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Another message from Dr. Katherine A. Dettwyler at Texas A&M University:
I looked up the two references 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:
lactated 3 months or less 0.85
lactated 4-12 months 0.78
lactated 13-24 months 0.66
lactated 24+ months 0.72
for all who lactated 0.78
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:
Health Status Relative Risk
Premenopausal breast cancer if not
breastfed 1.00
Premenopausal breast cancer if breastfed 0.76
Postmenopausal breast cancer if not
breastfed 1.00
Postmenopausal breast cancer if breastfed 0.73
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.
Katherine A. Dettwyler, Ph.D.
Associate Professor of Anthropology
Texas A&M University
co-editor of "Breastfeeding: Biocultural Perspectives" and specialist in
infant feeding and growth
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