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Etiology

Lactation was associated with a reduced risk for breast cancer in premenopausal women but not in postmenopausal women

ACP J Club. 1994 May-June;120:84. doi:10.7326/ACPJC-1994-120-3-084

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Source Citation

Newcomb PA, Storer BE, Longnecker MP, et al. Lactation and a reduced risk of premenopausal breast cancer. N Engl J Med. 1994 Jan 13;330:81-7.


Abstract

Objective

To determine if lactation is associated with a reduction in the risk for breast cancer.

Design

Case-control study drawn from state cancer, driver's license, and Medicare beneficiary registries.

Setting

4 states in the United States.

Patients

Women who were < 75 years of age and newly diagnosed with breast cancer between April 1989 and December 1991. Of 8532 eligible patients, 6888 were enrolled in the study. 11 319 women with no history of breast cancer were potential control participants; 9529 agreed to participate. Women who were nulliparous and, therefore, had never lactated or who could not provide a complete reproductive history were excluded from the analysis.

Assessment of risk factors

Reproductive history, duration of lactation, reasons for stopping breast feeding, medications used to inhibit lactation, hormone use, physical activity, alcohol use, selected dietary items, height and weight, medical history, and demographic factors were assessed. Inquiry into personal and family history of breast cancer was made at the end of the interview to maintain blinding. Interviewers were blind to the case-control status until the end of the interview for 78% of the case patients and 90% of the control participants.

Main outcome measure

Relative risk for breast cancer according to lactation history.

Main results

Among premenopausal women, after adjustment for other risk factors, a history of lactation was associated with a reduced risk for breast cancer (relative risk [RR] 0.78, 95% CI 0.66 to 0.91). Increased duration of lactation and younger age at first lactation were associated with a decreased risk for breast cancer among premenopausal women (P for trend = 0.003). Length of time since first lactation or last lactation (latency) was not associated with the risk for breast cancer. Overall, and for the subgroup of postmenopausal women, no association was noted between lactation history and the risk for breast cancer.

Conclusions

Lactation was associated with a reduced risk for breast cancer in premenopausal women. No reduction in risk was observed among postmenopausal women.

Source of funding: Public Health Service.

For article reprint: Dr. P.A. Newcomb, University of Wisconsin Comprehensive Cancer Center, 1300 University Avenue, No. 4780, Madison, WI 53706, USA. FAX 608-263-0415.


Commentary

The role of lactation in modifying breast cancer risk is controversial. This case-control study, in which lactation history and other risk factors for breast cancer were evaluated by telephone interview, helps clarify this issue. The main finding of the study was that lactation was associated with a reduction in the risk for breast cancer among premenopausal women (RR = 0.78) when compared with parous premenopausal women who had never lactated. This protective effect increased with longer duration of lactation and with earlier age at the time of lactation. This effect was not seen in postmenopausal women or in the overall study group.

The strengths of this study are its large size, relatively good response rates, evaluation and adjustment for other breast cancer risk factors, and evaluation for reasons for terminating lactation. The latter is important because the association between lack of lactation and increased breast cancer risk may be confounded by factors resulting in insufficient milk supply and by early termination of lactation. An important question for future research is why lactation is not protective in postmenopausal women.

The implications of this study are important. Almost 25% of cases of breast cancer occur in premenopausal women, and breast cancer in this group accounts for more than 40% of years of potential life lost among women dying of breast cancer. In addition to this, most of the major risk factors for breast cancer cannot be modified. Finally, the ability to reduce breast cancer deaths among premenopausal women with periodic screening is controversial. It is unlikely that screening will result in the same mortality reductions seen among postmenopausal women. Therefore, although the protective effect of lactation is not strong, it is a modifiable risk factor that is associated with other public health benefits. Thus, at a time when the incidence of breast cancer is increasing and the prevalence of many of its risk factors is increasing, it is heartening to learn that modifying lactation behavior may reduce the incidence of premenopausal breast cancer.

Linda Humphrey, MD, MPH
Department of Veterans Affairs Medical CenterPortland, Oregon, USA