Breastfeeding linked to reduced breast cancer risk

LONDON (Reuters Health) - The longer women breastfeed their children, the more they reduce their risk for breast cancer, UK researchers report in the July 20th issue of The Lancet. Dr. Valerie Beral from the Radcliffe Infirmary, Oxford, and colleagues from the Collaborative Group on Hormonal Factors in Breast Cancer reviewed data from 47 epidemiological studies from 30 countries that examined the connection between breastfeeding and breast cancer. These studies included 50,302 women with invasive breast cancer and 96,973 controls.

Among these women, those with breast cancer had fewer births compared with controls (2.2 versus 2.6). Also, fewer parous women with breast cancer had ever breastfed compared with compared with controls (71% versus 70%). Women with breast cancer also breastfed for a shorter time than controls (9.8 months versus 15.6 months), the researchers found.

For every 12 months of breastfeeding there was a 4.3% decrease in the relative risk of developing breast cancer (p<0.0001). Without breastfeeding, the risk of breast cancer is decreased by 7.0% with each birth (p<0.0001), Dr. Beral's team notes.

Once the duration of breastfeeding was taken into account, the risk for developing breast cancer did not differ significantly among women in developed and developing countries. Similarly, the risk also did not vary with age, menopausal status, ethnic origin, the number of births, age at the first birth nor other variables studied, they add.

 The UK researchers estimate that "the cumulative incidence of breast cancer in developed countries would be reduced by more than half, from 6.3 to 2.7 per 100 women by age 70, if women had the average number of births and lifetime duration of breastfeeding that had been prevalent in developing countries until recently."

 In a press statement, Dr. Beral said that "the results of this study are a major step forward in our understanding of why breast cancer is so common in developed countries. It has long been known that breast cancer was common in situations where women had few children breastfed for short periods. We have shown that these factors alone account for the high rates of breast cancer in more developed settings."

 Coauthor Dr. Gillian Reeves added that "to expect that substantial reductions in breast cancer incidence could be brought about today by women returning to the pattern of childbearing and breastfeeding that typified most societies until a century or so ago is unrealistic. However, even if women were to breastfeed each of their children for an additional 6 months, this could prevent about 5% of breast cancers each year." 

Lancet 2002;360:187-195.