It is not known exactly why alcohol increases the risk of breast cancer. Two current theories focus on hormonal effects and vitamins, namely folate.

Careful studies have found that regular, moderate use of alcohol affects the levels of important female hormones, especially for postmenopausal women whose bodies make much less estrogen and progesterone than before they entered menopause.

One study carefully monitored the food and alcohol consumed by a group of postmenopausal women. A form of estrogen in the blood increased in the women who drank alcohol compared to the women who did not receive alcohol as part of their diet. Taking the equivalent of one drink a day increased the hormone levels; taking the equivalent of two drinks a day increased the levels even more.

That means that the breast cells were exposed to higher levels of estrogen if the women consumed alcohol. This may in turn trigger the cells, which are estrogen sensitive in these women, to become cancerous.

Other studies have examined the role of the vitamin folate in either decreasing or preventing breast cancer in women who drink alcoholic beverages.

There have been reports that folate may counteract the effect of alcohol on increased breast cancer risk.

Research performed in China, where women get folate almost exclusively from their diet and not from vitamin pills, showed that increased amounts of folate in the diet decreased the risk of breast cancer. But, Chinese women don’t usually drink much alcohol, so whether having more folate in the diet would decrease the risk of breast cancer in United States women who drink alcohol remains unknown.

Another study found that women who had more than one alcoholic drink daily and who took less than the recommended daily amount of folate had much a much higher risk of breast cancer than women with the same alcohol intake, but who had adequate folate in their diet.