The research evidence to date suggests that there is a U-shaped relationship between selenium intake and health. According to a recent report by the long-time selenium researcher Professor Dr. Margaret P. Rayman, University of Surrey, UK, both selenium deficiency and selenium excess have been associated with adverse health effects.
Conditions Indicating a Need for Selenium Supplementation
Professor Rayman lists a number of conditions that have been associated in the research literature with selenium deficiency:
- Keshan disease (a heart muscle disease caused by a selenium deficiency together with a strain of Coxsackie virus)
- Kashin-Beck disease (a bone disease for which selenium deficiency is a factor)
- Increased viral virulence
- Increased mortality
- Poorer immune function
- Problematic fertility/reproduction
- Thyroid autoimmune disease
- Cognitive decline/dementia
- Type-2 diabetes
- Prostate cancer risk
- Colo-rectal cancer risk (in women)
- Increased risk of tuberculosis in HIV patients
Professor Rayman does not specify a plasma/serum selenium level for selenium deficiency.
She does mention a US National Health and Nutrition Examination Survey that measured the serum selenium levels in 13,887 adult participants and then followed up for mortality for up to 18 years. The mortality in that study showed a U-shaped association between serum selenium and death, with a serum selenium concentration of 135 micrograms per liter at the bottom of the U [Rayman 2019].
Suggested Plasma Selenium Levels for Adequacy
Professor C. D. Thomson has suggested the following plasma selenium concentrations as levels of minimum selenium adequacy:
- > 20 mcg/L To prevent the selenium deficiency disease Keshan disease
- > 79-95 mcg/L To maximize plasma GPx and SeppP selenoprotein levels
- > 118 mcg/L To protect against some cancers
Note: GPx refers to the selenium containing antioxidant enzymes glutathione peroxidase, which neutralize hydrogen peroxide and lipid peroxide radicals.
SeppP refers to the selenium containing selenoprotein that is the main plasma selenium transporter to body tissues.
Necessary Selenium Intakes to Raise Plasma Selenium Levels
Selenium intakes and plasma/serum selenium concentrations vary considerably from region to region depending upon the availability of selenium in the soil and in fodder and food. One estimate is that the mean European blood selenium levels range from 70 to 80 micrograms per liter while median US blood selenium concentrations are between 105 and 140 micrograms per liter [Outzen].
A study designed to find optimal selenium intakes showed that 55 micrograms of selenium daily from food plus 50 or 100 micrograms of selenium daily from supplements for 10 weeks will raise the plasma selenium levels as follows [Hurst 2010]:
- From 95.7 micrograms per liter up to 118.3 micrograms per liter with a daily selenium supplement of 50 micrograms
- From 95.7 micrograms per liter up to 152.0 micrograms per liter with a daily selenium supplement of 100 micrograms
Analysis of studies of the association between blood selenium levels and prostate cancer risk has shown the cancer risk slowly declining as the measured blood selenium levels increased from 60 to 170 micrograms per liter [Hurst 2012].
Long-Term Exposure to High Daily Dosages of Selenium
Figure 1 in Professor Rayman’s report shows that daily doses of selenium supplementation of 300 micrograms for five years in a country with moderately low selenium status (Denmark) are associated with increases in all-cause mortality 10 years later [Rayman 2019].
However, she goes on to explain that certain other population groups with high-selenium exposure over long periods appear able to cope with high selenium intakes without adverse effects, e.g. the Inuit in Greenland and Northern Canada as well as certain population groups in the Brazilian Amazon and in regions of China [Rayman].
She gives four possible reasons for this tolerance of high selenium exposures [Rayman 2019]:
- Some people may have polymorphisms in the genes that affect the ability to deal with high selenium exposure.
- High selenium status does confer health benefits without toxic effects in populations exposed to high levels of arsenic, cadmium, and mercury.
- The extent of the harm caused by high exposure to selenium may depend upon the form (species) of the selenium ingested.
- The composition of the gut microbiota may influence the effect of the high exposure to selenium.
Conclusion: Selenium intake/status and health
Professor Rayman concludes that the concept of a U-shaped relationship between selenium intake/status and health effects is useful but is also somewhat simplistic.
Clearly, the selenium intakes for optimal health vary from region to region and from population to population and even from individual to individual.
Logically, the best thing to do is to get one’s plasma or serum selenium level tested to see where it falls in the range from 60 to 170 micrograms per Liter, the range in which there is a gradual decrease in prostate cancer risk [Hurst 2012].
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Outzen, M., Tjønneland, A., Larsen, E. H., Friis, S., Larsen, S. B., Christensen, J., & Olsen, A. (2016). Selenium status and risk of prostate cancer in a Danish population. The British Journal of Nutrition, 115(9), 1669-1677.
Rayman, M.P. (2019). Selenium intake, status, and health: a complex relationship. Hormones, doi: 10.1007/s42000-019-00125-5. [Epub ahead of print].
Thomson, C.D. (2004). Assessment of requirements for selenium and adequacy of selenium status: a review. Eur J Clin Nutr, 58(3): 391-402.
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The information contained in this review article is not intended as medical advice and should not be used as medical advice.