Background: Selenium is considered to be an antioxidant, and its high levels have been inversely associated with cancer risk of several sites. This meta-analysis examined the relationship between levels of selenium measured in serum and toenails, and the risk of bladder cancer.

Methods: A meta-analysis using data from seven published epidemiologic studies (three case-control, three nested case-control, one case-cohort) published before March 2010 was done to examine the association between levels of selenium and bladder cancer. Fixed and random effects analyses were done to calculate meta-odds ratio (mOR) and 95% confidence intervals (CI). Heterogeneity among studies was measured by the I2 statistic.

Results: Overall, the risk of bladder cancer was inversely associated with elevated levels of selenium according to a random-effects model (mOR = 0.61; 95% CI, 0.42-0.87). The mORs were 0.95 (95% CI, 0.69-1.27) and 0.55 (95% CI, 0.32-0.95) among men and women, respectively. Sex, type of sample specimen, smoking status, and study design were found to be potential sources of heterogeneity.

Conclusions: A significant protective effect of selenium, observed mainly among women, may result from gender-specific differences in its accumulation and excretion. The heterogeneity found among studies was mainly linked to the different biological sample specimens used to measure the selenium concentrations and the small size of the studies. Although these results suggest a protective effect of selenium for bladder cancer risk, additional large studies are warranted to support these preliminary evidence.

Impact: The present results suggest a beneficial effect of high selenium intake for bladder cancer risk. Cancer Epidemiol Biomarkers Prev; 19(9); 2407–15. ©2010 AACR.

Worldwide, bladder cancer is one of the most common types of cancer, especially among men. The highest incidence is observed in Spain and Italy. The established risk factors for bladder cancer are smoking, which accounts for 60% of cases in men and 35% in women, occupational exposure to aromatic amines, high levels of arsenic intake, and schistosomiasis infection (1). Bladder cancer is a complex disease, and polymorphisms in low-penetrance genes are also involved in the development of this neoplasm. There is consistent evidence that NAT2 slow acetylator and GSTM1 null genotypes increase the risk of bladder cancer. Furthermore, NAT2 slow acetylator has been found to modulate the effect of smoking on bladder cancer, which represents one of the few known gene environment interactions involved in carcinogenesis (2). Although these factors explain more than half of the etiologic scenario of bladder cancer (3), a substantial fraction of the disease remains unexplained. The risk factors that remain may include a complex pattern of environmental exposures, difficult to measure with questionnaires. The increasing evidence of a role for trace metals and other environmental exposures in cancer acting through oxidative stress mechanisms (4, 5) suggests their further exploration in relation to bladder and other cancers.

Although the mechanism(s) by which selenium may act as an anticarcinogen are not fully known, several studies have observed an inverse association between selenium status and cancer, such as gastrointestinal, lung, and prostate cancers (6-9). However, for prostate cancer, there have been some conflicting results reported by clinical trials (10-12).

The present meta-analysis was conducted to summarize the association between high levels of selenium as measured in biological sample specimens and bladder cancer risk, and examine the possible causes of heterogeneity among published studies on this topic.

Study selection

Studies were identified by searching PubMed, ISI Web of Knowledge, Scopus, Cochrane Library, and Google Scholar databases before March 2010 for the terms “selenium” and “bladder cancer.” References from relevant articles were also used to identify studies that were not found in the database search.

Eligible studies included epidemiologic manuscripts reporting measures of association between selenium and bladder cancer risk by measuring selenium in any of the following biological sample specimens: blood/serum, nails, hair, and saliva. Studies were excluded if they were not written in English, Spanish, or Portuguese; if they presented insufficient data; if they were reviews; or if they were not epidemiologic studies. Furthermore, duplicate articles were excluded (Fig. 1).

From the selected articles, the information extracted was as follows: (a) first author's last name, year of publication, and country of the population studied; (b) study design; (c) sample size; (d) odds ratio (OR) or relative risk (RR) estimates with 95% confidence interval (CI), and adjustments for potential confounding factors, if applicable. Information on sex and type of sample specimen was also extracted from all the eligible publications, whenever possible. Relative risk was treated as if it was OR.

The corresponding authors of the selected studies were contacted if pertinent information on relevant study data was not available in the published article. To assess the quality of each study, the criteria used by Flores-Mateo et al. (13) were adapted (Supplementary Table S1). These major criteria include, for example, the assessment of exposure at the individual level, data collection in a similar manner for all participants, and the use of incident cases only.

Statistical analysis

The OR or relative risk and 95% CI for highest (exposed) versus lowest (reference) levels of selenium groups were extracted from the selected manuscripts. To obtain the suitable weight of each study related to the summary OR, the SEM for each logarithm of the OR was calculated by using the 95% CI. The square of the SEM was used as the estimated variance of the logarithm of the OR. Both fixed and random effects models were assessed, but the latter was preferentially used when heterogeneity was detected.

The I2 statistic, representing the proportion of total variation across study estimates due to heterogeneity, was used to determine the level of heterogeneity (14). Potential sources of heterogeneity were explored using stratified meta-analysis to check the influence of the following determinants: sex, type of sample specimen, smoking status, and study design. Additionally, the relative influence of each study on pooled estimates was assessed by omitting one study at a time. Publication bias was explored by analyzing funnel plots and Egger's regression asymmetry test. Statistical calculations were done with STATA/SE 10.1 (StataCorp).

The initial literature search identified 172 publications. After excluding duplicates and other publications according to selection criteria (Supplementary Table S2), three case-control, three nested case-control, and one case-cohort studies that examined the association between selenium status and bladder cancer risk were identified and used in the present analysis (Table 1). A nested case-control study of a Finnish population met almost all the inclusion criteria but could not be included because it lacked an adequate definition of reference and exposed groups (15). The total number of cases and controls/cohort members in the identified studies was 1,910 and 17,339, respectively. Four studies were done in the United States and three in Northern Europe. In four studies, selenium status was based on analysis of toenails, whereas in the remaining three, serum was the sample specimen used. The two oldest and smallest studies of the seven reported nonsignificant inverse associations (Table 1; refs. 16, 17). Three other studies reported the same tendency. Only two showed a significant decrease in the risk of bladder cancer among individuals with higher selenium levels (Table 1; refs. 18-22). In two case-control studies, toenail selenium concentrations were inversely associated with bladder cancer risk only among women, moderate smokers, and p53-positive cancers (Table 1; refs. 19, 22). Using a random effects model, the overall OR of bladder cancer risk for the highest compared with the lowest selenium status was 0.61 (95% CI, 0.42-0.87). A comparable result was found when using a fixed effects model (OR = 0.70; 95% CI, 0.58-0.84; Fig. 2).

Among the pooled studies, a moderate level of heterogeneity was detected (χ2 = 15.32; degrees of freedom = 6; P = 0.018; I2 = 60.8%; Fig. 2).

In the analysis stratified by gender, only women yielded significant decreased risk associated with selenium (OR = 0.55; 95% CI, 0.32-0.95) with a nonsignificant I2 of 23.7% (Table 2). The sample specimen in which selenium was determined (serum or toenails) was found to be a source of heterogeneity (Table 2). Although selenium in toenails and in serum provided significant results, serum levels of selenium showed a stronger protective effect (OR = 0.33; 95% CI, 0.21-0.51). Stratifying the analysis according to smoking status decreased the heterogeneity found in the overall analysis, and results became similar between never and ever smokers (Table 2). As for the type of study, the stratified results were consistent with the pooled overall estimate. Moreover, heterogeneity increased in the stratum of case-control studies (χ2 = 13.13; degrees of freedom = 2; P = 0.001; I2 = 84.8%; Table 2).

Publication bias was not detected in the meta-analysis (coefficient = −1.84, P = 0.357), even when stratifying by the several study determinants (Table 2). As expected, the most influential studies in the analysis were found to be the largest ones (18, 19, 22).

In the present meta-analysis, we observed a significant 39% decreased risk of bladder cancer associated with high levels of selenium by combining results from seven epidemiologic studies, conducted in different populations, which applied individual levels of selenium measured in serum or toenails. Although little is known on the role of selenium in bladder cancer, several systematic reviews and meta-analysis, both on observational (6-9) and interventional studies (10-12), have reported on the protective effect of selenium on cancer. The few clinical trials on selenium supplementation have reported conflicting and not yet conclusive results, with the Nutritional Prevention of Cancer (NPC) and the Supplémentation en Vitamines et Minéraux Antioxydants showing an inverse association between selenium and prostate cancer, and the Selenium and Vitamin E Cancer Prevention trial reporting nonsignificant increased risk of prostate cancer among those selenium-supplemented individuals (8, 10-12). The NPC also reported results on bladder cancer and other types of cancer, as secondary outcomes of the trial, but no clear association was found, probably due to insufficient number of incident cases (23).

The above-mentioned high selenium status refers to concentrations of selenium in toenails of ∼0.9 μg/g and in serum of ∼100 μg/L, or higher. Toenail and serum selenium levels are correlated4

4D. Behne. (personal communication) calculated from the data of the 22 test persons investigated in “Behne D, Alber D, Kyriakopoulos A. Long-term selenium supplementation of humans: Selenium status and relationships between selenium concentrations in skeletal muscle and indicator materials. J Trace Elem Med Biol 2010;24:99-105.”

(24, 25). Furthermore, a level of about 80 to 95 μg/L of selenium in serum is considered optimal for the maximization of glutathione peroxidases and selenoprotein P activities (26).

Selenium may exert anticarcinogenic effects mainly through selenoproteins, although the specific mechanisms are not yet fully known. Aberrant expression patterns of glutathione peroxidases and selenoprotein P, found in colorectal cancer, show that the antioxidant properties of selenoenzymes are relevant in carcinogenesis and tumor progression (27), particularly by scavenging reactive oxygen species and diminishing further oxidative damage. The protection of selenium against cancer is also linked to the activities of hydrogen selenide and selenomethionine present in cells, which may be responsible for modifying protein thiols and mimicking methionine, leading to higher methylating efficiency of RNA and thiols (28). Some studies suggest that this essential dietary trace element has antioxidant properties and that it produces effects on apoptosis, DNA repair, and carcinogen metabolism. To decrease the oxidative stress caused by exposure to arsenic, cadmium, or lead, the selenium requirement increases (29) as these metals act as selenium antagonists (30). Moreover, both organic and inorganic forms of selenium may enhance p53 activity toward either DNA repair or apoptosis (31). If selenium is in the form of seleno-l-methionine, the DNA repair branch of the p53 pathway is preferentially induced, which also involves Ref1 and Brca1 in a protein complex (32).

Stratified analyses by type of sample specimen (serum and toenails), smoking status, and study design also found that higher levels of selenium lead to a lower risk of bladder cancer, although each was identified as a potential source of heterogeneity. Serum and toenail measurements are both accepted as biomarkers of exposure to assess the exposure to selenium in the organism (33, 34), but they provide information on different time frames with toenail selenium reflecting longer term exposure (35, 36). Part of the heterogeneity found in the pooled overall analysis was also explained by the smoking status, which may be due to different ways of assessing that status and different definitions of smoking groups among studies. Although published evidence between selenium and smoking is not clear, several studies on healthy individuals show that smokers have lower levels of selenium (37, 38). Although study design explained some of the heterogeneity, more diversity was found among the case-control studies, which is probably the result of different criteria for selection of controls.

The summary effects for the two sexes were not similar in that a significant inverse association was observed for women but not men. An opposite gender pattern, with protective effects in men and not in women, was reported in a meta-analysis on selenium supplementation and primary cancer incidence and mortality (6). However, the trials in that meta-analysis were not specific for the effects of selenium on bladder cancer risk; two of the four trials that provided sex-specific data used a mix of compounds rather than selenium alone; and the effects of selenium on cancer incidence and mortality were based on levels of supplementation instead of internal levels of selenium. Moreover, only one of them assessed the internal levels of selenium of the individuals, and this showed that men had higher levels of selenium at baseline, which could perhaps explain the higher protection from cancer in those. Although no accepted explanation for these sex-specific differences exists, there are some hypotheses indicating different excretion rates, half-lives, and also sensitivity to potential toxic effects of selenium between genders (39, 40).

Publication bias was not evident in this meta-analysis. However, a note of caution is warranted because only seven epidemiologic studies were included in this meta-analysis and because they presented some heterogeneity. Nevertheless, the results of this meta-analysis and the heterogeneity found among studies are informative in the sense they emphasize the input of each study to the existing literature and the topics that entail further research.

In conclusion, this meta-analysis supports an inverse association between selenium concentration and bladder cancer risk. To further elucidate this relationship, efforts to quantify selenium and other trace metals in biological sample specimens at the individual level in large observational studies or randomized trials are needed. These are fundamental steps before suggesting selenium supplementation to bladder cancer patients.

No potential conflicts of interest were disclosed.

Grant Support: Red Temática de Investigación Cooperativa en Cáncer, Instituto de Salud Carlos III, Spanish Ministry of Science and Innovation, and by the Association for International Cancer Research (AICR09-0780).

The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

1
Silverman
D
,
Devesa
S
,
Moore
L
,
Rothman
N
. 
Bladder cancer
. In:
Schottenfeld
D
,
Fraumeni
J
 Jr.
, editors.
Cancer epidemiology and prevention
. 3rd ed.
New York (NY)
:
Oxford University Press
; 
2006
, p.
1101
27
.
2
Garcia-Closas
M
,
Malats
N
,
Silverman
D
, et al
. 
NAT2 slow acetylation, GSTM1 null genotype, and risk of bladder cancer: results from the Spanish Bladder Cancer Study and meta-analyses
.
Lancet
2005
;
366
:
649
59
.
3
Murta-Nascimento
C
,
Schmitz-Dräger
BJ
,
Zeegers
MP
, et al
. 
Epidemiology of urinary bladder cancer: from tumor development to patient's death
.
World J Urol
2007
;
25
:
285
95
.
4
Valko
M
,
Rhodes
CJ
,
Moncol
J
,
Izakovic
M
,
Mazur
M
. 
Free radicals, metals and antioxidants in oxidative stress-induced cancer
.
Chem Biol Interact
2006
;
160
:
1
40
.
5
Navarro Silvera
SA
,
Rohan
TE
. 
Trace elements and cancer risk: a review of the epidemiologic evidence
.
Cancer Causes Control
2007
;
18
:
7
27
.
6
Bardia
A
,
Tleyjeh
IM
,
Cerhan
JR
, et al
. 
Efficacy of antioxidant supplementation in reducing primary cancer incidence and mortality: systematic review and meta-analysis
.
Mayo Clin Proc
2008
;
83
:
23
34
.
7
Bjelakovic
G
,
Nikolova
D
,
Simonetti
RG
,
Gluud
C
. 
Antioxidant supplements for preventing gastrointestinal cancers
.
Cochrane Database Syst Rev
2008
:
CD004183
.
8
Zhuo
H
,
Smith
AH
,
Steinmaus
C
. 
Selenium and lung cancer: a quantitative analysis of heterogeneity in the current epidemiological literature
.
Cancer Epidemiol Biomarkers Prev
2004
;
13
:
771
8
.
9
Etminan
M
,
FitzGerald
JM
,
Gleave
M
,
Chambers
K
. 
Intake of selenium in the prevention of prostate cancer: a systematic review and meta-analysis
.
Cancer Causes Control
2005
;
16
:
1125
31
.
10
Meyer
F
,
Galan
P
,
Douville
P
, et al
. 
Antioxidant vitamin and mineral supplementation and prostate cancer prevention in the SU.VI.MAX trial
.
Int J Cancer
2005
;
116
:
182
6
.
11
Duffield-Lillico
AJ
,
Dalkin
BL
,
Reid
ME
, et al
. 
Selenium supplementation, baseline plasma selenium status and incidence of prostate cancer: an analysis of the complete treatment period of the Nutritional Prevention of Cancer Trial
.
BJU Int
2003
;
91
:
608
12
.
12
Lippman
SM
,
Klein
EA
,
Goodman
PJ
, et al
. 
Effect of selenium and vitamin E on risk of prostate cancer and other cancers: the Selenium and Vitamin E Cancer Prevention Trial (SELECT)
.
JAMA
2009
;
301
:
39
51
.
13
Flores-Mateo
G
,
Navas-Acien
A
,
Pastor-Barriuso
R
,
Guallar
E
. 
Selenium and coronary heart disease: a meta-analysis
.
Am J Clin Nutr
2006
;
84
:
762
73
.
14
Higgins
JP
,
Thompson
SG
. 
Quantifying heterogeneity in a meta-analysis
.
Stat Med
2002
;
21
:
1539
58
.
15
Knekt
P
,
Aromaa
A
,
Maatela
J
, et al
. 
Serum micronutrients and risk of cancers of low incidence in Finland
.
Am J Epidemiol
1991
;
134
:
356
61
.
16
Nomura
A
,
Heilbrun
LK
,
Morris
JS
,
Stemmermann
GN
. 
Serum selenium and the risk of cancer, by specific sites: case-control analysis of prospective data
.
J Natl Cancer Inst
1987
;
79
:
103
8
.
17
Helzlsouer
KJ
,
Comstock
GW
,
Morris
JS
. 
Selenium, lycopene, α-tocopherol, β-carotene, retinol, and subsequent bladder cancer
.
Cancer Res
1989
;
49
:
6144
8
.
18
Kellen
E
,
Zeegers
M
,
Buntinx
F
. 
Selenium is inversely associated with bladder cancer risk: a report from the Belgian case-control study on bladder cancer
.
Int J Urol
2006
;
13
:
1180
4
.
19
Michaud
DS
,
De Vivo
I
,
Morris
JS
,
Giovannucci
E
. 
Toenail selenium concentrations and bladder cancer risk in women and men
.
Br J Cancer
2005
;
93
:
804
6
.
20
Michaud
DS
,
Hartman
TJ
,
Taylor
PR
, et al
. 
No Association between toenail selenium levels and bladder cancer risk
.
Cancer Epidemiol Biomarkers Prev
2002
;
11
:
1505
6
.
21
Zeegers
MP
,
Goldbohm
RA
,
Bode
P
,
van den Brandt
PA
. 
Prediagnostic toenail selenium and risk of bladder cancer
.
Cancer Epidemiol Biomarkers Prev
2002
;
11
:
1292
7
.
22
Wallace
K
,
Kelsey
KT
,
Schned
A
,
Morris
JS
,
Andrew
AS
,
Karagas
MR
. 
Selenium and risk of bladder cancer: a population-based case-control study
.
Cancer Prev Res
2009
;
2
:
70
3
.
23
Clark
LC
,
Combs
GF
 Jr.
,
Turnbull
BW
, et al
. 
Effects of selenium supplementation for cancer prevention in patients with carcinoma of the skin. A randomized controlled trial. Nutritional Prevention of Cancer Study Group
.
JAMA
1996
;
276
:
1957
63
.
24
Longnecker
MP
,
Taylor
PR
,
Levander
OA
, et al
. 
Selenium in diet, blood, and toenails in relation to human health in a seleniferous area
.
Am J Clin Nutr
1991
;
53
:
1288
94
.
25
Satia
J
,
King
I
,
Morris
J
,
Stratton
K
,
White
E
. 
Toenail and plasma levels as biomarkers of selenium exposure
.
Ann Epidemiol
2006
;
16
:
53
8
.
26
Thomson
CD
. 
Assessment of requirements for selenium and adequacy of selenium status: a review
.
Eur J Clin Nutr
2004
;
58
:
391
402
.
27
Murawaki
Y
,
Tsuchiya
H
,
Kanbe
T
, et al
. 
Aberrant expression of selenoproteins in the progression of colorectal cancer
.
Cancer Lett
2008
;
259
:
218
30
.
28
Jackson
MI
,
Combs
GF
 Jr.
Selenium and anticarcinogenesis: underlying mechanisms
.
Curr Opin Clin Nutr Metab Care
2008
;
11
:
718
26
.
29
Fowler
BA
,
Whittaker
MH
,
Lipsky
M
,
Wang
G
,
Chen
XQ
. 
Oxidative stress induced by lead, cadmium and arsenic mixtures: 30-day, 90-day, and 180-day drinking water studies in rats: an overview
.
Biometals
2004
;
17
:
567
8
.
30
Schrauzer
GN
. 
Anticarcinogenic effects of selenium
.
Cell Mol Life Sci
2000
;
57
:
1864
73
.
31
Smith
ML
,
Lancia
JK
,
Mercer
TI
,
Ip
C
. 
Selenium compounds regulate p53 by common and distinctive mechanisms
.
Anticancer Res
2004
;
24
:
1401
8
.
32
Fischer
JL
,
Lancia
JK
,
Mathur
A
,
Smith
ML
. 
Selenium protection from DNA damage involves a Ref1/p53/Brca1 protein complex
.
Anticancer Res
2006
;
26
:
899
904
.
33
Slotnick
MJ
,
Nriagu
JO
. 
Validity of human nails as a biomarker of arsenic and selenium exposure: A review
.
Environ Res
2006
;
102
:
125
39
.
34
Arnaud
J
,
Bertrais
S
,
Roussel
AM
, et al
. 
Serum selenium determinants in French adults: the SU.VI.M.AX study
.
Br J Nutr
2006
;
95
:
313
20
.
35
Longnecker
MP
,
Stram
DO
,
Taylor
PR
, et al
. 
Use of selenium concentration in whole blood, serum, toenails, or urine as a surrogate measure of selenium intake
.
Epidemiology
1996
;
7
:
384
90
.
36
Garland
M
,
Morris
JS
,
Rosner
BA
, et al
. 
Toenail trace element levels as biomarkers: reproducibility over a 6-year period
.
Cancer Epidemiol Biomarkers Prev
1993
;
2
:
493
7
.
37
Sanchez
C
,
Lopez-Jurado
M
,
Aranda
P
,
Llopis
J
. 
Plasma levels of copper, manganese and selenium in an adult population in southern Spain: Influence of age, obesity and lifestyle factors
.
Sci Total Environ
2010
;
408
:
1014
20
.
38
Swanson
C
,
Longnecker
M
,
Veillon
C
, et al
. 
Selenium intake, age, gender, and smoking in relation to indices of selenium status of adults residing in a seleniferous area
.
Am J Clin Nutr
1990
;
52
:
858
62
.
39
Rodriguez Rodriguez
EM
,
Sanz Alaejos
MT
,
Diaz Romero
C
. 
Urinary selenium status of healthy people
.
Eur J Clin Chem Clin Biochem
1995
;
33
:
127
33
.
40
Patterson
B
,
Veillon
CC
,
Taylor
P
,
Patterson
K
,
Levander
OA
. 
Selenium metabolism in humans differs by gender: results from a stable isotope tracer study
.
FASEB J
2001
;
15
:
A969
.

Supplementary data