Kathy's Commentaries

Breastfeeding and Breast Cancer

by Katherine A. Dettwyler, Ph.D.

Department of Anthropology,
Texas A & M University

The post below was written in 1995. In 1999, I was diagnosed with breast cancer. As Eeyore would say, life's not fair. But I'm alive and well in 2004, and will be adding some essays about my "adventures with breast cancer" to my web site in the coming months (spring 2004)!

I looked up the two references [I mentioned in previous e-mail] on breastfeeding and breast cancer. The first was a study looking at whether breastfeeding her own children protects a woman against breast cancer. The reference is:

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.

In this article, they looked at women who had never lactated, and those who had lactated for varying lengths of time. If you set the frequency of premenopausal breast cancer among the women who never lactated at 1.00, then the relative risk of breast cancer for women who had lactated was:

The authors write: "An increasing duration of lactation was associated with a statistically significant trend toward a reduced risk of breast cancer (P < 0.001). Lactation at early ages and for long durations was associated with more substantial reductions in risk. If women who do not breastfeed or who breastfeed for less than 3 months were to do so for 4 to 12 months, breast cancer among parous premenopausal women could be reduced by 11 percent, judging from current rates. If all women with children lactated for 24 months or longer, however, then the incidence might be reduced by nearly 25 percent. This reduction would be even greater among women who first lactate at an early age."

Newcomb's study is merely the latest in a long series of studies that find protective effects of breastfeeding for mothers. It should also be pointed out that many women nurse far longer than the 24+ month limit in this study.

The second study I referred to in yesterday's post looked at whether having been breastfed protected women from breast cancer when they grew up. This study involved 1,130 women from two counties in Western New York. "Breastfeeding" was defined as ANY breastfeeding, so some of these women may have only been breastfed for a week or a month, and others for several years. The reference is:

Freudenheim, J. et al. 1994. "Exposure to breast milk in infancy and the risk of breast cancer." Epidemiology 5:324-331. (NOTE: epidemiologists use the term "exposure" to refer to both good and bad factors).

Their results showed:

Thus, for both premenopausal and postmenopausal breast cancer, women who were breastfed as children, even if only for a short time, had a 25% lower risk of developing breast cancer than women who were bottle-fed as an infant.

Between the two factors, having been breastfed oneself, and breastfeeding one's own children, one could reduce the risk of breast cancer by almost half. Now breast cancer strikes about 1 in 8 women over the course of their lifetimes. If one could reduce the chances to 1 in 16, that would be worth doing, I would think. These studies do not promise anyone that they won't get breast cancer if they were breastfed and breastfeed their own children, they merely lower the risk by half. Chances are good that you won't get breast cancer no matter what you do -- as 7 out of 8 women don't. You can play the odds, or you can change the way you live to reduce your risk.

It is interesting to look at the steady rise in incidence of breast cancer over the last few decades in light of this new information. Let me use my own mother as an example. She was born in 1920, when almost all babies were still breastfed for several years, and her mother breastfed her. Thus she got the first type of protection. By the time she started having children in the late 1940s and up to the mid 1950s, many women were not breastfeeding their children any more (although my mother did). That means that there was an entire cohort of women who had been breastfed as infants, but did not breastfeed their own children. Thus they got the first type of protection, but not the second. As they aged, they were at greater risk for breast cancer than their mothers and grandmothers had been (b/c their mothers and grandmothers had had both types of protection). They you come to my generation, most of whom were born in the 1950s and 1960s and were not breastfed as children, so they missed out on the first type of protection. Then when they started to have kids in the 1970s and 1980s, many still did not breastfeed their own children, thus missing out on the second type of protection. As this cohort ages, those who were neither breastfed nor breastfed their own children are at even greater risk than their mothers had been. Could it be that the steady erosion of these two sources of protection account for the steady rise in breast cancer incidence in the United States over the past 4 decades? At the moment, this is just speculation based on the timing of the two processes.

I hope I have given you more to think about.

Addendum

In the post I sent [earlier] describing Newcomb's study, several people noted the increase in risk for the "lactated more than 24 months" group. Let me see if I can clear this up -- I was trying to be succinct in the former post, and you know how difficult that is for me.

The differences between groups were

The study groups were largest for never lactated and lactated less than 3 months. The smallest group was the ones with >24 months of lactation. The statistical differences between the groups was significant for the "never lactated/lactated < 3 months" comparison, and the "trend" is clearly downward, but as I recall the differences between the other groups were only marginally statistically significant, or not statistically significant. In other words, the trend itself is significantly downward with longer duration of lactation, even though in this particular small group of women who lactated more than 24 months it was slightly higher (but not statistically significantly higher) than the group that lactated 13-24 months. Is that clear?

In order to do the definitive study, you would need to treat breastfeeding as a continuous variable with maybe monthly or even weekly increments and see how the rates changed, but it is very difficult to get a big enough sample of women who have breastfed for a long time. I myself am now entering my 99th month of lactation. Another problem with these studies is that it isn't at all clear if it makes a difference whether your 24 months are all with one child or spread across 2 or 3 or 4 or 5 or 6. Hope that clears up the confusion.

Last updated March 11, 2004, by kad. Contents copyright 1999-2004 Sue Ann Kendall and Kathy Dettwyler.   Thanks to Prairienet, the Free-Net of east-central Illinois, for hosting this site from 1999 through 2004.  

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