Selenium is a naturally occurring trace mineral found in soil, water, and various foods. It is considered an essential nutrient, meaning the human body cannot produce it and must obtain it through diet. Selenium plays a fundamental role in maintaining overall health by supporting numerous bodily processes.
Selenium’s General Function in the Body
Selenium performs diverse roles within the human body. It functions as a powerful antioxidant, helping to protect cells from damage caused by unstable molecules known as free radicals. This protective action is partly due to its presence in selenoproteins, such as glutathione peroxidase enzymes, which are involved in neutralizing oxidative stress.
Selenium also supports the immune system. It contributes to the production of signaling molecules called cytokines, which regulate immune responses, and helps maintain the function of white blood cells that fight infections. Additionally, selenium is important for thyroid hormone metabolism. The thyroid gland contains a high concentration of selenium, as many selenoproteins are involved in converting inactive thyroid hormone to its active form, regulating metabolism, growth, and development.
Selenium’s Connection to Prostate Health Research
The relationship between selenium intake and prostate health, particularly prostate cancer, has been a significant area of scientific investigation, yielding complex and sometimes contradictory findings. Early observational studies suggested a potential protective effect of higher selenium levels against prostate cancer, sparking interest in supplementation. This led to large-scale clinical trials.
One prominent study was the Selenium and Vitamin E Cancer Prevention Trial (SELECT), which enrolled over 35,000 healthy men across the United States, Canada, and Puerto Rico. Participants, aged 50 or older (or 55 for non-African American men), received daily doses of 200 micrograms of L-selenomethionine, 400 IU of synthetic vitamin E, a combination of both, or a placebo. The trial was stopped early after a median of 5.46 years because no significant benefit in prostate cancer prevention was observed.
The results from SELECT indicated that selenium supplementation, at the dose and form used, did not prevent prostate cancer in the general healthy male population. A later analysis suggested that men with already high baseline selenium levels who took selenium supplements had an almost doubled risk of developing high-grade prostate cancer compared to those with lower baseline levels. This finding highlights that selenium’s effect may not be universal and could depend on an individual’s pre-existing selenium status.
The complexity of selenium’s role might also be influenced by genetic factors or the specific form of selenium used. While some earlier, smaller studies, like the Nutritional Prevention of Cancer (NPC) trial, showed a reduction in prostate cancer incidence, these studies often involved populations with lower baseline selenium levels. Current scientific consensus suggests that more research is necessary to fully understand the interactions between selenium, individual genetic makeup, and prostate health. Therefore, universal selenium supplementation for prostate cancer prevention is not currently supported by the evidence.
Dietary Sources and Supplementation Guidance
Individuals can obtain selenium through a variety of dietary sources, as it is naturally present in many foods. Foods high in protein tend to be good sources because selenium is often bound to protein. Brazil nuts are particularly rich in selenium; one ounce, or about six to eight nuts, can contain around 544 micrograms. However, the selenium content in Brazil nuts can vary significantly depending on the soil where they are grown.
Other notable dietary sources of selenium include seafood like yellowfin tuna (approximately 92 micrograms per 3 ounces cooked) and sardines (about 45 micrograms per 3 ounces canned). Meat, poultry, and eggs also provide selenium, with a 3-ounce portion of roasted ham containing about 42 micrograms and a large hard-boiled egg providing around 15 micrograms. Whole grains, such as brown rice and whole-wheat bread, contribute to selenium intake.
When considering selenium supplementation, it is available in various forms, including selenomethionine, selenium-enriched yeast, sodium selenite, and sodium selenate. Multivitamin/mineral supplements often contain around 55 micrograms of selenium, while selenium-only supplements can range from 100 to 400 micrograms. The body generally absorbs a high percentage of selenium from these forms, up to approximately 90%. Given the nuanced research findings, particularly concerning prostate health, it is advised to consult with a healthcare professional before starting any selenium supplements. Individual needs, existing health conditions, and current dietary intake should guide supplementation decisions.
Potential Risks of Excessive Selenium
While selenium is an essential trace mineral, consuming it in excessively high doses can lead to adverse health effects, a condition known as selenosis. The margin between beneficial intake and toxic levels is relatively narrow. The tolerable upper intake level (UL) for selenium in adults is generally set at 400 micrograms per day, encompassing intake from all sources. Consistently exceeding this limit without medical supervision can lead to health problems.
Symptoms of selenosis can manifest in various ways. The most common signs of chronic high intake include hair loss and brittle or lost nails. Other reported symptoms encompass gastrointestinal upset such as nausea and diarrhea, fatigue, irritability, and nervous system abnormalities. In rare instances, very high acute doses, such as those from misformulated supplements, have led to more severe symptoms and even death.
For example, a liquid dietary supplement recall in 2008 involved products containing selenium at 200 times the labeled concentration, leading to symptoms like diarrhea, fatigue, hair and nail loss, and joint pain in affected users. This underscores the importance of adhering to recommended intake levels and seeking professional medical advice. Consulting a healthcare provider before taking selenium supplements is a prudent step to ensure safe dosing and avoid potential adverse effects, especially given the variability of selenium content in some foods like Brazil nuts.