Men who eat one portion of salmon per week are 43 percent less likely to develop prostate cancer than men who eat no salmon, says a study from Sweden that links the apparent benefits to the omega-3 content of the fish.
The study, published online ahead of print in the International Journal of Cancer (doi: 10.1002/ijc.22319), adds to an ever-growing body of science linking omega-3 fatty acids to a wide range of health benefits, including cardiovascular disease (CVD), good development of a baby during pregnancy, joint health, behaviour and mood, and certain cancers.
Genetics play a role
The researchers, led by Maria Hedelin from the Karolinska Institutet in Stockholm, also found that genetics play a part in the development of the cancer, and also in the potential benefits of the fish oil.
“This study shows that there is an interaction between dietary factors and our genes, but it’s always hard to say what role the genes play,” she said. “Omega-3 fatty acids can still be good for men who don’t carry this gene variant in completely different ways.”
Over half a million new cases of prostate cancer are diagnosed every year worldwide, and the cancer is the direct cause of over 200 000 deaths. More worryingly, the incidence of the disease is increasing with a rise of 1,7 percent over 15 years.
How the study was done
The Swedish researchers assessed dietary fish intake among 1 499 men with prostate cancer and compared this with dietary intake of 1 130 healthy men in the general Swedish population. Genetic variations in a key enzyme in fatty acid metabolism and inflammation, cyclooxygenase (COX)-2, were also assessed.
Hedelin and her co-workers report that men who ate salmon-type fatty fish at least once a week were associated with a 43 percent reduction in prostate cancer risk compared to men who never ate fish.
They also found a significant interaction between salmon-type fish intake and a single nucleotide polymorphisms (SNP) in a COX-2 gene carried by 60 percent of the population. Carriers of the variant allele who ate one or more oily fish servings per week had an associated reduced prostate cancer risk of 72 percent, while no link was observed among carriers of the more common allele.
“Frequent consumption of fatty fish and marine fatty acids appears to reduce the risk of prostate cancer, and this association is modified by genetic variation in the COX-2 gene,” said Hedelin.
The proposed mechanism
The researchers’ proposed mechanism is that the gene controls the outcome when omega-3 and omega-6 fatty acids, which are found in vegetable oils, compete for inclusion in hormone-like substances in the body known as prostaglandins. Prostaglandins derived from omega-3 fatty acids are anti-inflammatory and may protect against the development of cancer, said the researchers, while prostaglandins derived from omega-6 fatty acids are proposed to be pro-inflammatory.
The study supports others on the subject. Indeed, a study published in March in the British Journal of Cancer (doi: 10.1038/sj.bjc.6603030), reported that a metabolite of the omega-6 fatty acid, arachidonic acid (AA), prostaglandin E2, helped the spread of the prostate cancer cells to bone marrow cells. However, when EPA and DHA were present at just half the concentration of the omega-6 fatty acid, this spread of cancer cells was stopped.
Omega ratio important
Another study, published in August in the journal Clinical Cancer Research (Vol. 12, Issue 15), reported that by increasing the omega-3 to omega-6 fatty acid ratio in the diet of mice reduced the growth of prostate cancer growth by 22 percent, compared to mice with mostly omega-6 fatty acids in the diet.
“We may be able to use EPA and DHA supplements while also reducing omega-6 fatty acids in the diet as a cancer prevention tool or possibly to reduce progression in men with prostate cancer,” said the lead researcher of the Clinical Cancer Research study, Professor William Aronson from UCLA.
The risk of pollutants from oily fish, such a methyl mercury, dioxins, and polychlorinated biphenols (PCBs) have led to some claims to reduce fresh fish intake, especially for pregnant women who may damage the development of their babies.
Such advice has seen the number of omega-3 enriched or fortified products on the market increase. Most extracted fish oil are molecularly distilled and steam deodorised to remove contaminants. - (Decision News Media, November 2006)
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