For Patients & Caregivers
Tell your healthcare providers about any dietary supplements you’re taking, such as herbs, vitamins, minerals, and natural or home remedies. This will help them manage your care and keep you safe.
How It Works
There is no clear evidence to support the use of selenium for cancer prevention.
Selenium is an element obtained in the diet from seafood, poultry, meats, cereals, nuts, and grains. It is an essential part of cellular antioxidant systems that protects cells from DNA damage and mutations. For this reason, it has been studied for prevention of diseases caused or aggravated by this type of damage including cancer, heart disease, and diabetes. Selenium is also necessary for proper immune function, 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, and large doses can cause serious side effects.
To prevent or treat cancer
Clear evidence is lacking. Some studies suggest it may actually increase the risk of aggressive and secondary cancers.
To reduce cancer treatment side effects
Preliminary studies suggest it may reduce some side effects caused by cancer treatments, but additional studies are needed.
To prevent heart disease
Although low blood selenium levels have been associated with heart disease, studies do not support the use of selenium to protect against heart disease.
To prevent diabetes
Supplementation is not useful in preventing diabetes and in some cases may increase the risk for this disease.
To stimulate the immune system
A few studies suggest enhanced immune response, but more studies are needed.
With doses >1000 micrograms per day: Muscle weakness, fatigue, nerve pain, skin irritation, nail and hair changes/loss, garlic breath/body odor, irritability, and liver damage.
Accidental overdose: Gram quantities of selenium can cause severe GI and nerve problems, breathing distress, kidney or heart problems, and in one reported case led to death.
- Daily recommended intake: 55 micrograms, which is usually provided by seafood, meat, and fortified grain products.
- Tolerable upper level of selenium: 400 micrograms per day. Higher dosages can cause toxicity.
- Supplementation in patients with low levels or during active treatment should be under the guidance of healthcare practitioners.
For Healthcare Professionals
Selenium, an essential dietary trace mineral, is an important component of cellular antioxidant defense systems that include enzymes such as glutathione peroxidase (1). 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. It is sold in supplemental form to boost immune function and for disease prevention.
Studies on the role of selenium in reducing cardiovascular disease risk are inconclusive (23) (28), although a meta-analysis suggests benefit with antioxidant mixtures when they contain selenium (48). Data are also mixed on whether supplementation can reduce viral load in patients with HIV (19) (49). Although it may improve glucose metabolism and reduce markers of inflammation and oxidative stress (41), supplementation is not useful in preventing diabetes and in some cases may increase the risk (20) (30) (42) (50). Selenium alone or in combination with vitamin E also did not prevent dementia (43).
In studies on the relationship between selenium and cancer prevention, data suggest it may help prevent gastrointestinal (16) (36) and lung (17) cancers, although findings from bladder cancer research are conflicting (26) (38) (44). Other studies on whether selenium status impacts cancer risk are also mixed (51) (52) (53), and a review of RCTs did not find clear evidence of cancer preventive potential (29).
In addition, the Selenium and Vitamin E Cancer Prevention Trial (SELECT) failed to find protective effects against prostate cancer (8) (21) or colorectal adenomas (45). In other studies, selenium along with soy and vitamin E did not prevent prostate cancer progression (27), and selenium taken with lycopene did not modulate prostate cancer risk (46). Moreover, 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 non-small cell lung cancer did not show benefit, but did suggest a possible nonsignificant increase in secondary lung cancers (25). Long-term supplementation may increase the risk for certain types of skin cancer as well (13).
With respect to dietary intake, no association between dietary selenium and thyroid cancer incidence was found (35).
A few studies suggest selenium supplementation may reduce some cancer treatment side effects including hair loss, abdominal pain, loss of appetite (15), diarrhea (24) (39), fatigue and nausea (40), head and neck lymphedema (12) (14), and salivary gland damage (47). However, more studies are needed and supplementation in patients with low levels or during active treatment should be under the guidance of healthcare practitioners in cancer populations.
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).
In lab studies, selenium protected against adriamycin-induced cardiac dysfunction via restoring ATP-sensitive potassium channel expression (34). It induced a multitargeted cell death process characterized by unfolded protein response, ER-stress, and occurrence of large cytoplasmic vacuoles in addition to ROS formation (31). However, excessive selenium intake induces hepatic insulin resistance through an opposite regulation of ROS (32). The effects of dietary selenium on progression of malignant mesothelioma tumors depend on arising cancer cell redox metabolism and tumor ability to benefit from increased selenium intake (33).
Accidental overdose and death: Of a 75-year-old man from 10 g oral sodium selenite to self-treat prostate cancer after Internet reading on the subject (18). Consumption of gram quantities of selenium can cause severe GI and neurological disturbances, acute respiratory distress syndrome, myocardial infarction, and renal failure.
Chronic selenosis (doses >1000 mcg/day): Muscle weakness, fatigue, peripheral neuropathy, dermatitis, nail and hair changes/loss, garlic breath/body odor, irritability, growth retardation, hepatic necrosis.