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Ask Dr. K: Can eating fish prevent prostate cancer?

DEAR DOCTOR K: Does eating fish help prevent prostate cancer?

DEAR READER: You’ve certainly heard me encourage readers to eat plenty of fish, particularly fatty fish such as salmon and mackerel. That’s because many good studies have found that people who eat fish frequently have lower rates of many serious diseases, including heart disease and several types of cancer.

A recently published study from the Prostate Cancer Prevention Trial was described in the media as coming to the opposite conclusion. To explain why, I first need to talk about the substances in fish that are thought to be beneficial for humans.

Fish contain high levels of two omega-3 fatty acids – EPA and DHA – which have been shown to reduce the risk of heart attacks and strokes. EPA and DHA also calm inflammation – and inflammation contributes to the development and progression of prostate cancer.

The PCPT measured levels of various fatty acids in the blood of 3,461 men, 1,658 of whom developed prostate cancer during the PCPT study. They found that men with the highest levels of DHA were 2.5 times more likely to develop aggressive, high-grade prostate cancer over a seven-year period compared with men who had the lowest levels of DHA. What could explain these results?

It’s possible that some of the participants already had high-grade cancer before they entered the trial.

Most important, this was not a study of fish consumption at all. It was a study of the levels of certain nutrients in the blood, including omega-3 fatty acids such as DHA. These omega-3s could have come from supplements, from food other than fish, or from fish itself.

It’s also important to put the results in perspective. The researchers analyzed blood samples from the 1,658 men who developed prostate cancer during the PCPT study. But only 125 of these men – about 8 percent of the total – developed high-grade tumors. The vast majority of men developed low-grade cancer, and DHA levels had no relationship to low-grade cancer.