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Physicians' Health Study Reports Results Of "A Prospective Study Of Plasma Selenium Levels And Prostate Cancer Risk"
(June 2004)
In the JNCI May 5,
2004 issue the PHS researchers detailed 13 year follow-up data regarding
the 586 men who developed prostate cancer from the start of the study in
1982 up to 1995. These men were a subset of the 22,071 physician data
base that has been the source of many important health related findings.
The analysis in this study compares these 586 men with 577 well matched
control cases in whom no prostate cancer was diagnosed during that
period. The focus of the research is on the conclusions that can be
drawn based on the single measurement of plasma selenium obtained from
each participant at entrance into the study. The selenium levels were
categorized into five groups, the highest quintile to the lowest
quintile, and researchers were interested in noting the relationship
between the selenium levels and 1) the risk of developing prostate
cancer, and 2) the degree of aggressiveness of the prostate cancer.
Some details may help
in understanding "high" and "low" selenium levels, the fulcrum of this
study's analysis. The median baseline selenium level for both
prostate cancer patients and controls was essentially the same, about
104 mcg/L. Quintile groupings were established. The median selenium
value for the lowest quintile was ~90 mcg/mL, and the levels increased
by quintile at ~100, ~110, ~ 120, to the highest quintile median level
of ~130 mcg/L. For a perspective in interpreting selenium levels, it is
interesting to note that in the much discussed Clark study, where
selenium supplementation of 200 mcg daily was associated with a 63%
lower incidence of prostate cancer compared to the control group, the
baseline selenium level was 115mcg/L, and the supplemental 200 mcg
selenium boosted the level to ~190 mcg/L. The range of values for
the highest PHS quinitle was ~120 to ~190 mcg/mL. This comparative
observation points up that the baseline selenium levels for men in the
highest quintile of the PHS study were trending upward to the level
attainable with a 200 mcg selenium supplementation.
Now the results:
Overall, based on analysis of the co-mingled selenium levels of the
cancer and control groups, men in the highest quintile of pre-diagnostic
selenium levels had a statistically nonsignificant 22% lower risk
of developing prostate cancer during the follow-up period than the men
in the lowest quintile. However, subset evaluation further clarified
this overall observation by noting that "higher plasma selenium levels
were associated with a lower risk of prostate cancer only for
prostate cancer patients with increased baseline PSA levels [greater
than 4 ng/mL]". And an even more refined subset evaluation found that
this inverse relationship between selenium levels and prostate cancer
risk was particularly strong for men with PSA levels above 10 ng/mL,
where men with the highest levels enjoyed a 63% reduction in the risk of
developing prostate cancer.
The principle
conclusion, however, of this study was that, although having a low
selenium level did not significantly increase a man's overall chance of
developing prostate cancer in the 12 year follow-up, if a man did
develop cancer and his selenium level had been low at baseline,
he had a much greater likelihood of presenting with advanced
prostate cancer. Of those men who developed cancer, the men in the
highest quintile for selenium had a 48% lower risk of developing
advanced prostate cancer than the men in the lowest quintile.
The article offers a
conjecture based on the fact that "the prevalence of microscopic
or latent prostate tumors is similar in most populations" worldwide,
while the incidence of prostate cancer in the United State is
nearly the highest in the world. The authors then speculate that "higher
levels of selenium may slow prostate cancer progression", and
that optimal selenium intake may be an important modifiable factor that
affects prostate cancer development, particularly the more advanced
type.
Bottom Line:
Maintenance of an adequate selenium level may be an important modifiable
factor to prevent prostate cancer progression.
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