The effect of selenium supplementation on blood sugar levels and on the risk of diabetes is still an open question. However, the data from randomized controlled studies show that selenium supplementation is associated with either a beneficial effect or no effect at all on blood sugar levels, insulin sensitivity, and glucose tolerance [Jablonska 2016].
Study Participants with Type-2 Diabetes
Beneficial effect. In a 2019 study, the participants were 72 male and 22 female patients aged 48 to 64 years old with diabetes mellitus type 2. They were smokers, all of them, and they all followed a Mediterranean diet.
The researchers administered selenium 200 microg/day once daily on an empty stomach. The study data showed a statistically significant reduction in the blood levels of glucose and in HbA1c values at both three months and six months after the beginning of the treatment. The administration of selenium to type-2 diabetic patients seemed to improve the patients’ glycemic profile [Karalis 2019].
Note: HbA1c test values give an overall picture of average blood sugar levels over a period of weeks.
Beneficial effect. In a study of 20 males and 40 females diagnosed with type-2 diabetes and coronary heart disease, selenium supplementation at 200 micrograms/day for eight weeks was associated with significantly reduced values for insulin resistance as well as with significantly increased insulin sensitivity values. There was no significant selenium-associated change in fasting plasma glucose levels [Farrokhian 2016].
Beneficial effect. Researchers randomly assigned 60 patients with diabetic nephropathy to receive 200 microg/day of selenium (n = 30) or placebo (n = 30) for 12 weeks. Selenium supplementation resulted in significant decreases in serum insulin levels and insulin resistance values as well as in significant increases in plasma glutathione peroxidase levels compared with the placebo. The selenium supplementation had no significant effect on fasting plasma glucose [Bahmani 2016].
Adverse effect. A 2014 randomized controlled trial that enrolled patients with type-2 diabetes (34 males and 26 females) showed that three months of selenium supplementation at 200 microg/day was significantly associated with increased fasting glucose levels [Faghihi 2014]. The result is puzzling not least because patients with diabetes have been shown to have increased oxidative stress associated with decreased selenium concentration [Faghihi 2014].
Women with Gestational Diabetes
No effect. The researchers randomly assigned 60 pregnant women with gestational diabetes (mean age: 29 years) to take either 100 microg/day selenium supplements or placebos for 12 weeks starting in the 24th-28th week of pregnancy. Compared with the placebo group, the selenium supplementation group showed no significant difference in fasting plasma glucose, HbA1C, serum insulin level, and insulin resistance. The supplementation 100 micrograms of selenium did not affect glucose homeostasis in women with gestational diabetes [Sadat Najib 2020].
Beneficial effect. The researchers tested the effect of selenium supplementation on pregnant women with gestational diabetes. The study participants were not taking oral hypoglycemic agents. The patients were randomly assigned to take 200 microg/day selenium supplements (n = 35) or placebo (n = 35) for 6 weeks starting from weeks 24 to 28 of pregnancy. The selenium supplementation, compared with placebo, resulted in significant reductions in fasting plasma glucose, serum insulin levels, and insulin resistance. Overall, the selenium supplementation in pregnant women with gestational diabetes showed beneficial effects on glucose metabolism [Asemi 2015].
Study Participants Not Diagnosed with Type-2 Diabetes
No effect. In the Danish PRECISE study, researchers randomly assigned 491 volunteers aged 60 to 74 years to treatment with 100, 200 or 300 microg/day selenium or placebo. After 2 years of selenium supplementation, HbA1c values decreased significantly in all treatment groups, with no difference between active treatment and placebo. Selenium supplementation in an elderly European population of low selenium status did not significantly affect HbA1c levels after 2 years [Stranges 2019].
No effect. The results of the clinical trial did not support a causal role for selenium supplementation at 200 microg/day in the development of insulin resistance or Type 2 diabetes. For an average of 2.9 years, there were no differences between selenium and placebo supplementation in terms of beta-cell function and insulin sensitivity. In addition, oral glucose tolerance testing showed that fasting blood glucose concentrations were significantly higher for those in the placebo group compared with those in the selenium treatment group [Jacobs 2019].
Beneficial effect. Researchers assigned a group of 53 patients (aged 45-85 years) with congestive heart failure to take 200 microg/day of selenium (n=26) or placebo (n=27) for 12 weeks. Selenium supplementation for 12 weeks resulted in a significant decrease in serum insulin concentrations and in insulin resistance values and in a significant elevation of insulin sensitivity values compared with the placebo but had no significant effect on fasting plasma glucose concentrations [Raygan 2018].
No effect. Researchers analyzed the blood samples of 76 non-smoking and non-diabetic individuals (36 males and 40 females) who had low baseline plasma selenium concentrations (mean: 65 microg/L) and who were supplemented with 200 microg/day of selenium in the form of selenium yeast for six weeks. The selenium supplementation was associated with a significant decrease in the level of HbA1c. The reduced levels of HbA1c persisted at least four weeks after discontinuation of the supplementation. The researchers did not see any changes at the level of fasting plasma glucose as a result of the supplementation. However, HbA1c is regarded as a more reliable marker of changes in glycemic control [Jablonska 2016].
No effect. In an analysis of data from 699 men who took part in a Phase 3 clinical trial investigating the effect of selenium supplementation on the risk of prostate cancer, the researchers found that changes in serum glucose levels during the course of the five-year trial did not differ significantly between the placebo group and the selenium 200 microg/day and 400 microg/day groups. The researchers tested the study participants every 6 months for up to 5 years. They concluded that the study results do not support a relationship between selenium supplementation and risk of diabetes [Algotar 2013].
No effect. In another study, the researchers randomly assigned the study participants to receive selenium 200 microg/day (n=47), selenium 800 microg/day (n=47), or placebo (n=46). They measured serum glucose levels every 6 months for up to 5 years. The changes in serum glucose levels during the course of the trial were not statistically significantly different as compared with placebo for either selenium group. The study results do not show a relationship between selenium supplementation and changes in serum glucose levels [Algotar 2010].
Beneficial effect. In a study of 70 women with polycystic ovary syndrome, the researchers found that eight weeks of supplementation with selenium at 200 microg/day was significantly associated with decreases in serum insulin levels and insulin resistance while the levels of fasting plasma glucose remained unchanged. The selenium supplementation was also significantly associated with an increase in the QUICKI quantitative insulin sensitivity check index [Jamilian 2015].
Beneficial effect. In a study of the effect of selenium supplementation on women with excess visceral fat, the researchers did not measure levels of fasting plasma glucose or glucose tolerance or HbA1c. However, six weeks of selenium supplementation at 200 microg/day was associated with significant decreases in serum insulin levels and insulin resistance values [Alizadeh 2012].
No effect. In the French Su.Vi.Max study, supplementation with 100 microg/day of selenium in combination with zinc and vitamins C, E, and beta-carotene for 7.5 years did not affect the level of fasting plasma glucose in 3146 participants [Czernichow 2006].
Take-Home Message: Selenium and Glucose Levels
Why eight out of 13 observational studies show a statistically significant positive correlation between selenium concentrations and the odds for Type 2 diabetes while the aggregated results of three long-term randomized clinical trials and several shorter-term intervention studies do not is puzzling [Kohler 2018].
- One explanation could be reverse causation, in which it is the Type 2 diabetes that affects selenium concentrations, not the other way around.
- Another explanation could be the existence of an unidentified residual confounding variable in the observational studies.
Selenium Supplementation Tips
- The preferred form of selenium supplementation is selenium from selenium-enriched yeast [Richie 2014].
- There may be a U-shaped relationship between selenium status and good health. In Figure 3 in her review article in The Lancet, Professor Margaret P. Rayman shows best health outcomes associated with a selenium status of 100-170 microg/Liter [Rayman 2012].
Sources
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Algotar AM, Stratton MS, Stratton SP, Hsu CH, Ahmann FR. No effect of selenium supplementation on serum glucose levels in men with prostate cancer. Am J Med. 2010;123(8):765-768.
Alizadeh M, Safaeiyan A, Ostadrahimi A, et al. Effect of L-arginine and selenium added to a hypocaloric diet enriched with legumes on cardiovascular disease risk factors in women with central obesity: a randomized, double-blind, placebo-controlled trial. Ann Nutr Metab. 2012;60(2):157-168.
Asemi Z, Jamilian M, Mesdaghinia E, Esmaillzadeh A. Effects of selenium supplementation on glucose homeostasis, inflammation, and oxidative stress in gestational diabetes: Randomized, double-blind, placebo-controlled trial. Nutrition. 2015;31(10):1235-1242.
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Czernichow S, Couthouis A, Bertrais S, et al. Antioxidant supplementation does not affect fasting plasma glucose in the Supplementation with Antioxidant Vitamins and Minerals (SU.VI.MAX) study in France: association with dietary intake and plasma concentrations. Am J Clin Nutr. 2006;84(2):395-399.
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Stranges S, Rayman MP, Winther KH, Guallar E, Cold S, Pastor-Barriuso R. Effect of selenium supplementation on changes in HbA1c: Results from a multiple-dose, randomized controlled trial. Diabetes Obes Metab. 2019;21(3):541-549.
The information presented in this review article is not intended as medical advice and should not be construed as such.
30 October 2020