Coffee and general caffeine intake may affect a woman’s levels of estrogen and other sex hormones, a new study suggests - offering a potential explanation for findings that link caffeine to certain cancers.

Several studies have found connections between caffeine and breast and ovarian cancers, though the findings have not always been consistent.

For instance, different analyses of the Nurses’ Health Study (NHS) - a large, long-running study of U.S. female nurses - have linked higher caffeine intake to lower risks of breast and ovarian cancers in postmenopausal women, but to a higher risk of ovarian cancer before menopause.

No one knows whether caffeine directly affects the risks of the cancers. But since estrogen and other sex hormones play a role in both diseases, it’s possible that caffeine affects the risks of the cancers via hormonal influences, note investigators Dr. Joanne Kotsopoulos and colleagues at Harvard Medical School.

The team looked at the relationship between coffee and caffeine intake and hormone levels among more than 1,200 women involved in the NHS.

At various points during that study, the women had completed questionnaires on their diets and other lifestyle factors, and provided blood samples. Kotsopoulos and her colleagues used those stored samples to measure the women’s levels of estrogen and other sex-related hormones.

Overall, the researchers found, the more coffee and caffeine a premenopausal woman consumed, the lower her levels of estradiol, a form of estrogen, during the second half of the menstrual cycle.

Meanwhile, higher caffeine intake was related to higher levels of another sex hormone, progesterone, the researchers report in the journal Cancer.

  The findings were somewhat different among postmenopausal women. For them, greater coffee and caffeine consumption was linked only to higher levels of sex hormone-binding globulin, or SHBG. Some studies have linked higher levels of SHGB - which decreases the activity of estradiol and testosterone - to a lower risk of breast cancer in postmenopausal women, Kotsopoulos and her colleagues note.

Exactly what the current findings mean is far from clear, according to the researchers.

In theory, lower estrogen levels in premenopausal women would help protect against ovarian cancer - so the findings do not explain the earlier results linking higher caffeine intake to a higher risk of premenopausal ovarian cancer.

Still, the researchers write, the results do suggest that caffeine influences sex hormone levels. They say that more studies are needed to see how those influences may affect hormone-related cancers.

SOURCE: Cancer, June 15, 2009.