We love all the good our morning caffeine does for us, but coffee and tea have their dark side as well. Women especially should pay attention to their caffeine intake due to the effect it can have on their estrogen levels.

Research shows that more than one cup of coffee a day increased estrogen in women between the ages of 36 and 45 in the first stage of the menstrual cycle. Another study reported that caffeine intake decreased estradiol, one of the forms of estrogen, during the luteal phase – the last two weeks – of the menstrual cycle. And a new study finds that drinking tea appears to trigger epigenetic changes in women that are associated with cancer and the metabolism of estrogen.

Estrogen is the “female hormone.” It is responsible for both the development and regulation of the female reproductive system and secondary sex characteristics. Your body generates more of it during your menstrual cycle, stimulating the maturation of eggs and encouraging pregnancy. If you do not become pregnant, your estrogen levels will drop during your cycle's luteal phase.

A study by researchers at the National Institutes of Health indicates that the effect of caffeine on estrogen levels is influenced by race. Asian women who consumed the equivalent of roughly two cups of coffee daily had elevated estrogen levels when compared to women who consumed less. However, white women who consumed the same amount of caffeine had slightly reduced estrogen levels. There was no statistical difference in estrogen levels among caffeinated and decaffeinated black women.

But vary the source of the caffeine and you vary the results, the researchers learned. Caffeinated coffee was the baseline, but consumption of more than one cup each day of caffeinated soda or green tea was associated with a higher estrogen level in Asians, whites, and blacks.

In the most recent study on tea and estrogen, the researchers analyzed the data of 3,096 adults across four European cohorts. They learned that drinking tea appeared to alter the expression of two genes associated with estrogen metabolism and cancer among women. There were no such epigenetic changes noted among the male participants in the study. It is important to note that the researchers have not yet figured out whether these tea-induced genetic changes are positive or negative when it comes to cancer risk.