Written by Gabriele Dennert and the CAM-Cancer Consortium.
Updated October 20, 2013

Selenium prevention

What is it?

Description

The trace element selenium (Se) is nutritionally essential for human health and is naturally present in various food groups including grains, pulses, meat and fish. Although a number of health problems have been linked to selenium deficiency, selenium is also toxic in large doses. Selenium occurs naturally in a number of inorganic forms (e.g. selenite, selenate, selenide) and is also found in organic compounds (e.g. selenomethionine, selenocysteine). 1

Selenium in human nutrition

Humans usually ingest selenium in crop-grown or animal products. One slice of whole-wheat bread or a half-cup of boiled rice contains approximately 10µg selenium; one egg includes approximately 14µg selenium, and 100 g of meat or fish generally contains between 20µg and 40µg of selenium. 2 However, the selenium content of foods may vary significantly depending on plant and animal metabolism, growing conditions and animal nutrition, or even the geographic region from which the food originates. 3

The bioavailability of ingested organic and inorganic selenium – as a function of absorption, metabolism and excretion – is reported to be between 50% and 90%, depending on the form of selenium and type of preparation, as well as on age, physiological status and polymorphisms in selenium transport protein genes. 4

Subclinical selenium deficiency is sometimes defined as being a serum selenium level of below 75µg/l (0.95µM). 5,6 According to this definition, subclinical selenium deficiency is rare in middle and northern European countries (e.g. studies with volunteers have found prevalence rates below 2% in Switzerland and France) 5,6 but prevalence of subclinical selenium deficiency may be higher in Eastern and Southern European countries 7,8 and in populations with chronic diseases and cancer. 9 However, there is no consensus for using the serum selenium level as a marker of selenium status, and no universal normal reference values have yet been established.

History

The chemical element selenium was discovered in 1817 by the Swedish chemist Jöns Jakob Berzelius. In the 1930s, toxic properties of selenium were discovered in cattle, and in 1957 selenium was identified as an essential mammal nutrient.

Ingredients

Licensed preparations for the treatment of selenium deficiency contain sodium selenite and can be obtained as liquid solutions for oral intake or intravenous administration. In addition, a large number of selenium-containing nutritional supplements or dietary aids are marketed for medical or health purposes. These contain either selenium alone (in inorganic or organic forms) or selenium in combination with other trace minerals as well as vitamins. Fermentation of yeast in a medium that is rich in inorganic selenium yields selenised yeast which contains 80-90% organic forms such as selenomethione, Se-methylselenocysteine and γ-glutamyl-Se-methylselenocysteine. 10

Application and dosage

Selenium supplements are marketed as tablets, capsules, pellets and liquid solutions for oral intake. Mono-selenium preparations usually contain 50–300µg of selenium per single dose, while multi-component supplements usually contain 30–50µg.

Recommended daily intake of selenium differs between regulatory agencies. The US Institute of Medicine recommends the highest amount of daily selenium intake (55µg selenium for adult women and men), when compared to other regulatory agencies. 11 The World Health Organization (WHO) recommends a daily selenium intake range of 30–40µg/day for adults, depending on body weight. 12 The average dietary intake of selenium in European populations appears to meet the WHO recommendations, 3 with mean values of between 40µg and 93µg/day in men and women, respectively. However, estimation of individual daily selenium intake is difficult and the optimal amount of selenium intake for a person’s health is unknown.

Supplemental selenium is often taken in doses ranging from 50–800µg/day. The doses used in prevention trials have ranged from 200–400µg/day of selenised yeast to 500µg/day of sodium selenite.

Claims of efficacy/Alleged indication(s)

Selenium supplementation has been promoted for several distinct purposes in relation to cancer. Regarding primary prevention, selenium supplements have been claimed to protect against several types of cancer, in particular those of the prostate and the digestive tract.

Mechanism(s) of action

Selenium is bound to selenoproteins in the human body after intestinal absorption and incorporated into proteins either specifically, in catalytically active selenoenzymes, or non-specifically in tissue proteins. 13

More than 25 selenium-containing enzymes have been identified to date. There are at least two functional groups of selenoproteins. The first is involved in redox processes in the tissues (glutathione peroxidases, thioredoxin reductase), the second in thyroid hormone metabolism (iodothyronine deiodinases). Furthermore, several selenoproteins have been linked to a variety of health problems. 14

Purported mechanisms of action against cancer include, amongst others, the modulation of immune function, regulation of the cell cycle and apoptosis, alteration of DNA damage/repair 15, p1355, Figure 9. In-vitro studies showed that selenium affects Akt enzymatic activity 16 and p53 expression. 17 The understanding of selenium biology, however, is still incomplete.

Prevalence of use

The author is not aware of any studies investigating the prevalence of supplemental selenium use for cancer prevention in general populations.

Legal issues

Most, but not all, forms of selenium are freely marketable for human use within the EU (selenised yeast, for example, is not). Licensed drugs for the treatment of selenium deficiency are available over the counter in most European countries; a prescription is required in Denmark and Germany (although Germany, only for preparations containing more than 50µg selenium). 18

Cost(s) and expenditures

The price of 100µg of inorganic selenium, via Internet pharmacies, is about €0.50 (€0.15–1.60); this amounts to approximately €15 a month.

Expenditures are higher for prescription drugs than for over-the-counter nutritional supplements, but may be reimbursable in some countries if the individual has a clinical diagnosis of selenium deficiency.

Citation Gabriele Dennert, CAM-Cancer Consortium. Selenium prevention [online document]. http://ws.cam-cancer.org/The-Summaries/Dietary-approaches/Selenium-prevention. October 20, 2013.

References

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