Tuesday, May 10, 2011

Keep Fish on the Menu: Omega-3s and Prostate Cancer

Photo: courtesy practicalowl
If you tracked the health news last week, you may be suffering a bit of whiplash from all the double takes of unexpected headlines: “Mammograms plummet among women under 50” (MSNBC); “Study questions value of salt reduction in healthy people” (Boston Globe); and “Heart-healthy omega-3’s not healthy for prostate” (US News and World Report).

While we could write a post about each of these pieces, Dr. Len Lichtenfeld of the American Cancer Society did a nice job putting the small mammogram study in context (Dr Len's Blog), as did our Harvard colleagues at the Nutrition Source for the less-than-perfect salt/heart disease study (Nutrition Source).

But we'll say a few things about the fats and prostate cancer study by Brasky et al, which appeared in the American Journal of Epidemiology (study)(Brasky, 2011).

This was a case-control study that looked at blood levels of omega-3 fatty acids and how they might be related to prostate cancer risk. It was a fairly large study, with 1,658 men with prostate cancer (cases), 1,803 without prostate cancer (controls), and some pretty surprising results. 

For a little background, there are a number of types of omega-3 fatty acids.
  • Alpha-linolenic acid (ALA) is largely found in vegetable oils (especially soybean and canola oils), nuts (especially walnuts), and flax seeds (and their oil), and leafy green vegetables. 
  • Eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) are found mainly in fish.  The body can also convert ALA into these  types of omega-3s in the body, though this only accounts for a small amount of EPA and DHA in the body.
Omega-3's are generally seen as a "healthy" fat, since numerous studies have shown them to lower the risk of heart disease and stroke.   Omega-3's have anti-inflammatory properties and can help regulate genetic function, and these same properties have pushed the hypothesis that they could help reduce the risk of cancer as well.

Results for prostate cancer, though, have so far been a bit up-in-air.  Some studies have found that ALA (the type of omega-3 found in some vegetable oils) may increase the risk of serious prostate cancer (Giovanucci, 1993; Gann, 1994; Ramon, 2000), while others have found that DHA (the type of omega-3 largely from fish) actually lowers the risk of serious prostate cancer (Leitzmann, 2004; Szymanski, 2010).

The new Brasky et al study mixes these findings up further by showing nearly the opposite.  It found  that those with the highest blood levels of DHA had 2.5 times the risk of high grade prostate cancer compared to those with the lowest levels. 

It was a result opposite of what the author's - and what many other researchers -  expected.  And it's the type of finding that can leave people, once again, reeling from mixed messages, not knowing if they should embrace omega-3's or shun them.  

But this study on its own shouldn't change the healthy habits people follow. First, although it was a well-designed study, there were only a small number of cases of high-grade prostate cancer in the study population - just 125 total, compared to over 1,500 of low-grade cases. Such small high-grade numbers can leave more room for chance findings. The high-grade lesions were also generally small and unsymptomatic, so it's unclear if they'd develop further into something more serious or deadly.

Next, there are some pretty solid data from multiple studies showing protection from DHA. Ed Giovannucci, ScD, Professor of Nutrition and Epidemiology at the Harvard School of Public Health says that while the role of fatty acids in prostate cancer requires more study, currently there's more evidence of prostate cancer benefits than risks from consuming DHA-rich foods, like fish, noting that "a recent meta-analysis of fish intake and prostate cancer mortality found a highly significant 63 percent reduction in risk of fatal prostate cancer in the high fish consumers." (Szymanski, 2010).

So, for now, recommendation for a healthy diet should stay the same. It's important to focus on fruits, vegetables and whole grains. Keep red meat to a minimum. Avoid unhealthy fats, like trans fats and saturated fats.  And eat more healthy fats, like monounsaturated and polyunsaturated fats.  To be especially prostate-conscience, choose healthy fats that are low in ALA, like olive oil, corn oil, and peanut oil.

But, if you'd rather not track the alphabet soup of Omega-3's (ALA, EPA, DHA), that's OK for now, too. Overall, a diet rich in omega-3's of all types currently has a big health advantage over a diet low in omega-3's, since omega-3's have been clearly shown to lower the risk of heart disease, which kills over  400,000 people each year in the United States, compared to 28,000 for prostate cancer.

Other steps that can help lower the risk of prostate cancer include:
  • Eating a diet low in animal fat; cut back on red meat, whole milk and cheese
  • Eating more tomato-based foods like salsa and red spaghetti sauce
  • Avoiding too much calcium; stick to the recommended 1,000 to 1,200 milligrams a day for most adults

Related Websites
Your Disease Risk - Prostate Cancer


References
Brasky TM, Till C, White E, et al. Am J Epi. 2011; Early online, April 24.
Gann PH, Hennekens CH, Sacks FM, et al. J. Natl Cancer Inst. 1994; 86:281-6.
Giovannucci E, Rimm EB, Stampfer MJ, et al. J Natl Cancer Inst. 1993; 85:1571-9.
Leitzmann MF, Stampfer MJ, Michaud DS, et al. Dietary intake of n-3 and n-6 fatty acids and the risk of prostate cancer. Am J Clin Nutr. 2004; 80:204-16.
Ramon J, Bou R, Romea S, et al. Cancer Causes Control. 2000; 34:607-14.
Szymanski KM, Wheeler DC, Mucci LA. Am J Clin Nutr. 2010; 92:1223-33


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