Santa Monica - Eating fish and other foods high in omega-3 fatty acids doesn't have the wondrous effect on preventing malignancies that it seems to have in warding off heart disease.

As a cancer preventive strategy, omega-3 was left high and dry, reported Catherine H. MacLean, M.D., Ph.D., and colleagues at Rand Health in the Jan. 25 issue of the Journal of the American Medical Association.

The authors noted that their results seem to conflict with laboratory and animal studies suggesting that omega-3 fatty acids may have a protective effect against cancer, but they chalked up this discrepancy to the inadequacies of cancer models, and to differences between omega-3 forms found in foods and in dietary supplements.

The finding that omega-3 is a bust as a cancer preventive came just as the American Heart Association took the air out the sails of soy protein for heart disease, saying that it's not effective for lowering LDL cholesterol, does not improve HDL or triglycerides, and has no effect on blood pressure.

In a review of 38 articles describing the effects of omega-3 consumption (either from foods or from dietary supplements) on cancer, Dr. MacLean and colleagues found that "omega-3 fatty acids appear not to affect a mechanism of cancer development that is common across the different types of cancers evaluated in this report. Likewise, there is little to suggest that omega-3 fatty acids reduce the risk of any single type of cancer."

The researchers scoured the medical literature for studies that looked at the effects of omega-3 fatty acids on tumor incidence, and they narrowed their search to studies with a prospective cohort design that considered different levels of omega-3 consumption.

Two reviewers independently extracted from the studies detailed data about the incidence of cancer, the type of cancer, the number and characteristics of patients, details on the exposure to omega-3 fatty acids, and the elapsed time between the intervention and outcome measurements.

The reviewers identified 20 cohorts from seven countries for 11 different types of cancer, and up to six different ways to categorize omega-3 fatty acid consumption. The cohorts ranged in size from 6,000 to 121,000, and had follow-up periods ranging from 9,000 to 1.5 million person-years.

More than half of the studies focused on breast, colorectal or prostate cancer. The rest looked at the effects of omega-3 on either aerodigestive cancers (i.e., squamous cell carcinoma of the oral cavity or pharynx, esophagus, or larynx); or other malignancies, such as bladder, lung, non-Hodgkin's lymphoma, ovarian, pancreatic, basal cell carcinoma, and stomach.

The authors found that among 65 estimates of the association between omega-3 fatty acid consumption and cancer, only eight were statistically significant, and there was conflicting evidence for an association with cancer risk across many of the studies.

For example, they found one study indicating that for breast cancer there was an increased risk for cancer associated with omega-3 (incidence risk ratio 1.47; 95% confidence interval, 1.10-1.98), but three other suggesting a decreased risk (RR, 0.68-0.72). Still seven other estimates showed no significant association between breast cancer risk and omega-3 consumption.

Similarly, only one study showed a significant association between omega-3 and reduced risk of colorectal cancer (RR, 0.49; 95% CI, 0.27-0.89), whereas 17 other studies showed no association.

As for a possible association between lung cancer and omega-3, one study showed an increased risk (IRR, 3.0; 95% CI, 1.2-7.3), another showed a decreased risk (RR, 0.32; 95% CI, 0.13-0.76), and four others showed no significant associations.

In the case of advanced prostate cancer, again, one study showed an increased risk (RR, 1.98; 95% CI, 1.34-2.93), one estimated a decreased risk (RR, 0.43; 95% CI, 0.22-0.83) and 15 others found no association.

One study looking at basal cell carcinoma found a slightly increased risk (RR, 1.13; 95% CI, 1.01-1.27), but for all the other types of cancer there were no significant associations reported.

"A large body of literature spanning numerous cohorts from many countries and with different demographic characteristics does not provide evidence to suggest a significant association between omega-3 fatty acids and cancer incidence," Dr. MacLean and colleagues wrote.

"Dietary supplementation with omega-3 fatty acids is unlikely to prevent cancer," they concluded.

Primary source: Journal of the American Medical AssociationSource reference:MacLean CH et al. Effects of Omega-3 Fatty Acids

on Cancer Risk. A Systematic Review. JAMA. 2006;295:403-415