The cumulative amount of alcohol consumed increases the risk of breast cancer, even among those who average only three to six drinks a week, according to a new study.

Researchers at Brigham and Women's Hospital (Boston, MA, USA) and Harvard Medical School (Boston, MA, USA) conducted a prospective observational study of 105,986 women enrolled in the Nurses' Health Study that were followed up from 1980 until 2008, with an early adult alcohol assessment and 8 updated alcohol assessments. The main outcome measures were the relative risks of developing invasive breast cancer associated with alcohol consumption during adult life, including quantity, frequency, and age at consumption.

The results showed that during 2.4 million person-years of follow-up, 7,690 cases of invasive breast cancer were diagnosed. Increasing alcohol consumption was associated with a 15% higher risk of invasive breast cancer (compared with nondrinkers) that was statistically significant at levels as low as 5 g to 9.9 g per day, equivalent to 3-6 drinks per week. Binge drinking, but not frequency of drinking, was associated with an increased risk of 50% in women who averaged more than 30 drinks a week. Analysis yielded a trend for a 10% increase in breast cancer risk for each 10-g increase in alcohol consumption, and alcohol intake both earlier and later in adult life was independently associated with risk. The study was published in the November 2, 2011, issue of the Journal of the American Medical Association (JAMA).

“Consistent with other studies, we did not find any difference between type of alcohol beverage,” concluded lead author Wendy Chen, MD, and colleagues of the department of medicine. “Although the mechanism for the association between alcohol and breast cancer remains unclear, one ‘probable explanation’ relates to alcohol's tendency to increase circulating estrogen levels.”

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