More than 250 women, ages 18 to 44, took part in the study between 2005 and 2007. On average, they consumed 90 milligrams of caffeine a day, about the equivalent of one cup of caffeinated coffee.

Estrogen is the reproductive hormone produced by the ovaries.

Asian women who consumed an average of 200 milligrams or more of caffeine a day (equivalent to about two cups of coffee) had elevated estrogen levels compared to women who consumed less. But white women who consumed the same amount of caffeine had slightly lower estrogen levels than women who consumed less.

Black women who consumed 200 or more milligrams of caffeine daily had elevated estrogen levels, but this finding was not statistically significant, said the U.S. National Institutes of Health researchers and their colleagues.

The caffeine consumed by the women in the study came from any of these sources: coffee, black tea, green tea and caffeinated soda. The findings differed slightly when the researchers considered the source of caffeine individually.

Consuming 200 milligrams of caffeine from coffee mirrored the overall findings. But consumption of more than one cup each day of caffeinated soda or green tea was associated with higher estrogen in all three groups of women, according to the study published online in the February issue of the American Journal of Clinical Nutrition.

The caffeine-related changes in estrogen levels did not appear to affect women's ovulation, said the researchers, who followed the women for up to two menstrual cycles.

About 89 percent of U.S. women ages 18 to 34 consume the caffeine equivalent of 1.5 to two cups of coffee a day, according to the authors.

"The results indicate that caffeine consumption among women of child-bearing age influences estrogen levels," Enrique Schisterman, of the Division of Epidemiology, Statistics and Prevention Research at the U.S. National Institute of Child Health and Human Development, said in an NIH news release.

"Short term, these variations in estrogen levels among different groups do not appear to have any pronounced effects. We know that variations in estrogen level are associated with such disorders as endometriosis, osteoporosis, and endometrial, breast and ovarian cancers. Because long-term caffeine consumption has the potential to influence estrogen levels over a long period of time, it makes sense to take caffeine consumption into account when designing studies to understand these disorders," Schisterman said.

More information

Health Canada has more about caffeine.

SOURCE: U.S. National Institutes of Health, news release, Jan. 26, 2012

-- Robert Preidt