annualized favor: My take on the fish oil and prostate cancer study

Wednesday, July 17, 2013

My take on the fish oil and prostate cancer study | Dr Briffa's Blog - A ...

Every so often, a piece of health news will make a splash and trigger a mini-deluge of emails in my inbox. This was certainly the case last week, when a study linking certain omega-3 fats (?fish oils?) with an increased risk of prostate cancer. The study, published in the Journal of the National Cancer Institute, assessed the level omega-3 fat in the blood of 2227 men and risk of prostate cancer [1]. Men with the highest levels of omega-3 fats had a statistically significant increased risk of prostate cancer compared to those with the lowest levels. Specifically, increased risk of ?low-grade? (early) and ?high-grade? (more advanced) prostate cancer was found to be 44 and 71 per cent respectively.

One good thing about this study is that it assessed omega-3 fat levels in the blood. This, I think, is better than relying, say, on people?s memories of how much oily fish they ate over the preceding year or whatever. However, one fundamental deficiency of this study is the fact that it is epidemiological in nature. These studies can tell us about the relationships between things, but not that one thing is causing another.

Also, it often makes sense to view the results of any new study in the context of what has come before.

The authors themselves present data from similar studies grouped together in the form of what is known as a ?meta-analysis?. Meta-analyses are not perfect (for example, sometimes the methods used vary between studies which can make interpretation difficult), but they generally provide a better overview of the evidence than single studies. The meta-analyses (which included the present study) revealed:

A link between high omega-3 levels and increased risk of high-grade prostate cancer (higher levels were associate with a 51 per cent increased risk)
NO link between high omega-3 levels and increased risk of low-grade prostate cancer
NO link between high omega-3 levels and increased risk of prostate cancer overall (high- and low-grade prostate cancers combined)

Another way of getting an overview of the evidence is to broaden the enquiry beyond a single condition. Let?s use sunlight as an example. If we were, say, to focus purely on the relationship between sunlight exposure and certain skin cancers, we may be tempted to never go out in the sun unprotected. However, what is often missed is that sunlight exposure is also associated with a reduced risk of many different forms of cancer. Overall, the relationship between sunlight and cancer is one of protection. Also, sunlight is associated with a reduce risk of other conditions including heart disease and multiple sclerosis.

Probably the best and broadest way to judge the relationship between any factor and health is to assess its relationship with overall risk of death. Although still epidemiological in nature, these studies encompass all conditions. Do we have such evidence for omega-3 fats? Indeed we do.

In a study published earlier this year, researchers from the Harvard School of Public Health in the US measured blood levels of omega-3 fats in 2,692 adults and then followed them for an average of 11? years [2].

Those with the highest levels of omega-3 fats (compared to those with the lowest levels) were 27 per cent less likely to die over the course of the study.

Those with the highest levels lived an average of 26? months more than those with the lowest levels.

It would be difficult to make a case for this study not to trump the one from last week (which looked at risk of prostate cancer alone, and not even risk of death from this).

I saw the Harvard School of Public Health study when it came out but let it float by. Why? Primarily because it?s epidemiological in nature, and I?ve made it an unofficial policy of mine not to put too much store in research which, at the end of the day, doesn?t tell us that much.

References:

1. Brasky TM, et al. Plasma Phospholipid Fatty Acids and Prostate Cancer Risk in the SELECT Trial. J Natl Cancer Inst. 2013 Jul 10. [Epub ahead of print]

2. Mozaffarian D, et al. Plasma phospholipid long-chain ?-3 fatty acids and total and cause-specific mortality in older adults: a cohort study. Ann Intern Med. 2013;158(7):515-25

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