Women who drink four or more cups of coffee a day may have a reduced risk of developing cancer in the lining of their uterus, according to a US study.

Researchers who looked at more than 67,000 US 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, said the study, published in the journal Cancer Epidemiology, Biomarkers & Prevention.

The absolute risk that any one woman, coffee drinker or not, would develop the cancer was fairly small, with only 672 women -- or one percent of the study group -- being diagnosed with it over 26 years.  

While researchers could also not say for certain that coffee was the reason for the lower risk among those who drank a lot of coffee, the study adds to several others with similar results.

Coffee itself may have some benefits, said senior researcher Edward Giovannucci, of the Harvard School of Public Health in Boston.

"It can lower insulin levels and may lower levels of free estrogen circulating in the body," he added.

Higher concentrations of insulin and higher lifetime exposure to estrogen have both been linked to a higher risk of endometrial cancer.

Researchers looked at a number of other factors, such as differences in women's weight, since obesity is also linked to a higher risk of endometrial cancer, but that did not account for the lower cancer risk seen among coffee drinkers.

Nor did differences in women's childbirth history or hormone use, though birth control pills or hormone replacement therapy after menopause.

Of course, downing four cups of coffee a day may not be a good idea, especially for someone sensitive to the effects of caffeine. 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.

In theory, adding sugar and cream to coffee could be bad for the waistline. With obesity also tied to a higher risk of the cancer, that could wipe out any potential benefit of coffee drinking.

"It would be premature to make a recommendation that women drink coffee to lower their endometrial cancer risk," Giovannucci said.

The bottom line, he said, is that people who are already enjoying their coffee can probably continue to do so - but the biggest preventative for the cancer is maintaining a healthy weight through diet and regular exercise.