New study below [1].
Odd that the dog (man's best friend) is the only other species (I believe) that naturally develops PCa. Stop giving them table scraps! LOL
"Our discovery of a U-shaped dose-response between toenail selenium concentration and prostatic DNA damage in dogs remarkably parallels data on the relationship between selenium status and prostate cancer risk in men. Notably, the dog U-curve provides a plausible explanation for the unanticipated increase in prostate cancer incidence among men with highest baseline selenium who received selenium supplementation in the largest-ever prostate cancer prevention trial (SELECT)."
The SELECT trial was the expensive & futile "Selenium and Vitamin E Cancer Prevention Trial".
Every selenium epidemiological study that I have read shows that you don't want to be in the bottom group, but there is no advantage in having intakes higher than that which will get you out of the bottom group.
What does that mean? In America, due to ample selenium in the soil of regions where wheat is grown (ditto Canada), selenium status is generally good. The bottom tenth in terms of intake is where the risk is concentrated. The risk in the other nine groups is pretty much the same. In Europe, where there is no significant selenium source in the diet (wheat is grown in low-selenium soils), a larger section of the population is at risk for low selenium-related PCa.
The problem with the SELECT study was that selenium status did not affect the intervention dose. Why give selenium to those who are not deficient? Why, in particular, give selenium to those who already have the highest intake? & so, SELECT reduced risk at the lower end, while increasing it at the upper end. This created the U-shaped risk not apparent in population studies.
The interesting thing about the dog study is that a U-shaped risk curve was also seen, even though there was no intervention program to explain it. But dogs don't exactly eat a natural diet. I don't have a dog, so I have no idea what the average dog food product contains. The coyotes I sometimes hear at night - how much selenium do they get from their kills?
"According to AAFCO, the maximum amount of selenium used in a dog food should not exceed 2.0 mg/kg on a dry matter basis — or about 18 times the recommended minimum of 0.11 mg/kg.
"And on a caloric basis, the maximum suggested selenium content is 0.57 mg per 1000 calories of food — which is about 19 times the minimum 0.03 mg for the mineral." [2]
Back to the study:
"Dog studies can be relied upon to contribute important insights into dose-dependent and form-dependent effects - two critical aspects of selenium biology that will have to be disentangled if the burgeoning science of selenium is to be translated into effective strategies for human disease prevention."
Yes - there are a variety of forms of selenium, with differing bioavailability & toxicity profiles.
For our sons & others who might ask about selenium for protection, supplementation at the lowest level is prudent.
-Patrick
Alcohol Study - lower risk of PCa progression 
Vitamin D and PCa mortality risk. 
