This morning's papers are filled with reports that selenium reduces prostate cancer risk.
Here's the low-down.
The study is small, only 181 cases of advanced prostate cancer. The key result -- i.e., those with the highest selenium levels had one-third the rate of advanced prostate cancer -- is only based on 35 cases. But there's more.
The researchers kept adjusting the statistics until they got the desired result. When the data was adjusted only for age and smoking, no statistically significant trend was observed between selenium and prostate cancer risk. When the data was adjusted for lycopene, saturated fat, calcium, family history of prostate cancer, body mass index and vasectomy, the trend was only borderline statistically significant. To reach the touted result, the data were further adjusted for soil selenium content of the regions the subjects were from. I still don't see the link between soil selenium content and prostate cancer risk.
This type of trend analysis is nonsense anyway. In this study, a trend consists of five data points -- one for each quintile of selenium level (measured from toenail clippings!). Two of the trend points -- the relative risks for the second and fourth quintiles -- aren't statistically significant. So these are really nonsense data points and can't make up a "trend."
The relative risks for the third and fifth trend points are the same -- a fact which contradicts the researcher's hypothesis. If selenium really lowered prostate cancer risk, we would observe less risk with more selenium. But here the subjects in the third and fifth quintiles had the same risk even though the subjects in the fifth quintile had 40 percent more selenium!
Instead of wasting my time reading this study, I should have just looked at the study's authors. There I found Walter Willet. As readers of the Junk Science Home Page know, Walter Willet is a synonymous with "junk science."
Comments on this posting?
Click here to post a public comment on the Trash Talk Bulletin Board.
Click here to send a private comment to the Junkman.
Copyright © 1998 Steven J. Milloy. All rights reserved on original material. Material copyrighted by others is used either with permission or under a claim of "fair use." Site developed and hosted by WestLake Solutions, Inc.
Material presented on this home page constitutes opinion of Steven J. Milloy.