For Patients & Caregivers
How It Works
There is no clear evidence to support use of Selenium for cancer prevention.
Selenium is an element obtained in the diet from Brazil nuts, seafood, meats, cereals and grains. It is an essential part of the antioxidant enzyme glutathione-peroxidase, which protects cells from damage and DNA from mutations. For this reason, it has been studied for the prevention of diseases that are caused by or aggravated by this type of cellular damage, including cancer, heart disease, and rheumatoid arthritis. Selenium is also necessary for proper function of the immune system, but it is not known whether higher-than-normal levels of selenium can stimulate the immune system. Long-term use of selenium may increase the risk of certain types of cancer.
To prevent and treat cancer
Clear evidence is lacking. Some studies suggest it may actually increase the risk of aggressive and secondary cancers.
To reduce chemotherapy-induced adverse effects
Selenium may help reduce fatigue and nausea in young patients.
To reduce secondary lymphedema (swelling due to the accumulation of lymph)
A clinical trial suggests that selenium supplementation can reduce the swelling that occurs in patients who have undergone extensive surgery or radiation therapy.
To prevent heart disease
Although low blood selenium levels have been associated with heart disease, studies in the general population have not supported the use of selenium to protect against heart disease. This use remains controversial.
To stimulate the immune system
Selenium is essential for proper functioning of the immune system, and a few studies have shown an enhanced immune response in people taking selenium supplements.
To treat rheumatoid arthritis
Low blood levels of selenium have been found in patients with rheumatoid arthritis. However, treatment of rheumatoid arthritis with selenium has not shown benefit in a few clinical trials.
Do Not Take If
- Chronic selenosis can develop with doses greater than 1000 micrograms per day and results in muscle weakness, fatigue, peripheral neuropathy, dermatitis (redness and irritation of the skin), nail and hair changes/loss, garlic breath/body odor, irritability, growth retardation, and liver damage.
- Selenium poisoning has occurred with either accidental or suicidal ingestion of gun bluing solution or sheep drench. This usually involves ingestion of grams of selenium and can cause severe gastrointestinal and neurological disturbance, acute respiratory distress syndrome, myocardial infarction (heart attack), and kidney failure.
For Healthcare Professionals
Selenium, an essential dietary trace mineral, is an important component of antioxidant systems such as glutathione peroxidase (1) that neutralize and protect against damage caused by free radicals and reactive oxygen species. It also plays an important role in thyroid function. Selenium can be obtained from the diet by consuming whole grains, meats, seafood, poultry and nuts, and is sold in supplemental form to boost immune function and for prevention of cardiovascular, rheumatic diseases, and cancer.
Several studies have been conducted to determine the role of selenium in reducing the risk of cardiovascular disease but data are inconclusive (23) (28). Supplementation was found to reduce viral load in individuals infected with human immunodeficiency virus (HIV) (19). It was also shown to reduce markers of inflammation and oxidative stress in patients with diabetic nephropathy (41), may improve glucose metabolism, but was not useful in preventing type 2 diabetes and in some cases may increase the risk (20) (30) (42). In another study of asymptomatic older men, selenium alone or in combination with vitamin E was ineffective in preventing dementia (43).
The importance of selenium in cancer prevention has been documented in epidemiological studies and clinical intervention trials. Data suggest its benefits in preventing gastrointestinal (16) (36) and lung (17) cancers although findings from bladder cancer research are conflicting (26) (38) (44). Selenium supplementation also resulted in reduction of side effects including hair loss, abdominal pain, and loss of appetite in ovarian cancer patients undergoing chemotherapy (15). It was found effective in reducing head and neck lymphedema (12) (14); to help reduce diarrhea associated with radiation therapy (24) in gynecological cancer patients without influencing the effectiveness or long-term survival (39); and may decrease chemotherapy-induced fatigue and nausea in adolescents (40).
However, a recent review of randomized controlled trials did not find clear evidence of selenium’s cancer preventive potential (29). The large prevention study SELECT (Selenium and Vitamin E Cancer Prevention Trial) (8), based on previous data indicating that selenium and vitamin E reduced the incidence of prostate cancer, also failed to find evidence of protective effects of selenium. The trial was suspended in January 2009, when initial data analysis showed that selenium, by itself or when combined with vitamin E, did not prevent prostate cancer in healthy men (21). Further, a sub-group analysis failed to find any benefit for prevention of colorectal adenomas (45). In other studies, selenium when used together with soy and Vitamin E, did not prevent prostate cancer progression (27); and when used with lycopene, had no effect on modulating risk of prostate cancer (46). Moreover, results from a cross-sectional analysis of men with prostate cancer indicate that selenium levels may influence the risk of aggressive prostate cancer (22), and supplementation may increase prostate cancer mortality (37).
A chemoprevention trial of selenium in patients with NSCLC (non small-cell lung cancer) was also halted after interim analysis as data did not show any benefit of selenium supplementation; there was an increase in secondary lung cancers in selenium users, although it was statistically insignificant (25). Long-term use of selenium may increase the risk of certain types of skin cancer as well (13).
Additional studies indicate no association between dietary selenium intake and thyroid cancer incidence (35), but that it may protect radiation-induced damage to salivary glands in patients with differentiated thyroid cancer (47).
Mechanism of Action
Selenium is an essential structural element of the antioxidant enzyme glutathione-peroxidase that converts aggressive oxidation products and intracellular free radicals into less reactive or neutral components (3). Other biological functions of selenium include regulation of thyroid hormone action and regulation of the reduction status of vitamin C (2).
Selenium has been shown to induce a multi-targeted cell death process characterized by induction of unfolded protein response, ER-stress and occurrence of large cytoplasmic vacuoles, in addition to the formation of reactive oxygen species (ROS) (31). However, excessive intake of selenium induces hepatic insulin resistance through opposite regulation of (ROS) (32).
Recent data show that the effects of dietary selenium on progression of malignant mesothelioma tumors depend on arising cancer cells’ redox metabolism, and the tumors able to convert increased selenium into a stronger reducing capacity actually benefit from increased selenium intake (33). In another study, selenium supplementation was shown to afford protection against adriamycin-induced cardiac dysfunction via restoring ATP-sensitive potassium channels (KATP) expression (34).
Case Report: Oral consumption of 10 g of sodium selenite supplement for treatment of prostate cancer resulted in the death of a 75-year-old man (18).
Chronic selenosis (doses greater than 1000 mcg/day): Muscle weakness, fatigue, peripheral neuropathy, dermatitis, nail and hair changes/loss, garlic breath/body odor, irritability, growth retardation, hepatic necrosis.
Toxicity: Acute toxicity via selenium poisoning has been reported with either accidental or suicidal ingestion of gun bluing solution or sheep drench. Consumption of gram quantities of selenium can cause severe gastrointestinal and neurological disturbances, acute respiratory distress syndrome, myocardial infarction, and renal failure.