Oct. 13, 2008 -- There's good news for women who can't get through the day without their coffee fix.
Over 10 years of follow-up, women who drank four or more cups of coffee a day had the same overall risk of breast cancer as women who almost never drank coffee.
There was some suggestion that heavy caffeine consumption was associated with an increased risk for benign breast disease and that caffeine may speed the progression of aggressive forms of breast cancer.
But study co-author Shumin M. Zhang, MD, of Brigham and Women's Hospital and Harvard Medical School, says it is not clear if this observation was real or a chance finding.
"I would interpret this subgroup analysis with caution," she tells WebMD. "These findings really should reassure women. From this and other studies, I would say that it is pretty clear that there is no overall increase in risk."
Coffee, Caffeine, and Breast Cancer
Last May, one of those studies, which included nearly 86,000 female nurses followed for 22 years, also showed no overall increase in breast cancer risk associated with coffee or caffeine consumption.
The findings, by well-known nutrition researcher Walter Willett, MD, and colleagues from the Harvard School of Public Health, did suggest a slight increase in breast cancer risk among postmenopausal women.
But like Zhang, Willett says women should not be too worried by the subgroup finding.
"I wouldn't pay too much attention to this," he tells WebMD. "The consistent thing in all the research is that nobody has seen an overall increase in breast cancer risk."
In the study by Zhang and colleagues, 1,188 of the 38,432 study participants developed invasive breast cancer over 10 years of follow-up.
Based on the women's responses to surveys about the foods they ate, the researchers concluded that consumption of caffeine and caffeinated beverages and foods was not significantly associated with an overall increased risk of breast cancer.
Among women with benign breast disease, a nonsignificant positive association with breast cancer risk was seen in those who drank four or more cups of coffee a day and in those with the highest caffeine consumption overall.
There was also a suggestion in heavy caffeine consumers of an increased risk for tumors that are negative for both estrogen receptor (ER) and progesterone receptor (PR) and for tumors larger than 2 centimeters -- factors associated with a poorer prognosis.
This finding suggests that caffeine may speed the progression of existing tumors, but Zhang says more research is needed to determine if this finding is real.
The study appears in the Oct. 13 issue of the Archives of Internal Medicine.
Decades of Study on Caffeine and Health
Willett's study did not find a link between caffeine and ER/PR negative or larger tumors.
And after decades of research examining caffeine, coffee, and health, Willett says there is very little to suggest an impact, either positive or negative, on any cancer.
"In fact, it is very hard to pin anything on coffee or caffeine at this time," he says. "There is a hint of an increased risk of fractures, but there doesn't seem to be much there."
He adds that coffee and caffeine have been found to be protective against type 2 diabetes in several studies and a research analysis.
"It is fair to say that, so far, the overall balance of risks and benefits are on the side of benefits," he says.