Selenium is suggested to be involved in central anti-carcinogenic processes. Selenium supplements are widely marketed with many health claims, the prevention of cancer being one of them. There is a worldwide debate about the association between selenium exposure and cancer risk or whether selenium supplements are effective in decreasing the incidence or mortality of cancer. Epidemiologic and other data suggest differential effects in men and women and there are hints that selenium supplements might even have harmful effects, this especially being the case in certain populations.
Selenium is a trace element essential to humans. Humans usually ingest selenium with crop and animal products and sometimes as functional foods or supplements. Speciation and concentration of selenium in food sources vary considerably, depending on plant and animal metabolism and growth conditions or animal nutrition (Duffield 1999).
The recommended daily allowance differs between regulatory agencies. For example, the highest amount of daily intake (55 μg selenium for adults) has been recommended by the US Institute of Medicine (Institute of Medicine 2009), whereas the WHO (World Health Organization) recommendations range between 30 and 40 μg/day for men and women (WHO 2004).
Different selenium levels within populations have been found to be related to ethnicity (Kant 2007), gender, age or smoking behavior. Smoking tends to lower selenium biomarker concentrations despite being a source of selenium exposure (Kafai 2003).
Fifty-five studies with more than one million participants were included in this systematic review. Forty-nine studies observed and analyzed whether healthy people with high selenium levels in blood or toenail samples or with a high selenium intake developed cancer more or less often than other people. We found that people with higher selenium levels or intake had a lower frequency of certain cancers (such as bladder or prostate cancer) but no difference for other cancers such as breast cancer. However, it was not possible to determine from these studies that selenium levels or selenium intake was really the reason for the lower risk of cancer in some people. Factors apart from higher selenium levels could also influence the cancer risk: They might have had a healthier nutritional intake or lifestyle, have had a more favorable job or overall living conditions.
Six randomized controlled trials (RCT’s) assessed whether the use of selenium supplements might prevent cancer. In general, there are two types of selenium supplements: one type uses the salt of selenium as main ingredient; the other type uses organic selenium. These two types may act differently in the human body when ingested. We assessed the quality of each trial according to four established methodological criteria. The trials with the most reliable results found that organic selenium did not prevent prostate cancer in men and increased the risk of non-melanoma skin cancer in women and men. Other trials found that participants using selenium salt or organic supplements had a decrease in liver cancer cases. However, due to methodological shortcomings this evidence was less convincing. We advise further investigation of selenium for liver cancer prevention before translating results into public health recommendations. We also recommend that there should be further evaluation of the effects of selenium supplements in populations according to their nutritional status as they may differ between undernourished and adequately nourished groups of people.
To maintain or improve health, access to healthy food and a healthy diet is important. Currently, there is no convincing evidence that individuals, particularly those who are adequately nourished, will benefit from selenium supplementation with regard to their cancer risk.
Currently, regular intake of selenium supplements for cancer prevention cannot be recommended to either the selenium-replete or deficient populations.