Alcohol consumption and risk of breast cancer in postmenopausal women: the NIH-AARP Diet and Health Study

One of the largest studies of its kind has found that alcohol is a substantial risk factor for development of the most common type of breast cancer - the 70 percent of tumours that are classified as positive for both the oestrogen and progesterone receptors (ER+/PR+).

Researchers report that even moderate alcohol consumption, defined as one or two drinks per day, increased risk of developing this kind of cancer, and the more a woman drank, the higher her risk. Compared to women who did not drink at all, women who had three or more glasses of alcohol daily had as much as a 51 percent increased risk of ER+/PR+ breast cancer.

"This suggests that a woman should evaluate consumption of alcohol along with other known breast cancer risk factors, such as use of hormone replacement therapy," said the study's first author, Jasmine Q. Lew, a fourth-year medical student at the University of Chicago who is conducting this research at the National Cancer Institute's (NCI) Division of Cancer Epidemiology and Genetics.

Lew and her research colleagues from NCI say their analysis could not support a definitive conclusion as to whether alcohol influences development of other breast cancer tumour types, "but we have enough numbers to study alcohol's influence on ER+/PR+ breast cancer," she said.

Epidemiologic studies have long suggested that use of alcohol may increase a woman's risk for developing breast cancer, and laboratory studies have shown that alcohol increases the amount of estrogen metabolites available in a woman's body, which can then act as a fuel for hormone-sensitive breast cancer. But few studies have looked at alcohol's effect on tumour type.

In this study, the researchers reviewed data from the NIH-AARP Diet and Health Study, which began in 1995. Lew and her colleagues analyzed 184,418 postmenopausal women who enrolled in this cohort study, and who answered questions about their daily alcohol consumption. During an average of seven years of follow-up, they found that 70 percent of women in the study drank alcohol; the average amount was a little less than a drink a day. Overall, the authors found that moderate drinking in women increased risk of developing breast cancer.

They then identified 5,461 cases of invasive breast cancer, for which they had tumor type information on 2,391 cases. In all, they analyzed data on 1,641 ER+/PR+, 366 ER-/PR-, 336 ER+/PR-, and 48 ER-/PR+ cases of invasive breast cancer.

The researchers found that ER+/PR+ cancers showed a stronger association with alcohol than that seen in the overall group. Compared to non-drinkers, women who consumed less than one drink daily, one to two drinks, and three or more daily drinks, the increase in relative risk for developing ER+/PR+ breast cancer was 7 percent, 32 percent, and 51 percent, respectively. Although the data suggested increased risks among the women with ER+/PR- breast cancer, the number of cases was relatively small, and this finding was not statistically significant.

The increased risk of invasive breast cancer was observed across different types of alcohol consumed.

"Our study at this point provides evidence for the notion that alcohol affects estrogen metabolism, which increases risk of hormone sensitive breast cancer," Lew said. "Still, more study is needed to clarify the effect of alcohol on other tumor types."