Women who take fish oil supplements may have a lower risk of developing breast cancer than those who don't, a study published Thursday suggests — though whether the supplement itself bestows the benefit is not yet clear.
In a study of more than 35,000 postmenopausal women, researchers found that those who said they regularly used fish oil supplements were one-third less likely than nonusers to develop breast cancer over the next six years.
The lower risk was seen even with a number of known and suspected risk factors for breast cancer taken into account — including older age, obesity, heavy drinking, and sedentary lifestyle.
Still, the study is limited in the fact that it is what is known as an "observational" study — where researchers look at the relationship between an "exposure" (like supplement use) and a disease risk.
"There are a lot of cautions with this type of study," said senior researcher Dr. Emily White, of the Fred Hutchinson Cancer Research Center in Seattle. "It cannot show cause and effect."
So it is too soon to recommend that postmenopausal women start taking fish oil for the sake of lowering breast cancer risk, White told Reuters Health in an interview.
Any such recommendations may come when, and if, evidence from clinical trials supports a benefit of fish oil. "Fortunately, there is going to be a clinical trial," White said.
She was referring to a Harvard University trial just underway that will look at whether fish oil and vitamin supplements affect the risks of cancer, heart disease, and stroke in older men and women.
In that study, which aims to enroll 20,000 U.S. adults, researchers will randomly assign participants to take one or both supplements, or placebo pills to serve as a comparison. This type of study is considered the "gold standard" for demonstrating cause and effect.
Fish oil, a rich source of healthy fatty acids known as omega-3, is best known as a potential boon for heart health. A number of clinical trials have shown that fish oil may help lower triglycerides (a type of blood fat), high blood pressure, and the risk of heart attack in people with established heart disease; high fish consumption has also been linked to a lower risk of developing heart disease.
So experts generally recommend that adults aim to eat fish at least twice a week — preferably fattier fish like salmon, mackerel, and trout.
But whether fish or fish oil have any effects on cancer risk remains unclear.
The new study, published in the journal Cancer Epidemiology, Biomarkers & Prevention, looked at the question by asking 35,016 women ages 50 to 76 about their current and past use of fish oil and certain other supplements.
The researchers then followed the women for an average of six years to document any diagnoses of breast cancer. During that time, 880 women were diagnosed with the disease.
Among women who developed breast cancer, 5 percent had reported regularly using fish oil at the study's outset. Among women who remained cancer-free, 8 percent had been current fish oil users at the outset.
Overall, fish oil use at the start of the study was related to a one-third lower risk of developing breast cancer compared with nonuse — with factors like age, weight, self-reported diet, and exercise habits, and family history of breast cancer taken into account.
It is biologically plausible that fish oil could affect breast cancer development, according to White. Fish oil is known to have anti-inflammatory effects, and chronic inflammation in the body is thought to play a role in the growth and spread of cancer cells, White explained.
The pieces of evidence are there, she said, but they have not yet come together into a strong enough whole to recommend fish oil for trimming breast cancer risk.
Dr. Edward Giovannucci, of the Harvard School of Public Health in Boston, agreed.
"It is very rare that a single study should be used to make a broad recommendation," Giovannucci, a member of the journal's editorial board, noted in a written statement. "Over a period of time, as the studies confirm each other, we can start to make recommendations."
And while fish oil is generally considered safe when taken as directed, it can have side effects, such as stomach upset, heartburn, and, at least at higher doses, bleeding.
White's team also found that among a subgroup of women with a history of heart disease, fish oil use was actually related to an increased breast cancer risk versus nonuse.
The finding, White said, is surprising and not readily explained. She added, though, that it was based on only a small number of women, and may represent a chance finding rather than a true effect of fish oil.
The current findings cannot hint at whether a fish-rich diet might help lower breast cancer risk. But, White and her colleagues note, two previous large studies did look at the relationship between women's reported omega-3 intake from food and their risk of breast cancer. Neither study uncovered any link.
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