Breastfeeding was not protective against breast cancer
ACP J Club. 1996 July-Aug;125:24. doi:10.7326/ACPJC-1996-125-1-024
Michels KB, Willett WC, Rosner BA, et al. Prospective assessment of breastfeeding and breast cancer incidence among 89 887 women. Lancet. 1996 Feb 17;347:431-6.
To determine whether a protective association exists between breastfeeding and breast cancer.
6-year cohort analysis of women in the Nurses' Health Study who were alive and free of breast cancer in 1986.
Community-based study in 11 U.S. states.
121 701 female nurses aged 20 to 55 years who completed mailed questionnaires in 1976 that included questions about reproductive history and risk factors for cancer. Subsequent questionnaires were completed every 2 years until 1992. Participants were excluded from the analysis if they died before 1986; had a previous diagnosis of cancer, reported a diagnosis of cancer between 1978 and 1986, or had in situ breast cancer; were nulliparous in 1984; or did not report breastfeeding status. 89 887 women were followed.
Assessment of risk factors
Data collection since 1976 included parity, age at menarche, personal and family history of breast cancer, personal history of benign breast disease, weight, height, oral contraceptive use, menopausal status, postmenopausal hormone use, physical activity, diet, and alcohol and vitamin intake. Data on breastfeeding for all births (never, < 1 mo to ≥ 48 mo, and unknown) were collected in 1986.
Main outcome measure
Relative risk (RR) for breast cancer in relation to lifetime duration of breastfeeding.
No overall association was found between breast cancer and ever having breastfed (age-adjusted RR 0.95, 95% CI 0.86 to 1.06; P for trend = 0.76). The strength of association did not substantially change when other risk factors were controlled for in a multivariate analysis (RR 0.93, CI 0.83 to 1.03; P for trend = 0.65). The adjusted RR according to multivariate analysis for breast cancer among premenopausal women who had ever breastfed was 1.14 (CI 0.87 to 1.50) and among postmenopausal women was 0.89 (CI 0.79 to 1.00). Risk was not substantially affected by duration of breastfeeding in either group. Women who had 1 birth and breastfed had a RR for breast cancer of 0.68 (CI 0.46 to 1.00). The risk increased with every level of parity to 1.15 (CI 0.91 to 1.46) for women who had ≥ 4 births (P for trend = 0.02).
No association was found between breastfeeding and incidence of breast cancer. A lower risk for breast cancer was observed among women who had only 1 child and breastfed compared with women who had 1 child and did not breastfeed.
Sources of funding: National Institutes of Health and American Cancer Society Special Institutional Grant 18.
For article reprint: Dr. K.B. Michels, Department of Epidemiology, Harvard School of Public Health, 677 Huntington Avenue, Boston, MA 02115, USA.
Although breast cancer is a leading cause of cancer death in North American women, factors leading to its development remain largely unknown. Breastfeeding has received attention because of its direct effect on the breast and its association with reproductive factors that have been linked with breast cancer risk. Early studies suggested that breastfeeding reduced risk; much of this risk reduction is now thought to be from the effects of pregnancy per se.
In the U.S. Nurses' Health Study, a large, well-designed, prospective cohort study, the examination of the association of breastfeeding with the risk for breast cancer yielded largely negative results overall and in subgroups of patients according to menopausal status. A protective effect of breastfeeding (RR = 0.68) was seen in women who breastfed after their only pregnancy.
These observations are in keeping with those of most published studies. The major conflicting study, a case-control study by Newcomb and colleagues (1), identified a protective effect of breastfeeding on the risk for premenopausal breast cancer. This discrepancy may be related to differences in age (premenopausal women in the Nurses' Health Study were older than those in the Newcomb study) or to temporal differences in breastfeeding practices, particularly the more recent practice of demand feeding rather than scheduled feeding (the latter leading to less breastfeeding). Further investigation of the influence of breastfeeding on early premenopausal breast cancer is needed.
Existing evidence suggests that breastfeeding does not exert a strong overall effect on the risk for breast cancer, although a small degree of protection against the relatively uncommon early premenopausal breast cancer cannot be excluded. Women should base their decisions about breastfeeding on factors other than breast cancer. These factors include mother-child bonding and improved infant health (2).
Pamela J. Goodwin, MD
Mount Sinai HospitalToronto, Ontario, Canada