Drinking more than five cups of coffee a day? You might be jittery, but you might also be reducing your risk for a certain type of aggressive breast cancer, new research suggests.

Swedish researchers have found that women who drink five cups of coffee a day or more have a 57 per cent reduced risk of developing estrogen-receptor negative cancers.

While the research is intriguing, it's important to note that the finding was held only after the researchers had accounted for all the other factors that can affect the risk for developing breast cancer, such as excess weight, a lack of regular exercise and hormone use.

Experts say women looking to reduce their breast cancer risk should still focus on lifestyle habits they can change and that have also been shown to cut cancer risk.

The study, published online in the journal Breast Cancer Research, involved 5,929 Swedish women who were between the ages of 50 and 74. About half the women had been diagnosed with breast cancer while the other half were healthy.

Researchers from the Karolinska Institutet asked the women about their coffee drinking habits, and collected info on family history of breast cancer, reproductive history, hormone therapy use and habits such as smoking, drinking alcohol, and exercising.

The researchers found that, compared with women who drank less than one cup of coffee a day, those who consumed five cups of more were 20 per cent less likely to develop breast cancer – after the findings were adjusted only for age. (The risk of breast cancer increases with age.).

When the researchers took into account the other factors, such as family history and lifestyle factors, they found a 57 per cent reduction in risk estrogen-receptor negative cancers.

ER-negative breast cancer affects about one in four women with breast cancer. It's often resistant to drug treatment and requires intensive chemotherapy.

While heavy coffee drinkers were also less slightly likely to be diagnosed with ER-positive breast cancer, the difference in risk was so small, the researchers say it could have been a chance finding.

It's not clear why coffee drinking might affect breast cancer risk, if it does. Coffee has been shown to boost blood levels of the enterolactone, a phytoestrogen that has been associated with a lower risk of ER-negative breast cancer.

But the researchers aren't convinced that enterolactone might explain the finding.

"It is unlikely that the protective effect is due to phytoestrogens present in coffee since there was no reduction in the incidence of ER-positive cancer in this study," they said in a news release.

Previous research has produced conflicting results about coffee and breast cancer risk. One way this study might differ, the authors suggest, is the way coffee in Sweden is made. Swedes typically don't use coffee filters; instead, the water is boiled with the coffee grounds. So the resulting drink could contain more fatty acids.

"There is often conflicting information about the beneficial effects of coffee – when we compared our results to that of a German study we discovered that their data showed the same trend, but the relationship was much weaker. We suggest that this may have something to do with the way the coffee was prepared, or the type of bean preferred," the researchers said.