Why does breastfeeding reduce our breast cancer risk?

A few weeks ago Dr. Kathleen Arcaro from the University of Massachusetts visited the breastfeeding support group I run.  She is an environmental toxicologist who studies, among other things, the relationship between breastfeeding and breast cancer.*

She is working on a study now and stopped by to see if anyone was interested in donating milk for the study.  If you live in my area (Pioneer Valley, Western Massachusetts) and are interested in donating milk, see the study website for more information.  They've made it very easy to donate.

She was also nice enough to answer a question about breastfeeding and breast cancer, and also environmental toxics in breastmilk:

Could you describe the current theories about why breastfeeding protects against breast cancer?

There are several hypotheses about why breastfeeding provides protection against breast cancer but at this time we do not know which is/are correct.  The simplest explanation is that for many women the longer they breastfeed the fewer menstrual cycles they experience.  Since with every menstrual cycle cells in the breast grow and divide and could by chance accumulate mutations which might lead to cancer, reducing the total number of cycles a woman experiences in her lifetime should lead to a decreased risk of breast cancer.  This is supported by data showing that younger age at time of first menstruation and older age at menopause (greater total number of years menstruating) is associated with an increased risk of breast cancer.  Another hypothesis is that breastfeeding results in a permanent change in either breast morphology or the expression of genes in the breast, and that this change provides protection against breast cancer.  Finally it has been suggested that breastfeeding reduces the level of pollutants in the breast that may be associated with increased breast cancer risk.

You also study environmental toxics in breastmilk.  Can you describe any trends you're seeing in the concentrations of toxics in breastmilk?

The good news is that the concentration of some lipophilic (fat-loving) environmental pollutants in breast milk is decreasing.  For instance the level of many pesticides (DDT and its metabolites) and polychlorinated biphenyls (PCBs) has greatly decreased over the last 30 years.  However, the levels of some other compounds used in household and personal care products including flame retardants and synthetic musks are increasing.  In general, if a compound does not easily degrade, and accumulates in fatty tissue it is likely it will be in breast milk.  But considering that the pollutants are widely distributed and therefore are in cow’s milk and formula, breast milk clearly remains the best food for most infants. 

*If you're wondering if it's possible to breastfeed after breast cancer, here's one mother's story.  And if you're wondering how much breastfeeding reduces breast cancer risk, see this post.

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Comments

[Butterfly Mama]

Thanks for this terrific email. I know my sister's hubby was saying to her that there are many toxins found in breastmilk. It's nice to have a rebuttal - these environmental toxins are all around us daily too.

Heidi

[tanya@motherwearblog]

Heidi,

Right - I always ask, 'compared to what?' when I hear people talk about toxins in breastmilk. What makes us think that cow's milk is any less exposed than human milk, especially when it's then prepared in a factory?

Tanya

[Jenna]

Interesting. (Our family is still dealing with my mother's recent BC diagnosis.) My mother breastfed. However, she did have an early onset of menstruation (as did I).

I'm curious, of course, as to how certain hormonal birth controls such as Seasonale (which give you four periods per year as opposed to monthly periods) are being viewed with the hypothesis that less periods = less of a chance of breast cancer... when another risk factor is actually taking birth control for 5+ years. (More on risk factors: http://www.komen.org/stellent/groups/harvard_group/@dallas/documents/-komen_site_documents/rfapfactors.pdf)

I wonder if the two then cancel each other out? Or if one is worse than the other? Or one is more beneficial than the other?

It's all very confusing.

[Beth]

It's really interesting to hear about the studies Dr Arcaro is conducting and the theories behind how breastfeeding may reduce the risk of breast cancer.

Thanks for the great reminder that although there are often reports about the toxins found in breastmilk, the reports leave out the fact that they're also in cow's milk and formula. It's a good reminder that breastmilk is still best!

[Wani]

I just participated in a Breastfeeding Carnival on my blog if anyone wants to check it out:
http://wanibug.blogspot.com/2008/03/breastfeeding-carnival.html