July 9, 2010

Fish Oil May Lower Breast Cancer Risk

Fish oil, a rich source of healthy omega-3 fatty acids, is perhaps best known for its heart benefits. But a new study by researchers at the Fred Hutchinson Cancer Research Center suggests that fish oil supplements may also help postmenopausal women lower their risk of breast cancer.

In the study of 35,016 postmenopausal women aged 50 to 76, researchers found that those who reported taking fish oil supplements were 32 percent less likely than non-users to develop breast cancer over the following six years.

The lower risk was observed even after a number of known or suspected breast cancer risk factors were taken into account, such as family history, obesity, advancing age, alcohol consumption and sedentary lifestyle.

However, since the study was "observational", meaning it looked at the relationship between an exposure (such as supplement use) and a disease risk, it is not yet clear whether the fish oil supplements themselves are responsible for the lower breast cancer risk.

"There are a lot of cautions with this type of study," said Dr. Emily White, an epidemiologist at the Fred Hutchinson Cancer Research Center in Seattle and the lead author of the study.

"It cannot show cause-and-effect."

For that reason, it is too soon to advise women to begin taking fish oil supplements to stave off breast cancer, she said. 

Such recommendations might come only when evidence from clinical trials support the beneficial effects of fish oil.

"Fortunately, there is going to be a clinical trial," Dr. White said, referring to an upcoming Harvard University trial examining whether fish oil and vitamin supplements influence the risk of cancer, heart disease and stroke in older adults.

The Harvard Vitamin D and Omega-3 Trial (known as VITAL) will enlist 20,000 older adults to assess the impact of fish oil supplements and vitamin D on cancer, heart disease and stroke.

The participants will be randomly assigned to take one or both supplements, or placebo pills.  A study of this kind is considered the "gold standard" for establishing a cause-and-effect relationship.

Several clinical trials have shown the benefits of fish oil in lowering high blood pressure, triglycerides and the risk of heart attack among those with established cardiovascular disease.  And consumption of fish has long been associated with a lower risk of heart disease, with experts generally recommending that adults consume fish such as salmon, mackerel and trout two or more times a week.

However, whether fish or fish oil has any impact on cancer risk is not yet clear.

In the current study, Dr. White and colleagues asked 35,016 postmenopausal women about their current and past use of fish oil and certain other supplements, and then followed the participants for an average of six years to record any breast cancer diagnoses.

During the follow up period, 880 women were diagnosed with the disease, of which just 5 percent had reported regularly using fish oil at the study's outset.  Among women not diagnosed with breast cancer, 8 percent reported being fish oil users at the study's outset.

The researchers concluded that fish oil use at the beginning of the study was related to a 32 percent lower risk of developing breast cancer compared with non-use. 

The one-third reduction in risk was seen even after taking into account other risk factors such as weight, age, diet and exercise habits and family history of breast cancer.

Given the anti-inflammatory effects of fish oil, and the fact that chronic inflammation in the body is believed to play a role in the growth and spread of cancer cells, it is biologically plausible that fish oil could affect the development of breast cancer, Dr. White said.

The fragments of evidence are there, but they have not yet come together in their entirety to recommend fish oil for reducing breast cancer risk, she said in an interview with Reuters.

Dr. Edward Giovannucci of the Harvard School of Public Health concurred with Dr. White.

"It is very rare that a single study should be used to make a broad recommendation," Giovannucci said in a written statement.

"Over a period of time, as the studies confirm each other, we can start to make recommendations."

When taken as directed, fish oil is typically considered safe.  However, it can have side effects such as heartburn, indigestion and even bleeding when taken at high doses.

Surprisingly, Dr. White's team found that fish oil use was related to an increased risk of breast cancer among a subgroup of women with a history of heart disease.

Those results were surprising and not easily explained, Dr. White said.   But it is possible it may represent a coincidental finding, rather than a true effect of fish oil, since it was based on such a small number of women.
Dr. White's study is the first to demonstrate a link between the use of fish oil supplements and a reduction in breast cancer.   However, the study does not conclusively prove whether a diet rich in fish might help to lower breast cancer risk, and other studies of dietary intake of fish or omega-3 fatty acids have not been consistent.

Dr. White and her colleagues noted two previous large-scale studies that examined the relationship between women's reported omega-3 food intake and their risk of breast cancer.  Neither study uncovered a link.

Dr. White's study was published in the journal Cancer Epidemiology, Biomarkers & Prevention, July 2010. 


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