Blood Serum Levels of Omega-3 and Prostate Cancer Risk
Brasky,T.M. et al., Am. J. Epidemiol., 173: 1429-1439 , 2011
Fred Hutchinson Research Ctr., Seattle, WA, USA
In this population study on males (aged 55-84 years) across various centers in the US, the possible relation between polyunsaturated levels (and trans fatty acids) in blood serum lipid samples and the 7-year prevalence of prostate cancer was monitored.
The authors reported that higher levels of DHA (one of the two predominant omega-3 fatty acids found in fish ) as measured in blood samples were positively correlated with the risk of high-grade prostate cancer but not low-grade prostate cancer in US men. It is noted that 7.5 % of all prostate cancers in this population study were considered as high grade whereas 92.5 % were low-grade . Further, higher intakes of ‘trans’ fatty acids (common constituents of processed and fast foods containing partially-hydrogenated vegetable oils) were associated with a lower risk of high-grade prostate cancer.
Both of these findings (ie, positive relation of blood DHA omega-3 levels and negative relation of trans fatty acids to high-grade prostate cancer risk ) were most surprising (on first notice) since they were contrary to expectations from almost all the many previous studies on this topic.
The following points need to be considered:
1) The authors did not directly measure the intakes of fish or the intake of the fish/seafood -derived omega-3 fatty acids (DHA plus EPA).
2) Many previous studies have reported protective effects of both fish and dietary (DHA plus EPA) for total prostate cancer risk. Some have shown no increase in risk and others a decrease in prostate-cancer related mortality. It is noted that a major US study from the Harvard Medical School as published in 2008 (Am. J. Clin. Nutr., 88: 1297-1303(2008)) on 20,167 men found no relation of fish intakes to prostate cancer incidence with a 48 % lower risk of death from prostate cancer in those with the disease who consumed fish 5 or more times per week. A similar association was found for seafood-derived omega-3 fatty acid intakes and prostate cancer mortality. The following quote is taken from a very recent review of the many studies on the topic wrt fish intakes from Szymanski et al. (Am. J. Clin. Nutr., 92: 1223-1233 (2010) who concluded: “Our analyses provide no strong evidence of a protective association of fish consumption with prostate cancer incidence but showed a significant 63% reduction in prostate cancer-specific mortality.”
3) It is noted that it was the ‘blood level’ of DHA which (if higher) was associated with a greater risk of high-grade cancer. It is also noted that the average blood level of DHA was generally ‘low’ which is typical of American men who consume an average of one fish serving every 7-10 days (favourite type being tuna, a fish with much lower levels of DHA and EPA than salmon and many other fish and much lower than many fish oil supplements). Thus , the range of DHA levels in the blood went from ‘lower to higher’ with the ‘higher’ in fact being very much lower as compared to those who would consume fish (especially salmon, others) at least 2-3 times/week or take fish oil supplements containing DHA/EPA on a regular basis. Further, the blood levels of DHA were very much lower across the board than the Japanese who consume several levels of fatty fish/seafood/week.
4) It is noted that when 2-3 fatty fish servings/week or more or supplements containing fish oil are consumed, a significant rise in the blood levels of both DHA and EPA are seen . This is expected since both DHA plus EPA are found in fish and fish oil supplements. The EPA blood levels were very low in this study (typical of North Americans) who eat little fish . Further, the authors found no statistically-significant relation between blood levels of EPA or (EPA plus DHA) and the risk of high-grade prostate cancer. An earlier study (from Harvard) found no relation between blood levels of marine-derived omega-3 fatty acids ands prostate cancer risk (Gann et al., J. Nat. Cancer Inst., 86: 281-286(1994).
5) It is noted that a very recent paper from this same group (Brasky et al., Nutr. And Cancer, 63: 573-582 (2011) reported no relation between the intake of fish oil supplements and prostate cancer risk.
6) While the bulk of the published literature on higher fish intakes and higher (DHA plus EPA) intakes from fish/seafood or supplements show protective effects or no harm wrt prostate cancer , it is ‘possible’ that at very low DHA intakes and very low EPA intakes (ie, the lowest sub-set of American men when compared to the higher with the realization that essentially all are low by a global perspective), blood DHA levels (when moderately elevated ) may be a risk for advanced prostate cancer when the EPA levels remain low. Based on the literature in bulk, this is not likely with higher dietary intakes of DHA plus EPA and correspondingly higher blood levels of DHA plus EPA than found currently in American men (as in this study).
Finally, a major US analyses from various institutions which collaborated in the data analyses on lifestyle factors related to mortality concluded that low intakes of DHA plus EPA omega-3 from marine sources and high intakes of trans fatty acids were major risk factors for all-cause mortality (PLoS Med., 6: 1-23 (2009).).