Breastfeeding may help reduce risk of aggressive cancer: study
Last Updated: Monday, August 25, 2008 | 2:39 PM ET
CBC News
Breastfeeding for at least six months may help reduce a woman's risk of an aggressive form of breast cancer, according to a study that explored how reproductive factors influence different types of breast cancer.
In the aggressive "triple negative" breast cancer, tumours do not respond to a key regulatory protein called HER2 or to the hormones estrogen and progestin.
Because of that, hormone-based treatments are of little help for triple negative breast cancers, which is more common among younger women. Previous studies have found women with triple negative cancer are more likely to have a relapse.
The latest study looked for patterns among women with three types of breast cancer:
- Estrogen-sensitive, also called luminal — the majority of breast cancers that tend to grow slowly.
- HER-2 positive — cancers that grow rapidly and tend to have poor prognosis, but can often be treated successfully with Herceptin, according to the American Cancer Society.
- Triple negatives.
In the Oct. 1 issue of the journal Cancer, Amanda Phipps, of the Fred Hutchinson Cancer Research Center in Seattle and her colleagues found triple-negative breast cancer was half as common among women who said they breastfed for six months or more compared with women who hadn't breastfed.
Estrogen-sensitive or luminal breast cancers were also 20 per cent less common among those who breast fed, the researchers said.
"Our data do support the premise that risk factor profiles vary by breast cancer subtype and that hormonal risk factors have a greater impact on luminal-type breast cancers than HER 2-overexpressing or triple-negative tumors," the researchers said.
Hormone exposure
Until recently, researchers have grouped breast cancer subtypes together when looking for risk factors, but that could miss those found in only certain types.
To help fill this gap, Phipps's team looked at how reproductive factors such as breastfeeding and age of first menstruation and menopause influenced subtypes of breast cancer.
Starting to menstruate before age 13 was linked to a higher risk of HER2 breast cancer, the researchers found.
Entering menopause late, after age 55, and taking estrogen-plus-progesterone hormone therapy were linked to risk of estrogen-sensitive breast cancer.
The study involved women ages 55 to 79 and included 1,023 cases of luminal breast cancer, 39 cases of HER 2-overexpressing breast cancer, 78 cases of triple-negative disease, and 1,476 controls.
Such an observational study does not prove that breastfeeding helps prevent cancer or that not doing so increases the risk. The small number of triple-negative and HER-2 cases also limits the researchers' ability to draw conclusions, and it is possible that some subtypes were misclassified.
Researchers have speculated that since women don't menstruate while breastfeeding, they are exposed to fewer hormones. Another possibility is that the structure of breast cells change during breastfeeding, making them less susceptible to mutate.
