect against the damage to lung tissue and enzymes caused by the free radicals produced by inflammatory cells in asthmatic airways. As a component of the enzyme glutathione peroxidase it helps to stabilize cell membranes.
In a New Zealand study done in 1994, researchers examined 708 children for symptoms of wheezing. They then measured selenium levels in blood samples taken eight years previously from 26 of the children with current wheezing, and compared these with levels in 61 healthy children. The results showed that wheezing was more common in those with low levels of selenium.9 Another New Zealand study, done in 1990, showed that whole blood selenium concentrations and glutathione peroxidase activity were lower in adults with asthma than in those without.10
Rheumatoid arthritis
Free oxygen radicals are involved in the inflammatory process seen in rheumatoid arthritis and are generated mainly by white blood cells. Selenium is important to the functioning of the immune system and to the inflammatory process. Low selenium levels among patients with rheumatoid arthritis have been reported from areas with both high and low natural selenium intake. The reduction seems to be related to the clinical disease activity in arthritis patients, and selenium concentrations have been found to fluctuate during the disease.
Cataracts
Reduced antioxidant defenses seem to play a role in cataract formation and selenium deficiency may play a part in this. Glutathione peroxidase is found in high concentrations in the lens and selenium levels in lenses with cataracts have been found to be lower than in normal lenses.11
Anemia
Selenium deficiency may play a role in causing or aggravating anemia as glutathione peroxidase protects red blood cells from free radical damage and destruction.
Other effects
A 1996 study done at the USDA Human Nutrition Research Center in San Francisco suggests that people with low selenium levels might experience depressed moods, supporting the idea that selenium plays a special role in the brain. However, the study did not find improvements with selenium supplementation in people eating a typical American diet.12
Sources
Good food sources of selenium include organ meats, fish and shellfish, muscle meats, whole grains, cereals, dairy products and vegetables such as broccoli, mushrooms, cabbage and celery. The selenium content of foods depends on the soil in which they are grown. Food processing techniques can remove selenium.
| Food | Amount | Selenium (mg) |
|---|---|---|
| Brazil nuts | 6-8 kernels | 840 |
| Pork kidney, cooked | 1 cup, sliced | 271 |
| Beef kidney, cooked | 1 cup, sliced | 212 |
| Lamb kidney, raw | 1 cup, sliced | 185 |
| Lamb liver, raw | 1 cup, sliced | 118 |
| Tuna, canned, drained, in water | 1 can | 133 |
| Tuna, canned, drained, in oil | 1 can | 130 |
| Flounder, cooked | 1 fillet | 73.9 |
| Pink salmon, raw | ½ fillet | 70.9 |
| Macaroni pasta, dry | 1cup | 62.0 |
| Oysters, cooked | 6 oysters | 60.1 |
| Turkey, dark meat | 1 cup | 54.3 |
| Beef liver, pan-fried | 100g | 57 |
| Mackerel, baked | 1 fillet | 46.5 |
| Pork, chops, sirloin | 85g | 43.9 |
| Wheat flour, wholegrain | ½ cup | 40.3 |
| Wholewheat pita bread | 1 pita | 28.2 |
| Ocean perch, raw | 1 fillet | 27.7 |
| Rolled oats | 1 cup | 26.2 |
| White bread flour | ½ cup | 26.0 |
| Wheat bran | ½ cup | 22.1 |
| Wheatgerm | ¼ cup | 21.9 |
| Oat bran | ½ cup | 20.1 |
| Special K | 1 cup | 19.2 |
Recommended dietary allowances (RDA)
| Men | Women | Pregnancy | Lactation | |
|---|---|---|---|---|
| USA | 70 mcg | 55 mcg | 65 mcg | 75 mcg |
| UK | 75 mcg | 60 mcg | 75 mcg | 75 mcg |
| Australia | 85 mcg | 70 mcg | 80 mcg | 85 mcg |
Supplements
There are various forms of selenium supplements including organic selenium rich yeast, selenium in the form of selenomethionine, and inorganic sodium selenite. These different types of selenium may act differently, with selenium yeast raising blood selenium levels and sodium selenite more effective at increasing the activity of glutathione peroxidase. Organic selenium seems to be better absorbed and less toxic than the inorganic forms.
Young adults, vegetarians, the elderly, smokers, pregnant women and nursing mothers may benefit from supplements.
Toxic effects of excess intake
Selenium toxicity can occur at doses of 600 to 750 mcg. Early signs of selenium toxicity include fatigue, irritability and dry hair. Other symptoms of excess intake include dental caries in children, hair loss, skin depigmentation, abnormal nails, vomiting, nervous system problems, and bad breath.
Therapeutic uses of supplements
Cancer
Some studies have shown that selenium supplements protect against some types of cancer such as rectal, ovarian, colon, lung and cervical cancers. However there are also studies, including the Nurses Health Study at Harvard, which do not show a protective role for selenium against cancers at any major site. Laboratory studies have shown that selenium can slow tumor cell growth.
A 1996 study looking at the effect of selenium supplements on cancer has found a 50 per cent reduction in deaths from cancer in those taking supplements. Researchers at the Arizona Cancer Center set out to test the effectiveness of selenium supplements on the prevention of skin cancer in over 1300 patients. Participants received a placebo or 200 mcg selenium per day over a period of 4.5 years and a total follow-up of 6.4 years. While the results did not show any reduction in skin cancer risk, the selenium group had a 37 per cent reduction in cancer incidence and a 50 per cent reduction in cancer mortality. The effects appeared strongest for prostate (63 per cent lower risk), colorectal (58 per cent lower risk) and lung (53 per cent lower risk) cancers.13
A recent report in the Journal of the National Cancer Institute suggests that selenium compounds may inhibit colon cancer in rats. Researchers at the American Health Foundation gave the synthetic organoselenium compounds to rats with high fat diets and found inhibition of tumor incidence. The effects were more pronounced with a low fat diet. There were no toxic effects with either compound.14
Heart disease
Selenium may reduce heart disease by protecting against oxidative damage to blood cholesterol. Selenium supplements have been shown to increase HDL cholesterol levels and decrease LDL cholesterol levels. Selenium can also inhibit platelet aggregation, thus reducing the risk of build-up of atherosclerotic plaques in the arteries.
Finnish researchers evaluated the effect of selenium supplementation on 81 patients with heart attacks. Patients received either selenium-rich yeast (100 mcg per day) or placebo in addition to conventional drug therapy for a six- month period. During the follow-up period there were four cardiac deaths in the placebo group but none in the selenium group. There were two nonfatal heart attacks in the placebo group and one nonfatal attack in the selenium group.15
A small 1997 German study indicated improvements that patients who were given selenium supplements after heart attacks showed greater improvements in heart function than patients not given supplements.16
Asthma
In 1993 Swedish researchers conducted a study of 24 adults with asthma in which half of the patients received 100 mcg of selenium per day for 14 weeks, while the other half received a placebo. Six patients from the selenium-supplemented group and one from the placebo group noticed significant clinical improvement, although neither group showed improvement in laboratory measures.17
Rheumatoid arthritis
Selenium supplements may be beneficial in the treatment of rheumatoid arthritis, especially when combined with vitamin E treatment. In some trials, symptoms have been shown to improve as blood selenium levels increase. However, the results of studies are mixed. Selenium may reduce inflammation through its antioxidant action, and through control of prostaglandins, hormone-like compounds that regulate the inflammation process.
In a three month study done in 1997 in Germany, 70 patients with rheumatoid arthritis were randomly divided into two groups. One group was given 200 mcg per day of sodium selenite while the other group was given a placebo. Selenium concentrations in red blood cells of patients with rheumatoid arthritis were significantly lower than found in an average German population. At the end of the experimental period, the selenium-supplemented group showed less tender or swollen joints, and morning stiffness. Selenium-supplemented patients needed less cortisone and other anti-inflammatory medications than the placebo group. Analysis also showed a decrease in laboratory indicators of inflammation.18
Other uses
Selenium supplements have been used in the detoxification of arsenic, cadmium and mercury; to treat angina; high blood pressure in pregnancy; and hair, nail and skin problems. Selenium may also play a role in preventing anemia; cataracts; periodontal disease; and improving mood, anxiety, depression and fatigue in some people. Selenium supplements may benefit those with low immune function, such as the elderly.
Shampoos or prescription solutions containing selenium sulfide are used for the treatment of fungal infections, including tinea capitis.
Interactions
Interactions with other nutrients
The amino acid, methionine, is essential for the absorption, transport and bioavailability of selenium. Combined selenium and vitamin E seems to have synergistic effects in the treatment of heart disease, tissue damage due to restricted blood flow, and cancer. Vitamin C has also been found to have added effects. Large doses of vitamin C can interfere with the absorption and use of inorganic selenium such as sodium selenite.19