The science of nutrition is never simple.
Exhibit A: omega-3 fatty acids, the type of healthy fat found in abundance in oily fish such as salmon.
Previous research has indicated these fats reduce inflammation and help guard against heart disease.
But a new study published this week suggests omega-3s have a dark side: They may foster the growth of aggressive forms of prostate cancer.
Even the researchers were surprised by their findings. "Frankly, we were stunned," concedes the lead author of the study, Theodore Brasky, at the Fred Hutchinson Cancer Research Center in Seattle. "Our hypothesis going into this study was that omega-3 fatty acids would reduce prostate cancer - and we found the opposite."
The results are based on data from the Prostate Cancer Prevention Trial, involving almost 19,000 men over the age of 55. The main purpose of the trial was to see if the drug finasteride (known by the brand name of Proscar), could help prevent prostate cancer. Over the course of the seven-year trial, 1,668 men developed cancer, which included 1,533 cases of low-grade, or slow-growing prostate cancer, and 125 cases of high-grade, aggressive prostate cancer.
As part of the study, the men underwent regular blood tests; Dr. Brasky's team used this information to examine the relationship between dietary fats and prostate cancer risk.
The analysis revealed that men with the highest blood levels of docosahexaenoic acid (DHA), a type of omega-3 fatty acid found in fish, were 2.5 times more likely to develop aggressive prostate cancer than those with the lowest DHA levels.
Equally baffling, the study also showed that the risk of aggressive prostate cancer was 50 per cent lower in men with the highest blood levels of unhealthy trans fats - which are found in processed foods and are associated with increased inflammation and heart disease.
Dr. Brasky is at a loss to explain the findings, which were published in the American Journal of Epidemiology.
"It is pretty interesting stuff, but I think the bottom line is that nutrition and nutritional research is incredibly complex and it is entirely possible that something that is good for your heart might be bad for your prostate," he said.
Does this mean that men should shun fish and eat more trans fats? Absolutely not, says Dr. Brasky. "You have to weigh the actual risks." Because men are more likely to die of heart disease than an aggressive form of prostate cancer, their dietary choices should remained focused on foods that may help reduce the risks of developing cardiovascular disease.
Although many men are diagnosed with prostate cancer, relatively few succumb to the disease. Only one in 27 men will die of this form of cancer. Most prostate tumours are slow-growing and non-fatal. Plus, dietary fats don't seem to have an effect on these low-grade cases.
Heart disease, on the other hand, is one of the leading killers of Canadian men, accounting for almost 22 per cent of male deaths annually.
"If you are eating fish, continue to do so. It's better for you than not eating fish," he advises.