The quality of evidence is downgraded by study limitations (unclear allocation concealment), and by inconsistency (variability in results).
A Cochrane review [Abstract] 1 included 12 studies with a total of 19 715 subjects (7 studies with duration of at least 3 months). The vast majority of participants involved in the studies were male individuals from the USA, where people are already well nourished and take large amounts of selenium from natural foods.
There were no statistically significant effects of selenium supplementation on mortality or cardiovascular disease (CVD) events. There was a small, statistically nonsignificant, increased risk of type 2 diabetes with selenium supplementation. There was no statistically significant effect on total cholesterol, and mean HDL levels were unchanged. There was a statistically significant reduction in non-HDL cholesterol in one study of varying selenium dosage. None of the longer term trials examined effects on blood pressure. One study (the large SELECT study) found significantly increased risk of alopecia (RR 1.28, 95% CI 1.01 to 1.62) and dermatitis grade 1 to 2 (RR 1.17, 95% CI 1.0 to 1.35) with selenium supplementation.
| Outcome | Number of participants (studies) | Effect size (95% CI) |
|---|---|---|
| All cause mortality | 18 452 (2) | RR 0.97 (0.88 to 1.08) |
| Cardiovascular disease (CVD) mortality | 18 452 (2) | RR 0.97 (0.79 to 1.2) |
| Non-fatal CVD events | 18 452 (2) | RR 0.96 (0.89 to 1.04) |
| All CVD events (fatal and non-fatal) | 18 452 (2) | RR 1.03 (0.95 to 1.11) |
| Type 2 diabetes | 18 790 (3) | RR 1.06 (0.97 to 1.15) |
| Total cholesterol | 576 (3) | WMD - 0.11 mmol/L (- 0.3 to 0.07) |
| HDL cholesterol | 576 (3) | WMD 0.01 mmol/L (- 0.06 to 0.08) |
| Non-HDL cholesterol | 472 (1) | WMD - 0.2 mmol/L (- 0.41 to 0.00) |
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