Selenium is an essential trace mineral required for various functions, including metabolism, DNA synthesis, and immune defense. Its potential role in cancer prevention has been a subject of extensive scientific inquiry and debate. Research continues to clarify whether selenium supplementation offers protective benefits against cancer or poses risks.
The Antioxidant Hypothesis for Cancer Prevention
Initial interest in selenium’s cancer-preventive properties stemmed from its role in the body’s antioxidant defense system. Selenium is incorporated into specialized proteins called selenoproteins, which include enzymes like glutathione peroxidases and thioredoxin reductases. These selenoproteins neutralize reactive oxygen species, or free radicals, unstable molecules that cause oxidative stress and damage to cellular components, including DNA. Oxidative damage to DNA is a known pathway that can lead to cancer development. By mitigating this damage, selenoproteins were hypothesized to protect cells from malignant transformation and reduce cancer risk.
Major Clinical Trial Findings
The Nutritional Prevention of Cancer (NPC) trial was one of the first large-scale human studies to explore this link. Participants receiving 200 micrograms of selenium daily in the form of selenized yeast showed a significant reduction in the incidence of total cancer, particularly in men with lower baseline selenium levels.
However, the subsequent Selenium and Vitamin E Cancer Prevention Trial (SELECT), a larger study involving over 35,000 men, yielded different results. SELECT was designed to specifically investigate selenium and vitamin E’s effect on prostate cancer prevention. The trial found no evidence that selenium supplementation alone or in combination with vitamin E prevented prostate cancer. Furthermore, the SELECT trial raised concerns by suggesting a potential increase in the risk of prostate cancer in the vitamin E group.
Influence on Specific Cancer Types
Research has explored selenium’s potential influence on various cancer types, revealing nuanced and often inconsistent findings. Prostate cancer received significant attention due to early observational studies and the NPC trial, which suggested a protective effect. However, the large-scale SELECT trial later challenged this, finding no overall reduction in prostate cancer risk with selenium supplementation. Subsequent analysis of SELECT data even indicated a possible increased risk of high-grade prostate cancer in men with higher baseline selenium status who took supplements.
For other cancer types, results have been similarly varied. The NPC trial suggested reductions in lung and colorectal cancer incidence, but a later reanalysis of the complete data did not find these reductions to be statistically significant. A 2018 Cochrane Review, synthesizing evidence from multiple studies, concluded that selenium supplementation had little to no effect on the incidence of prostate, colorectal, lung, or bladder cancers. While some observational studies have indicated an inverse association between higher selenium status and a lower risk of certain cancers like liver cancer, overall consistent evidence for a broad preventative effect across all cancer types remains elusive.
Risks of Selenium Supplementation
Supplementing with selenium, particularly at high doses or without a diagnosed deficiency, carries potential health risks. Selenium has a narrow therapeutic window, meaning the difference between a beneficial dose and a toxic dose is relatively small. Excessive intake can lead to a condition called selenosis, which manifests with symptoms such as hair loss, nail brittleness or loss, gastrointestinal upset, fatigue, and neurological abnormalities.
Findings from large studies like SELECT have also highlighted additional concerns. While the initial SELECT results showed a statistically non-significant trend, further analyses and other studies have indicated that high-dose selenium supplementation may increase the risk of developing type 2 diabetes. This unexpected finding emphasizes that more is not necessarily better when it comes to trace minerals. For the general population, obtaining selenium through a balanced diet, which includes foods like nuts, seafood, and meat, is generally considered the safest and most effective approach to maintaining adequate selenium status without risking toxicity.