Certain kinds of coldwater fish, including trout, salmon, tuna, halibut, swordfish and mackerel contain especially high levels of omega-3 fatty acids. These acids have been shown to reduce the formation of blood clots, which can diminish the risk of heart attacks and strokes. Two- to three-weekly servings of these fish are recommended for healthy hearts.

But the value of omega-3s for cancer prevention has been less clear, with some studies suggesting they are beneficial, while others showed no effect. The research letter in Lancet concludes that consumption of fatty fish "could be associated with a decreased risk of prostate cancer."

This study was conducted by Paul Terry, PhD, and his colleagues at the Karolinska Institute in Stockholm, Sweden; it involved more than 6,000 Swedish twins who were first contacted in 1967 and followed for 30 years. Their diet was assessed only once, however, at the start of the study.

"We asked participants whether various food types accounted to none, or a very little part of their diet, a small part, a moderate part, or a large part," the authors write. They also questioned the men about their smoking, alcohol consumption, and exercise habits. The researchers then tracked the men using a national cancer registry to determine their rate of prostate cancer. There were 466 cases of prostate cancer, of which 340 were fatal. The average age at diagnosis was about 77 years old.

"Men who ate no fish had a two-fold to three-fold higher frequency of prostate cancer than those who ate moderate or high amounts did," according to Terry, of the Karolinska Institute’s epidemiology department. This held true regardless of their other lifestyle habits.

Terry adds that these findings could be helpful because to date little is known about how to reduce the risk of prostate cancer.

"Our study was done in Sweden, a country with traditionally high consumption of fatty fish from Northern — cold — waters, which contain high amounts of omega-3 fatty acids," Terry says. "Since few dietary and other modifiable factors seem to be associated with lower risk of prostate cancer, our results may indicate an important means by which this disease might be prevented."

"These results are promising, but they should be followed up with other studies," says Marji McCullough, ScD, a senior epidemiologist and registered dietitian with the American Cancer Society (ACS). "It’s hard to make specific recommendations based on this study, although it does seem like a good study. It’s like putting the pieces of a puzzle together."

McCullough notes that current ACS dietary guidelines don’t specifically call for the consumption of fatty fish, although the American Heart Association has made this recommendation. But the ACS guidelines are updated as new research becomes available, she says.

"I would still say that fish is part of a healthy diet, and recommend limiting your intake of animal fat, especially from red meat," McCullough adds.