Their study linked consumption of five or more cups of coffee a day to a relatively marked reduction in the non-hormone-responsive disease known as ER-negative breast cancer. However, coffee consumption did not appear to lower the risk for developing ER-positive breast cancer, a hormone-responsive oestrogen receptor form of the disease.
Daily consumption of coffee may protect against the most aggressive type of breast cancer, ER-negative, said study co-author Dr Per Hal, a professor in the medical epidemiology and biostatistics department at the Karolinska Institute in Stockholm.
"Now, we don't have all the details," he cautioned. "We don't know, for example, what specific type of coffee we're talking about here. But what we do know is that the protective effect is quite striking and remains even after adjusting for a lot of other factors that have the potential to play a protective role. And we know that we're talking about what we could call a relatively normal amount of coffee drinking. Certainly we're not talking about consuming gigantic amounts of coffee. So, this is a very intriguing finding."
The study, reported online in Breast Cancer Research, involved 5,929 Swedish women, aged 50 to 74. About half of the women had breast cancer.
Drinking coffee and breast cancer risk
Questionnaires were used to assess behavioural and health characteristics, including smoking and drinking patterns, physical activity routines, family history of breast cancer, hormone therapy protocols, nutritional intake, body mass index, education level and coffee consumption habits. Both tumour status and breast cancer type were also noted.
The principle finding: Drinking coffee appeared to spur a "strong reduction" in risk for ER-negative breast cancer, the researchers wrote. Women who drank five cups of coffee a day had a 33 to 57% lower risk for ER-negative cancer than did those who drank less than one cup a day.
The study revealed an apparent association between coffee consumption and a reduction in breast cancer risk, but not a cause-and-effect relationship.
And Hal was not eager for consumers to jump to conclusions.
"There are one or two other studies that have pointed in the same direction as ours - but not many, just a few," he cautioned. "So before I would go to tell my neighbours to start drinking more coffee than they already do, I would like to know what is the biological mechanism at work here. And that's not yet clear."
Hal noted that he and his colleagues are now working on a new study to tease out that information.
Coffee protects against cancer?
Dr Stephanie Bernik, chief of surgical oncology at Lenox Hill Hospital in New York City, described the findings as both "interesting" and "provocative," given that the kind of cancer coffee appears to protect against is one for which there are relatively few effective treatments.
"It is this kind of study that opens the door to improving treatment, as scientists try to uncover what biologic factors in a substance are beneficial, and then attempt to extract these factors and use them to defend against cancers," Bernik noted. "The goal would be to try and discover what it is in coffee that may be beneficial."
"The next step is to find out what chemical factors in coffee cause the decreased rate of cancer and then attempt to see if these same chemicals can be used to treat a patient once they are already diagnosed with cancer," she said.
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