Tuesday, January 15, 2008

Dairy Food And Prostate Cancer

Here's another study I stumbled across while investigating the milk and cancer link that keeps cropping up.

Dairy Products, Calcium, and Vitamin D and Risk of Prostate Cancer

It's a 2001 Harvard review of the body of evidence at that time on dairy intake and prostate cancer.
"Seven of 14 case-control and five of nine cohort studies have reported statistically significant positive associations between some aspect of dairy intake and prostate cancer risk. Overall, 12 of the 14 case-control studies and seven of the nine cohort studies observed a positive association for some measure of dairy products and prostate cancer; this is one of the most consistent dietary predictors for prostate cancer in the published literature."
Now, I realize Michael Pollan and others find reductionist nutritional science misleading and useless when it comes to choosing a diet. I think he has a point. You have to look at nutrients within context. However, if we were to accept the above excerpt without trying to understand what's going on, it might lead us to toss the milk out altogether.

What's Going On?

We don't know:
"It remains unknown which compounds in dairy products might be responsible for this association."
But we do have evidence that active vitamin D (1, 25 D produced in the kidneys from D precursors) is protective against prostate cancer:
"1, 25 D has consistently been shown to inhibit prostate cancer growth."
So why are dairy foods which are a good source of active vit D precursors increasingly linked to prostate cancer? A new line of thinking has emerged fingering calcium:
"When serum levels of calcium are low, 1, 25 D acts on the bones, kidneys, and intestines to increase retention and absorption of calcium until serum levels return to a normal range. Similarly, if serum levels of calcium are high, production of 1, 25 D is suppressed by reduced parathyroid hormone production."
Here's some evidence of that mechanism:
"The strongest evidence for an association between calcium intake and risk of prostate cancer comes from the Health Professionals Follow-up Study. ... In this study, men who consumed more than 2000 mg of calcium daily [from all sources, including supplements] had a multivariate relative risk of 4.6 for metastatic and fatal prostate cancer compared with men consuming less than 500 mg calcium daily."
That means there was a 4-and-a-half times greater probability that a high calcium consumer would experience metastatic and fatal prostate cancer than a low calcium consumer.

That's a pretty strong association. It could be the calcium ... there's both epidemiological evidence and a biologically plausible mechanism. Other components being discussed are IGF-1 and phosphorous. But we really don't know. Maybe Michael Pollen is right. Maybe we should just toss out the milk altogether. Or, well, keep it if our grandmothers drank it.

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