Breastfeeding and Cancer Risk

Does breastfeeding reduce the risk of cancer? While we generally focus on the positive benefits of breastfeeding for the infants, breastfeeding has been found to provide a measure of protection against uterine, cervical and ovarian cancers as well as breast cancer.

A study by Yale University researchers showed that women who breastfed for two years or longer reduced their risk of breast cancer by 50 percent. The researchers studied the medical history of 808 Chinese women in the rural Shandong province from 1997 to 1999. The women were aged 30 to 80 and half had breast cancer and half did not. The study was published in the American Journal of Epidemiology. Although the study did not explore the reasons why breastfeeding appears to lower the risk of breast cancer, some researchers say it could be because breastfeeding reduces exposure to estrogen, and yet another theory is that fat-soluble pollutants and carcinogens are not stored as much in lactating breasts than in non-lactation breasts.

One theory of how breastfeeding may help protect women from ovarian cancer is that breastfeeding reduces the total number of ovulations. The full mechanism by which breastfeeding provides protection is not fully understood, but we can measure the results. In the following you will find studies on breastfeeding's protective effects on mothers. These studies were originally published in such journals as the 'New England Journal of Medicine', 'American Journal of Epidemiology' and 'Journal of Clinical Epidemiology'.

References:

Breast Cancer

In 1999, a group of researchers at the Fred Hutchinson Cancer Research Center in Seattle published the findings of their study of postmenopausal breast cancer risk according to breastfeeding characteristics. For the study, the researchers contacted 3,633 breast cancer patients aged 50 to 79 from Massachusetts, New Hampshire and Wisconsin. More than 3,700 women of similar age were randomly selected to serve as the control group. The researchers obtained the women's lactation histories and breast cancer risk factors though telephone interviews.

After adjusting the statistics for age, parity, age at first birth and other breast cancer risk factors, the researchers found that breastfeeding for at least two weeks was associated with a slightly reduced risk of breast cancer compared to women who have never lactated. Their findings also modestly suggested that women who breastfed longer had a greater reduced risk of breast cancer than those who breastfed for shorter periods of time.

The researchers found no evidence that suggested age at first lactation was associated with breast cancer risk. Also, the use of hormones to suppress lactation was not associated with postmenopausal breast cancer, nor was the inability to breastfed related to risk. The researchers did conclude, however, that lactation may have a slight and perhaps long-lasting protective effect on postmenopausal breast cancer risk.

Among both premenopausal and postmenopausal women, risk of breast cancer decrease with increasing duration of lifetime lactation experience although the effect was consistently stronger for premenopausal women. Source: McTieman, A., "Evidence of Protective Effect of Lactation on Risk of Breast Cancer in Young Women." American Journal of Epidemiology, 1986

After controlling for age at first full term pregnancy and other potentially compounding factors, parity and duration of breast feeding also had a strong influence on the risk of breast cancer. Compared with parous women who never breast fed, women who had breast fed for 25 months or more had a lower relative risk. Source: Layde, P.M., "The Independent Associations of Parity Age at First full Term Pregnancy, and Duration of Breast Feeding with the Risk of Breast Cancer." Journal of Clinical Epidemiol, 1989.

If women who do not breastfeed or who breastfed for less than 3 months were to do so for 4 to 12 months, breast cancer among parous premenopausal women could be reduce by 11%; if all women with children lactated for 24 months or longer, the incidence might be reduced by nearly 25%. Source: Newcomb,P. etal. "Lactation and reduced risk of premenopausal breast cancer." N Engl J Med 1994; 330(2):81-87.

Women who were breastfed as infants, even if only for a short time, showed an approximate 25% lower risk of developing premenopausal or postmenopausal breast cancer, compared to women who were bottle-fed as an infant. Source: Freudenheim, J. "Exposure to breast milk in infancy and the risk of breast cancer." Epidemiology 1994 5:324-331

An increasing duration of lactation was associated with a statistically significant trend toward a reduced risk of breast cancer. Sources: Newcomb, P.A. et al. 1994 Lactation and a reduced risk of premenopausal breast cancer. The New England Journal of Medicine 330(2):81-87. (P Byers T, et al. Lactation and breast cancer: evidence for a negative association in premenopausal women. American Journal of Epidemiology Vol 121, pp664-74, 1985)

Uterine Cancer

A protective effect against uterine cancer was found for women who breastfeed. Source: Brock, K.E., "Sexual, Reproductive, and Contraceptive Risk Factors for Carcinoma-in-Situ of the Uterine Cervix in Sidney. " Medical Journal of Australia, 1989

Ovarian Cancer

Breastfeeding should be added to the list of factors that decrease ovulatory age and thereby decrease the risk of ovarian cancer. Source: Schneider, A.P. "Risk Factor for Ovarian Cancer. "New England Journal of Medicine, 1987

Endometrial Cancer

Lactation provides a hypoestrogenic effect with less stimulation of the endometrial lining. This event may offer a protective effect from endometrial cancer. Source: Petterson B, et al. "Menstruation span- a time limited risk factor for endometrial carcinoma". Acta Obstet Gyneocol Scand 1986;65:247-55