Omega-3 has been identified as one of the super-nutrients taking the food and supplements industry by storm. Much of its healthy reputation that is seeping into consumer consciousness is based largely on evidence that it can aid cognitive function and may help protect the heart against cardiovascular disease.
But one area in which it seems the evidence amassed so far does not appear to stand up is the fatty acid's role in reducing the risk of cancer.
Researchers led by Catherine MacLean, MD, PhD of RAND Health in Santa Monica, California, scrutinized 38 studies published between 1966 and October 2005 that investigated the purported link between omega-3 and different types of cancer and met certain criteria. The studies had to describe the effects of omega-3 fatty acid consumption on tumour incidence, be prospective cohort in design, and be conducted on a human population.
The findings are published in the January 25 issue of the Journal of the American Medical Association (2006;295:403-415).
Despite finding 65 estimates of association across 20 different cohorts for 11 different types of cancer and six different ways of assessing omega-3 consumption, only eight of these were found to be statistically significant.
Three studies showed decreased risk of breast cancer with omega-3 consumption, one for colorectal cancer, one for lung cancer and one for prostate cancer. But for each type there were also significant associations for decreased risk, and more estimates that did not identify any association.
Only one study looked at omega-3 consumption and skin cancer, and this found that there was an increased risk. No significant associations were found between omega-3 fatty acid consumption and incidence of aerodigestive, bladder, ovarian, pancreatic or stomach cancer or lymphoma.
According to the researchers, some dietary supplement products containing omega-3 and claiming to protect against conditions including cancer are being sold in some markets. These, they say, based their claims on the results of the positive studies - even though the reported results were mixed.
Despite concluding that "dietary supplementation with omega-3 fatty acids is unlikely to prevent cancer," MacLean did not say that an association can be ruled out entirely.
"For most types of cancer, the data are not sufficient to exclude with confidence an association between omega-3 fatty acid consumption and cancer incidence," she wrote.
Paul Coates, PhD, director of the NIH's Office of Dietary Supplements which, with the HHS' Agency for Healthcare Research and Quality (AHRQ), was a supporter of the study, said: "These findings, however, should serve as an important signal of the need for rigorous, well-designed preclinical and clinical studies in the field."
The study forms part of a series of reviews looking at the potential of omega-3 fatty acids in the prevention of a variety of health conditions. MacLean's team is also looking at the possibility of a link between omega-3 and cancer treatment.
As for the current results, AHRQ director Carolyn Clancy, MD, said: "These findings will help health care professionals and the public understand what the science shows for the effects of omega-3 fatty acids on cancer risk. This information will help them make informed, evidence-based decisions about their health and heath care."
In the past, however, some within the scientific community have raised questions about the value of meta-analyses. Indeed, when taken on a case-by-case basis, some of the evidence in support of an association does appear compelling.
In particular, several animal studies have delivered results that would appear to support an association. But MacLean said several factors mean it is unclear how much light these may shed on an association in humans.
These do not come close to replicating human exposure levels, she said, and they have not elucidated any mechanism for the effect of omega-3 and do not consider the stage of tumor development.
Moreover she called the methods used to modify omega-3 fatty acid consumption in the animal models "controversial".
Josephine Querido, science information officer at Cancer Research UK, said: "The jury is still out as to whether eating more omega-3 fatty acids will reduce your risk of developing cancer, and the evidence from this study is largely inconclusive. But, previous studies have suggested that diets high in fish oils can reduce the risk of bowel cancer. "The best way to reduce your risk of many cancers remains to eat a healthy, balanced diet, with plenty of fruit and vegetables." A study published in the June issue of the Journal of the National Cancer Institute (vol 97, no 12) (not included in the review) concluded from data from 1 million participants in the European Prospective Investigation into Cancer and Nutrition trial that people eating less than 14g of fish a day were 40 per cent more likely to develop colorectal cancer than those eating more than 50g per day.
However the researchers were unable to differentiate between fatty fish, which contains the majority of omega-3 fatty acids, and other fish.