"It is very rare that a single study should be used to make a broad recommendation," said Giovannucci. "Over a period of time, as the studies confirm each other, we can start to make recommendations."

Still, fish oil continues to excite many, as evidence emerges about its protective effect on cardiovascular disease and now cancer.

Harvard researchers are currently enrolling patients for the randomized Vitamin D and Omega-3 Trial (also called VITAL), which will assess the impact of fish oil supplements and vitamin D on cancer, heart disease and stroke.

The researchers plan to enroll 20,000 U.S. men aged 60 years and older and women aged 65 years and older who do not have a history of these diseases and have never taken supplements.

Recruitment for this National Institutes of Health funded study began in January, and more information can be found at www.vitalstudy.

Source: American Association for Cancer Research