NEW YORK (Reuters Health) - Women who down four or more cups of coffee a day may have a reduced risk of developing cancer in the lining of their uterus, researchers reported Tuesday.
A study of more than 67,000 U.S. nurses found that women who drank that much coffee were one-quarter less likely to develop endometrial cancer than women who averaged less than a cup a day.
The absolute risk that any one woman, coffee drinker or not, would develop the cancer was fairly small: over 26 years, 672 women -- or one percent of the whole study group -- were diagnosed with endometrial cancer.
And the researchers cannot say for certain that coffee was the reason for the lower risk among heavy java drinkers.
"It would be premature to make a recommendation that women drink coffee to lower their endometrial cancer risk," said senior researcher Edward Giovannucci, of the Harvard School of Public Health in Boston.
Still, the study, published in the journal Cancer Epidemiology, Biomarkers & Prevention, adds to several others that have found coffee drinkers to have a lower risk of endometrial cancer.
A strength of the current one, Giovannucci said, is that it was large, long-term and allowed the researchers to account for a number of other factors that could explain the coffee connection.
They looked at differences in women's weight, because obesity is linked to a higher risk of endometrial cancer. But that did not account for the lower cancer risk seen among heavy coffee drinkers.
Nor did differences in women's childbirth history or hormone use (through birth control pills or hormone replacement after menopause). Those things matter because higher lifetime exposure to estrogen is thought to raise the risk of endometrial cancer.
It's still possible there are other reasons for this coffee-cancer link, according to Giovannucci.
But it's also plausible, he said, that coffee itself has some benefit. "It can lower insulin levels and may lower levels of free estrogen circulating in the body," Giovannucci explained.
Like high estrogen levels, higher concentrations of insulin -- a hormone that regulates blood sugar -- have been linked to an increased risk of endometrial cancer.
Of course, downing four cups of coffee per day may not be a good idea, especially for someone sensitive to the effects of caffeine. In this study, the researchers found that while caffeinated coffee was tied to a lower cancer risk, there was no statistically significant link with decaf -- though there was a "suggestive" trend in that direction.
Giovannucci pointed out that few women drank large amounts of decaf, which may be why the researchers could not weed out a clear correlation.
Although they didn't look at the women's use of sweeteners or other coffee additives, in theory, drinking a lot of coffee could be bad for your waistline if you added sugar and cream each time. Since obesity is linked to a higher endometrial cancer risk, Giovannucci noted, that could wipe out any potential benefit of coffee drinking.
The bottom line, the researcher said, is that "people who are already enjoying their coffee" can probably continue to do so. But it's too early to recommend that anyone start drinking coffee hoping to get health benefits.
A researcher with the American Cancer Society (ACS) agreed.
"If a woman drinks coffee currently, this may be one benefit," Marji McCullough, strategic director of nutritional epidemiology for the ACS, told Reuters Health in an email.
But McCullough added that further studies are needed -- in part to see whether coffee has different effects on endometrial cancer risk in different groups of women. She pointed out that in this study, the link between coffee and lower cancer risk was weaker among women who had never smoked, versus those who had.
According to the ACS, the average U.S. woman has about a one in 40 chance of developing endometrial cancer in her lifetime.
Both McCullough and Giovannucci said that one of the best things women can do to curb their risk of endometrial cancer is to maintain a healthy weight though diet and regular exercise.
To put it in context, Giovannucci said, obesity has been tied to a several-fold increase in a woman's risk of the cancer -- as high as five- to 10-fold in some studies.
"Even if the coffee finding is causal," he said, "the most important thing would be weight management through diet and exercise."
