Summary of pooled RRs for selenium and subgroup analyses
| Groups | No. of results | No. of cases* | RR | 95% CI | Adjusted 95% CI | Heterogeneity P | ||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| RRs | ||||||||||||
| Overall | 13 | 1973 | 0.74 | 0.62–0.88 | 0.57–0.97 | <0.01 | ||||||
| Overall with NPC study | 14 | 2033 | 0.74 | 0.63–0.87 | 0.58–0.94 | <0.01 | ||||||
| Study design† | ||||||||||||
| Cohort | 4 | 501 | 0.81 | 0.60–1.10 | 0.51–1.30 | 0.07 | ||||||
| NCC | 9 | 1076 | 0.72 | 0.58–0.90 | 0.50–1.04 | <0.01 | ||||||
| Case-control | 2 | 517 | 0.93 | 0.65–1.33 | 0.56–1.55 | 0.15 | ||||||
| Gender | ||||||||||||
| Men | 8 | 912 | 0.68 | 0.54–0.86 | 0.46–1.02 | <0.01 | ||||||
| Women | 5 | 572 | 0.74 | 0.53–1.04 | NA | 0.41 | ||||||
| Assessment† | ||||||||||||
| Diet history/FFQ | 5 | 701 | 1.00 | 0.77–1.30 | NA | 0.47 | ||||||
| Serum selenium | 6 | 1029 | 0.80 | 0.63–1.03 | 0.58–1.10 | 0.13 | ||||||
| Toenail selenium | 4 | 364 | 0.46 | 0.32–0.66 | 0.24–0.87 | 0.03 | ||||||
| Smoking status | ||||||||||||
| Current smokers | 7 | 620 | 0.56 | 0.42–0.77 | 0.33–0.98 | <0.01 | ||||||
| Noncurrent smokers‡ | 5 | 274 | 0.72 | 0.46–1.12 | NA | 0.74 | ||||||
| Former smokers | 2 | 223 | 0.73 | 0.42–1.26 | 0.34–1.55 | 0.17 | ||||||
| Follow-up time§ | ||||||||||||
| <10 years | 12 | 762 | 0.70 | 0.55–0.90 | 0.43–1.15 | <0.01 | ||||||
| ≥10 years | 6 | 596 | 0.79 | 0.60–1.03 | 0.57–1.09 | 0.21 | ||||||
| Population selenium level∥ | ||||||||||||
| High | 4 | 754 | 0.86 | 0.65–1.15 | 0.61–1.22 | 0.22 | ||||||
| Low | 6 | 855 | 0.72 | 0.56–0.93 | 0.45–1.16 | <0.01 | ||||||
| Smoking adjustment | ||||||||||||
| Crude | 5 | 718 | 0.79 | 0.60–1.05 | 0.52–1.21 | 0.06 | ||||||
| Smoking adjusted | 5 | 718 | 0.82 | 0.61–1.12 | 0.53–1.28 | 0.08 | ||||||
| Mean differences | ||||||||||||
| Serum selenium∥ | 8 | 876 | 1.02 ng/ml¶ | −0.47 to 2.51 | 0.05 | |||||||
| High | 3 | 526 | −1.69 ng/ml¶ | −3.84 to 0.46 | 0.75 | |||||||
| Low | 5 | 350 | 3.90 ng/ml¶ | 1.84–5.96 | 0.99 | |||||||
| Toenail selenium | 4 | 667 | 0.02 μg/g¶ | 0.01–0.04 | 0.31 | |||||||
Note: NA, not applicable.
↵* The actual numbers maybe higher because the Hartman et al. study did not provide case numbers.
↵† Total number of studies and cases is greater than the number of studies and cases in the overall analysis because one cohort study assessing diet history of selenium and one NCC study assessing serum selenium were done in the same cohort (26, 27). Only the NCC study was included in the overall analysis, but diet history and serum results were included in the appropriate subgroup analyses. In the cohort study involving diet history, results were presented separately for smokers and nonsmokers.
↵‡ Includes never smokers and former smokers.
↵§ Total number of studies is greater than the number of studies in the overall analysis because some studies presented data for more than one follow-up period.
↵∥ Mean selenium levels in the controls. Classified as “high” if serum selenium ≥ 100 ng/ml or daily dietary selenium intake ≥ 55 μg. Classified as “low” if serum selenium < 100 ng/ml or daily dietary selenium intake < 55 μg.
↵¶ Mean difference defined as mean selenium level in controls minus mean selenium level in cases.