The biology of selenium in humans is complex. What is known is that selenium is widely distributed in body tissues and in physiological processes [Wrobel].
Particularly of interest is the preferential maintenance of selenium concentrations in the brain, even in circumstances in which selenium stores are deleted in other organs such as the liver and kidneys [Wrobel].
What Does Selenium Do in the Body?
Dr. Wrobel, University of Miami Department of Biochemistry and Molecular Biology, lists the following functions of selenium and selenoproteins in the body:
- Adequate levels of selenium are needed to modulate the function of the thyroid gland.
- Selenium plays an important role in male and female fertility.
- Selenium deficiency is associated with cardiac and skeletal muscle disorders, with changes in muscle fibers leading to the impairment of muscle contraction and to muscle atrophy.
- Low levels of selenium are associated with impaired cognitive function and neurological disorders.
- There exists an inverse relationship between selenium concentrations and the risk of coronary heart disease and the risk of certain types of cancer.
- Low selenium levels are associated with high HIV mortality.
- Selenium supplementation reduced the number of hospital admissions for HIV patients.
Selenium and the Prevention of Cancer
Epidemiological studies show generally that higher exposure to selenium is associated with a reduced risk of most forms of cancer. For example, people living in higher soil selenium regions tend to have lower incidence of cancer [Wrobel].
The results from intervention studies have been inconsistent. Daily supplementation with 200 micrograms of a selenium-enriched yeast preparation for 4.5 years in the Nutritional Prevention of Cancer Study was associated with significant reductions in the incidence of several cancers including prostate cancer as well as in overall cancer mortality [Wrobel].
On the other hand, daily intervention with 200 micrograms of a synthetic selenomethionine preparation in the SELECT Study did not seem to reduce the risk of cancer [Wrobel].
The difference in the outcomes of the NPC Study and the SELECT Study has been attributed to the difference in the basal selenium status of the participants – much lower in the NPC Study – and to the difference in the species in the selenium supplements – only one species in the SELECT Study preparation and many species of selenium in the NPC Study preparation [Wrobel].
Mechanism of Cancer Prevention with Selenium
The exact mechanisms of cancer prevention with selenium are not known. One hypothesis is that the antioxidative properties of the selenium containing proteins, the glutathione peroxidases and the thioredoxin reductases, are a factor but not the only factor in cancer prevention [Wrobel].
More specifically, other proposed mechanisms of action for the prevention of cancer with selenium include the following roles for selenium [Wrobel]:
- alteration of carcinogen metabolism
- stimulation of DNA repair
- modulation of inflammatory and immune responses
- regulation of cell cycle
- attenuation of cell proliferation
- inhibition of cell motility and angiogenesis
- stimulation of apoptosis
The relationship between selenium intake and cancer prevention may be U-shaped rather than linear. There is a relatively narrow window between selenium deficiency and selenium toxicity [Wrobel].
Selenium supplementation may benefit individuals with low selenium status but not individuals with high selenium status. Some selenium researchers recommend that individuals with plasma selenium concentrations above 122 micrograms per liter do not need a selenium supplement [Wrobel].
A systematic review of selenium and prostate cancer studies has shown that the risk of prostate cancer decreases with increasing plasma/serum selenium levels up to 170 micrograms per liter [Hurst].
Selenium and the Treatment of Cancer
Studies of the use of selenium supplements in the treatment of cancer have led to the conclusion that selenium supplementation can have the following effects without reducing the effectiveness of the primary cancer treatment [Wrobel]:
- enhances the efficacy of the chemotherapy
- limits side effects of cancer treatment
- improves the general condition of the cancer patients
Selenium and the Brain
The following is known about selenium and the brain:
- The brain is a privileged organ with respect to selenium. Selenium is retained longer in the brains of animals fed selenium-deficient diets than in other organs such as the liver and the kidneys.
- In selenium-deficient animals fed selenium, the brain impounds large portions of the available selenium.
- Selenoproteins protect against neurodegeneration by providing antioxidant defense in the brain.
- Changes in plasma and brain selenium concentrations are associated with increased risk of Alzheimer’s disease and brain tumors.
Selenium and Coenzyme Q10 and Heart Disease
Four years of daily supplementation with a combination of an organic high-selenium yeast preparation and Coenzyme Q10 supplements significantly reduced cardiovascular mortality, improved heart function, and increased health-related quality of life for senior citizens. The beneficial effects persisted long after the four years of supplementation [Alehagen].
Conclusions: Selenium and Selenoproteins
- The effects of selenium on human health are manifold and complex.
- Selenium supplementation can reduce the risk of developing certain types of cancer and may be useful in the treatment of cancer.
- Selenium accumulation and metabolism is especially important to the brain, and selenium supplementation may be a factor in the prevention of brain disorders.
- An organic selenium-enriched yeast formulation containing many and various species of selenium provides greater potential for health benefits and is the formulation used in most clinical trials.
Sources
Alehagen, U., Johansson, P., Björnstedt, M., Rosén, A., & Dahlström, U. (2013). Cardiovascular mortality and N-terminal-proBNP reduced after combined selenium and Coenzyme Q10 supplementation: a 5-year prospective randomized double-blind placebo-controlled trial among elderly Swedish citizens. International Journal of Cardiology, 167(5), 1860-1866.
Alehagen, U., Aaseth, J., Alexander, J., & Johansson, P. (2018). Still reduced cardiovascular mortality 12 years after supplementation with selenium and coenzyme Q10 for four years: A validation of previous 10-year follow-up results of a prospective randomized double-blind placebo-controlled trial in elderly. Plos One, 13(4), e0193120.
Hurst, R., Hooper, L., Norat, T., Lau, R., Aune, D., Greenwood, D. C., & Fairweather-Tait, S. J. (2012). Selenium and prostate cancer: systematic review and meta-analysis. The American Journal of Clinical Nutrition, 96(1), 111-122.
Wrobel, J. K., Power, R. & Toborek, M. (2016). Biological activity of selenium: revisited. International Union of Biochemistry and Molecular Biology, 68(2): 97-105.
The information presented in this review article is not intended as medical advice and should not be construed as such.