New research from USC and the Cancer Prevention Institute of California (CPIC) has found that eating salmon and other dark, fatty fish may decrease the risk of developing prostate cancer, while consuming flounder and other white, lean fish may increase the risk.
It depends on how the fish is cooked, according to the researchers.
Previous studies have emphasized the health benefits of dark fish – rich in omega-3 fatty acids – linking their consumption to the prevention of various diseases.
“One would expect eating dark and oily fish would be beneficial in preventing prostate cancer, but that protective effect seems lost if fish are cooked with high-temperature methods, in particular pan-frying,” said Mariana Stern, associate professor of preventive medicine at the Keck School of Medicine of USC. “Similarly, diets high in lean, white fish seem to mostly increase risk when the fish is pan-fried and appear to offer no protective benefit when cooked using other methods.”
Stern led the analysis, which appears on the website for the journal Cancer Causes and Control. It is the first study to show that fish type and its method of cooking may be relevant in terms of protecting against or increasing the risk for prostate cancer, she added.
In the United States, more than 240,000 men are diagnosed annually with prostate cancer and about 33,720 die from the disease, according to the National Cancer Institute. Only lung cancer kills more American men.
According to the Prostate Cancer Foundation, there are no proven strategies for preventing the disease, but changes in diet and lifestyle appear to have reduced the risk of disease progression.
The researchers analyzed data from nearly 3,000 men who participated in the California Collaborative Prostate Cancer Study in the Los Angeles and San Francisco Bay areas. Sue A. Ingles, associate professor of preventive medicine at the Keck School and Esther M. John of the CPIC headed that collaborative project.
Study participants completed a comprehensive survey that included questions about the amount and types of fish they consumed on a weekly basis and how the fish was cooked. More than 60 percent of the men were diagnosed with advanced prostate cancer.
The analyses found that diets high in dark fish like salmon, mackerel and sardines reduced the risk of prostate cancer if the fish were cooked at low temperatures, like baking or boiling. This suggested protective effect disappeared when the fish was cooked at high temperatures, such as broiling, grilling or pan-frying.
Surprisingly, men who ate two or more servings per week of white fish cooked using high-temperature methods were twice as likely to develop advanced prostate cancer than men who never ate any fish. The study found no association between cancer and diets high in white fish cooked using low-temperature methods.
The study also noted that high intake of deep-fried fish, such as fish sticks and fish sandwiches, was linked to an increased risk of prostate cancer among Hispanic men, but not among non-Hispanic whites or African Americans, who reported the highest intake of fried fish than any other ethnic group studied.
The researchers do not know what causes the disparities they observed between dark and white fish, but they proposed two hypotheses.
One, carcinogens may form while cooking fish at high temperatures, harm from which may be negated by the omega-3 fatty acids in dark fish. Alternatively, given that white fish absorbs more oil than dark fish when pan-fried, this cooking method could alter the ratio of good fats to bad ones.
“It’s too early to make any dietary recommendations but, given the few known risk factors for prostate cancer, the results of this study emphasize that diet may be a relevant modifiable factor for prostate cancer risk,” Stern said.
Co-authors included Amit D. Joshi MPH ’04, MS ’06; Ingles of USC; and John and Jocelyn Koo, also of the CPIC.
The research received financial support from the California Cancer Research program, the National Cancer Institute, the National Institute of Environmental Health Sciences and the Prostate Cancer Foundation.