The latest in a series of diet-health population studies – this one by Harvard researchers – indicates that women who eat lots of fruits and vegetables may cut their risk of breast cancer.
And a simultaneous study from Germany provides the first evidence that diets rich in lignans – a fiber abundant in most vegetables, in wheat and (especially) in flaxseed – may cut their risk of certain kinds of breast cancer.
In both studies, the risk reductions related only to one major class of breast cancer, called “estrogen receptor negative”.
Breast cancer is classified as estrogen receptor (ER) positive or negative:
ER-positive cancer is fueled by the female hormones estrogen or progesterone.
ER-negative breast cancer has no hormone receptors, and needs no hormones to grow.
While most (three out of four) breast cancers are ER-positive, the one in four women who develop ER-negative breast tumors have a worse prognosis because these grow more quickly.
In brief, the Harvard study linked diets high in fruits and vegetables to a reduced risk of ER-negative breast cancer.
And a simultaneous German study linked those diet-related reductions in risk of ER-negative breast cancer to plant fibers called lignans.
Harvard analysis links veggies to lower risk of ER-negative breast cancer
The Harvard team analyzed diet and health data from 86,621 postmenopausal women in the Nurses' Health Study, to look for any associations between dietary habits and breast cancer risk over the course of that 26-year study.
The researchers' analysis linked diet patterns similar to the Dietary Approaches to Stop Hypertension (DASH) eating plan to a significant, 20 percent lower risk of ER-negative breast cancer.
Compared with the standard American diet, the DASH plan features more vegetables, fruits and low-fat dairy, but less total fat, saturated fat, cholesterol, and sodium.
The Harvard analysis linked most of this reduction to the DASH diet's high fruit and vegetable intake.
No link was found between ER-positive tumors and higher-than-average fruit or vegetable consumption.
German study links breast risk reduction to lignans
Phytoestrogens are plant compounds which, in the human body, can attach to the receptors for the female sexual hormone estrogen.
The results of cell and epidemiological studies suggest that they exert a cancer-protective effect.
For example, Asian women – who eat lots of soy, which is rich in phytoestrogens called isoflavones – get fewer breast cancers compared with American women.
On the other hand, scientists fear that isoflavones might imitate the growth-promoting properties of real hormones and, thus, accelerate growth of ER-positive breast cancers.
Last year, a team led by Prof. Dr. Jenny Chang-Claude summarized the results of several studies in a meta-analysis that linked diets rich in phytoestrogens to reduced risk of developing breast cancer after menopause.
For the new study, her Heidelberg-based team analyzed data from an earlier study involving 1,140 women diagnosed with postmenopausal breast cancer. (Buck K et al. 2011)
They looked for any links between the women's cancer status and their blood levels of enterolactone … which is a metabolic breakdown product of dietary lignans and a good indicator of the level of lignans in someone's diet.
Lignans are found in wheat and vegetables, but are highly concentrated in seeds, particularly flaxseeds.
Compared to the women with the lowest enterolactone levels, the women with the highest blood levels of enterolactone were about 40 percent less likely to have died by the end of the six-year study.
And when the scientists looked at cancer progression in the women – as measured by the rate of metastasis and secondary tumors – they found that the women with the highest enterolactone levels were also at lower risk for cancer progression.
As Professor Chang-Claude said, “We now have first clear evidence showing that lignans lower not only the risk of developing postmenopausal breast cancer, but also the mortality risk.” (HAGRC 2011)
She went on to stress a key point: “The result was significant only for the group of tumors that have no receptor for the estrogen hormone (ER-negative tumors).” (HAGRC 2011)
Results of German study undermine estrogenic theory of lignans' breast benefit
This finding suggests that lignans – via their breakdown product, enterolactone – do not curb cancer by binding to estrogen receptors on tumor cells.
Instead, cell and animal studies suggest that enterolactone produced from lignans inhibits cancer growth by promoting “suicide” (apoptosis) among cancer cells and by inhibiting growth of the new blood vessels needed to sustain tumors … an effect known as anti-angiogeneis.
Clearly, more study is needed on this entire subject.
But in the meantime, it seems clear that diets rich in plant foods can help protect against a major kind of breast cancer.
And substantial amounts of lab and epidemiological research indicate that the omega-3s and vitamin D3 in fatty fish and dietary supplements also support breast health.
For more on that, see the “Omega-3s & Immunity,” “Omega-3s & Women's Health,” and “Vitamin D & Immunity” sections of our news archive.
Boggs DA, Palmer JR, Wise LA, Spiegelman D, Stampfer MJ, Adams-Campbell LL, Rosenberg L. Fruit and vegetable intake in relation to risk of breast cancer in the Black Women's Health Study. Am J Epidemiol. 2010 Dec 1;172(11):1268-79. Epub 2010 Oct 11
Buck K, Zaineddin AK, Vrieling A, Heinz J, Linseisen J, Flesch-Janys D, Chang-Claude J. Estimated enterolignans, lignan-rich foods, and fibre in relation to survival after postmenopausal breast cancer. Br J Cancer. 2011 Sep 13. doi: 10.1038/bjc.2011.374. [Epub ahead of print]
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Helmholtz Association of German Research Centres (HAGRC). Plant compound reduces breast cancer mortality. September 13, 2011. Accessed at http://www.eurekalert.org/pub_releases/2011-09/haog-pcr091311.phpPark Y, Brinton LA, Subar AF, Hollenbeck A, Schatzkin A. Dietary fiber intake and risk of breast cancer in postmenopausal women: the National Institutes of Health-AARP Diet and Health Study. Am J Clin Nutr. 2009 Sep;90(3):664-71. Epub 2009 Jul 22.