Selenium is an essential trace mineral required by the human body for a wide range of biological functions. Since the body cannot produce it, selenium must be obtained entirely through diet or supplementation. Scientists have long investigated whether increasing selenium intake could offer protection against diseases, particularly prostate cancer. Research has yielded complex and contradictory results, leading to ongoing debate about using selenium supplements for prostate health. The evidence suggests the relationship between this mineral and cancer prevention depends heavily on a person’s existing selenium status and the dosage consumed.
Selenium’s General Role in Cellular Function
Selenium’s influence on health stems primarily from its incorporation into a group of proteins known as selenoproteins. Selenium is integrated into the amino acid selenocysteine, which acts as an active site in these specialized proteins. The human body encodes for approximately 25 different selenoproteins, each performing distinct roles in maintaining cellular health.
Many of these selenoproteins function as powerful antioxidants that help regulate the body’s redox state. A notable example is the enzyme glutathione peroxidase (GPx), which uses selenium to neutralize harmful reactive oxygen species. GPx converts peroxides into harmless substances, protecting cell membranes and DNA from oxidative damage.
Another group of selenoproteins are the thioredoxin reductases (TxnRd), which help regenerate other cellular antioxidants. Selenium also plays a part in immune system maintenance and DNA repair mechanisms. Due to these fundamental roles in protecting cellular integrity, researchers hypothesized that adequate selenium intake could suppress the cell growth characteristic of cancer.
The Research Landscape on Prostate Health
Interest in selenium’s potential began with observational research. Some large population studies found that men with lower baseline concentrations of selenium showed a greater incidence of prostate cancer compared to those with higher concentrations. This suggested that adequate selenium status might be protective, especially in regions where the mineral content in the soil was naturally low.
An early, small-scale clinical trial, the Nutritional Prevention of Cancer (NPC) trial, provided promising initial results. Although designed to evaluate skin cancer, a secondary analysis indicated that participants receiving selenium supplementation experienced a notable reduction in prostate, lung, and colorectal cancers. This protective effect was most pronounced in men who had low selenium levels at the start of the trial, suggesting supplementation might only benefit those with a deficiency.
These findings led to the launch of the Selenium and Vitamin E Cancer Prevention Trial (SELECT), a large-scale, randomized, placebo-controlled intervention trial designed specifically to test the hypothesis. SELECT enrolled over 35,000 healthy men to receive either selenium (200 micrograms per day of L-selenomethionine), vitamin E, a combination of both, or a placebo. Crucially, the SELECT population was generally well-nourished, with an average selenium status higher than participants in the NPC trial.
The results of SELECT failed to demonstrate that selenium supplementation reduced the risk of prostate cancer. The trial data showed that the incidence rate in the selenium group was not significantly different from the placebo group. There was even a non-significant trend toward a slight increase in risk for the selenium group, leading to the early cessation of the intervention phase.
Analysis of these conflicting results highlights the importance of initial selenium status. The men in the SELECT trial generally had sufficient selenium levels at the start of the study, suggesting that supplementing an already adequate intake does not confer additional protection and may even be detrimental. This indicates that selenium’s role might be to correct a deficiency that increases risk rather than acting as a universal preventative agent at higher doses. Therefore, the current scientific consensus from large-scale trials does not support the recommendation of selenium supplementation for the general population to prevent prostate cancer.
Safe Intake, Dietary Sources, and Toxicity
For most healthy adults, the Recommended Dietary Allowance (RDA) for selenium is 55 micrograms (µg) per day. This level is sufficient to maintain maximum activity of the antioxidant enzyme glutathione peroxidase. Most people in developed countries easily meet this requirement through a balanced diet, as severe selenium deficiency is rare.
Selenium-rich foods include:
- Seafood, such as tuna and shrimp.
- Lean meats and poultry.
- Plant-based sources like whole grains, cereals, and dairy products.
- Brazil nuts, which are highly concentrated sources, though their content varies based on soil quality.
Excessive selenium intake can lead to selenosis, a form of toxicity. The Tolerable Upper Intake Level (UL) for adults is 400 micrograms per day, representing the maximum daily intake unlikely to cause adverse health effects. Chronic intake above this level, often from high-dose supplements, can cause symptoms like hair loss, nail brittleness, and skin lesions. Early signs of overconsumption may also include a metallic taste or a distinctive garlic odor on the breath.