Breast feeding and breast cancer risk

• a meta-analysis has been undertaken by the Collaborative Group on Hormonal Factors in Breast Cancer investigating the risk of breast cancer associated with breast feeding. In the meta-analysis the results of 42 case control studies were combined with those of 5 cohort studies (included by using a nested case control design) to obtain odds ratios - note that the incidence of breast cancer cannot be computed from case control studies and so the calculation of relative risk was approximated from odds ratios
  • in all parous women, the relative risk (RR) of breast cancer decreased with increasing duration of breast feeding (reduction in the RR per 12 months of breastfeeding 4.3%, 95% CI 2.9% to 5.8%)
  • in the absence of breast feeding each birth reduced the RR of breast cancer by 7.0% (CI 5% to 9%)
  • the overall effect of breast feeding did not vary by country (developed vs developing), education, age at diagnosis, previous use of hormonal contraceptives, use of alcohol or tobacco, ethnic origin, menopausal status, body mass index, parity, age at menarche, and age at first birth of the women
• the study authors conclude that the relative risk of breast cancer decreases with increasing duration of breast feeding. The effect is consistent after adjustment of possible confounders

Reference:

1. Collaborative Group on Hormonal Factors in Breast Cancer. Breast cancer and breast feeding: collaborative reanalysis of individual data from 47 epidemiological studies in 30 countries, including 50,302 women with breast cancer and 96,973 without the disease. Lancet 2002;20:187-95