Quote: “Pluck almost any cell from your body, and it will have a million or more selenium atoms in it, yet until recently nobody had any idea what they were there for. We now know that selenium makes two vital enzymes, deficiency in which has been linked to hypertension, arthritis, anaemia, some cancers, and, even, possibly reduced sperm counts. So, clearly, it is a good idea to get some selenium inside you (it is found particularly in nuts, whole meal bread, and fish), but at the same time, if you take too much, you can irremediably poison your liver. As with much of life, getting the balances right is a delicate business.” End Quote.
The above lines are quoted from Bill Bryson’s book The Body: A Guide for Occupants. ISBN-13: 978-0385539302. I can recommend the book highly. Bryson writes an English that is a pleasure to read, and the book is full of facts and relationships. You may already know him from his earlier book about science and technology, The Short History of Almost Everything.
Selenium and Seleno-Enzymes and Good Health
For quick basic information about the micronutrient selenium, you can go to the Linus Pauling Institute’s Micronutrient Information Center. There you will find the following basic facts about selenium:
- The chemical forms and levels of selenium in plant-based food vary according to the selenium content of soil; there is considerable variation in selenium levels from region to region of the world. For example, selenium intakes from food are relatively high in Canada, Japan, the United States, and Venezuela and much lower in Europe, particularly in northern and eastern Europe. China has regions characterized by selenium deficiency and other regions seemingly with an excess of selenium [Rayman 2012].
- The tolerable upper intake level for selenium from both food and supplements is 400 μg/day for adolescents and adults. One estimate is that adults in the US have average selenium intakes of about 100 μg/day; however, in the US, there is also considerable variation in the selenium content in food. By comparison, the average daily intake of selenium from food in the UK and in the Scandinavian countries is likely to be about 50 micrograms [Hurst 2010; Larsen 2002].
- The trace element selenium’s biological functions are carried out primarily with the selenium incorporated into the amino acid, selenocysteine.
- Selenocysteine is a component of 25 identified selenoproteins.
- We humans need sufficient selenium intakes for the appropriate functioning of these selenoproteins.
Seleno-enzymes: The most studied and most important selenoproteins
- Glutathione peroxidases: The GPx enzymes are mainly important as antioxidants that neutralize hydrogen peroxide and lipid hydroperoxides [Rayman 2012].
- Iodothyronine deiodinases: The seleno-enzymes are involved in the production of active thyroid hormone T3 [Rayman 2012].
- Thioredoxin reductases: Among other things, Trx-R-1 reduces the oxidized Coenzyme Q10 form ubiquinone to the reduced form ubiquinol and thus regenerates the antioxidant form and protects cells from oxidative stress [Nordman 2003].
- Selenoprotein P: The major function of SEPP is to transport and supply selenium in the body; it also has some antioxidant functions [Rayman 2012].
Selenium and Type-2 Diabetes: The Evidence
In a 2013 interview with Dr. Richard Passwater, Dr. Gerhard N. Schrauzer pointed out that the selenium–diabetes claim that was made in 2007 was poorly supported and was based on research results that seemed to indicate that diabetics have plasma selenium levels that tend to be higher than the plasma selenium levels of healthy people [Passwater 2013].
However, Dr. Schrauzer explained that this correlation between selenium levels and the risk of diabetes was the result of disease-related changes in the diabetics’ plasma proteins, not the result of the selenium’s allegedly having caused the diabetes. Dr. Schrauzer drew attention to newer research results from 2010 that showed that selenium may actually protect against the development of diabetes [Passwater 2013].
Dr. Passwater remarked that the regions of the world with higher selenium intakes tend to have low incidences of type-2 diabetes [Passwater 2013].
- In 2019, Jacobs et al published findings from a sub-set analysis of data from the Selenium Trial in Arizona. The findings showed no statistically significant differences in insulin sensitivity and beta-cell function between the study participants who received 200 micrograms of selenium in selenium-enriched yeast preparations and the study participants who received placebo preparations. Participants in both groups took the assigned preparation once daily for a median period of 33.6 months [Jacobs 2019].
- In 2018, Kohler et al reported the results of a systematic review and meta-analysis to investigate any possible association between selenium levels and the incidence of type-2 diabetes. The results showed that, in randomized controlled trials, there is no significant effect of selenium on the incidence of Type-2 diabetes [Kohler 2018]. Kohler et al concluded that their analysis demonstrates no consistent evidence that selenium supplementation plays a role in the development of Type-2 diabetes in adults.
- In 2014, Mao et al reported the results of a meta-analysis of four randomized controlled trials enrolling 20,294 participants. Their results suggest that selenium supplementation has no effect on the risk of Type-2 diabetes in Caucasians [Mao 2014].
- In a 2011 follow-up report on the results from the SELECT Trial, Klein et al reported no statistically significant increased risk of Type-2 diabetes in the selenium supplementation group compared to the placebo group. In the SELECT Trial, 8752 study participants took 200 micrograms of synthetic selenomethionine daily; 8696 study participants took placebos [Klein 2011].
- However, observational studies demonstrate a correlation between blood selenium levels and the risk of Type-2 diabetes [Kim 2019].
- Results from observational studies do not establish a cause-effect relationship. More weight should be placed on the results of randomized controlled trials, and the randomized controlled trials do not show any significant effect of selenium supplementation on the risk of developing Type-2 diabetes [Kohler 2018; Mao 2014; Klein 2011].
- It may be that the pathology of Type-2 diabetes has an effect on blood selenium levels such that it is Type-2 diabetes that is causing higher blood selenium levels rather than the other way around [Schrauzer in Passwater 2013].
- The discrepancy between the observational study results and the randomized controlled trial results may be related to a hitherto undiscovered confounding variable that affects both the exposure to selenium and the Type-2 diabetes endpoint.
- Studies from 2008-2010 have shown that higher blood selenium levels 1) may be protective against diabetes and metabolic syndrome, 2) may reduce damage from diabetes, and 3) may be positively associated with a lower occurrence of sugar metabolism imbalance [Kornhauser 2008; Puchau 2009; Akbaraly 2010].
Take-home Message: Why We Need Sufficient Selenium Intakes
Dr. Schrauzer made the following points in a 2010 interview with Dr. Passwater [Passwater 2010]:
- Selenium yeast preparations most closely duplicate the natural sources of selenium in our food. Selenium-enriched yeast is the best supplement choice.
- For selenium supplementation to give effective reduction of cancer risk, the supplementation should begin as early in adulthood and should be continued over the entire life span.
- The cancer-protective properties of selenium are undermined by exposure to arsenic, cadmium, lead, and mercury. We must limit our exposure to these elements in our food, drinking water, and environment.
Sources
Akbaraly TN, Arnaud J, Rayman MP, et al. Plasma selenium and risk of dysglycemia in an elderly French population: results from the prospective Epidemiology of Vascular Ageing Study. Nutr Metab (Lond). 2010;7:21.
Hurst R, Armah CN, Dainty JR, et al. Establishing optimal selenium status: results of a randomized, double-blind, placebo-controlled trial. Am J Clin Nutr. 2010;91(4):923-931.
Jacobs ET, Lance P, Mandarino LJ, et al. Selenium supplementation and insulin resistance in a randomized, clinical trial. BMJ Open Diabetes Res Care. 2019;7(1):e000613. Published 2019 Feb 7.
Kim J, Chung HS, Choi MK, et al. Association between Serum Selenium Level and the Presence of Diabetes Mellitus: A Meta-Analysis of Observational Studies. Diabetes Metab J. 2019;43(4):447-460.
Klein EA, Thompson IM Jr, Tangen CM, et al. Vitamin E and the risk of prostate cancer: the Selenium and Vitamin E Cancer Prevention Trial (SELECT). JAMA. 2011;306(14):1549-1556.
Kohler LN, Foote J, Kelley CP, et al. Selenium and Type 2 Diabetes: Systematic Review. Nutrients. 2018;10(12):1924.
Kornhauser C, Garcia-Ramirez JR, Wrobel K, Pérez-Luque EL, Garay-Sevilla ME, Wrobel K. Serum selenium and glutathione peroxidase concentrations in type 2 diabetes mellitus patients. Prim Care Diabetes. 2008;2(2):81-85.
Larsen EH, Andersen NL, Møller A, Petersen A, Mortensen GK, Petersen J. Monitoring the content and intake of trace elements from food in Denmark. Food Addit Contam. 2002;19(1):33-46.
Mao S, Zhang A, Huang S. Selenium supplementation and the risk of type 2 diabetes mellitus: a meta-analysis of randomized controlled trials. Endocrine. 2014;47(3):758-763.
Passwater R.A. Selenium and Human Health: An Interview with Gerhard Schrauzer, Ph.D. WholeFoods. 2013. Retrieved from http://www.drpasswater.com/nutrition_library/SeleniumHumanHealth.html.
Passwater RA. New FDA Qualified Health Claims for Selenium: An Interview with Gerhard Schrauzer. WholeFoods. Retrieved from https://wholefoodsmagazine.com/columns/vitamin-connection/new-fda-qualified-health-claims-selenium/.
Puchau B, Zulet MA, González de Echávarri A, Navarro-Blasco I, Martínez JA. Selenium intake reduces serum C3, an early marker of metabolic syndrome manifestations, in healthy young adults. Eur J Clin Nutr. 2009;63(7):858-864.
Rayman MP. Selenium and Human Health. Lancet. 2012;379(9822):1256-1268.
Thompson PA, Ashbeck EL, Roe DJ, et al. Selenium Supplementation for Prevention of Colorectal Adenomas and Risk of Associated Type 2 Diabetes. J Natl Cancer Inst. 2016;108(12):djw152. Published 2016 Aug 16
The information presented in this review article is not intended as medical advice and should not be used as such.
10 July 2020