This finding was particular to white women — because not enough black and Asian women could be included in the analysis — strengthened with longer times spent breastfeeding, and lasted for decades, its researchers wrote.
The study, “Association Between Breastfeeding and Ovarian Cancer Risk,” was published in the journal JAMA Oncology.
Because ovarian cancer is often detected late, its five-year survival rate is below 50%. Better and more personalized ways of preventing this cancer are needed, including the discovery of modifiable risk factors other than the use of oral contraceptives.
“Numerous studies have investigated the association between breastfeeding and ovarian cancer risk, with some showing a significant decrease in risk and others showing no association, leading the World Cancer Research Fund International to describe evidence of the association as limited,” the researchers wrote.
Observed associations between the duration and timing of breastfeeding and ovarian cancer risk have also been inconsistent across different studies.
Investigators in the U.S., working with colleagues in Australia and Europe, analyzed data from 13 case-control studies of ovarian cancer.
Pooled data from these studies, which were all part of the Ovarian Cancer Association Consortium, included information on 9,973 women (mean age of 57.4) diagnosed with ovarian cancer, and 13,843 healthy women serving as controls (mean age of 56.4). All had given birth at least once.
Investigators evaluated several breastfeeding parameters, including its duration following all pregnancies, a woman’s age at her first and last breastfeeding, and the number of years since a last breastfeeding. These measure were gathered via questionnaires or through interviews performed during the studies.
Data collection spanned from November 1989 to December 2009, and data analysis from September 2017 to July 2019.
Breastfeeding was linked to a 24% lower risk of invasive ovarian cancer, and to a 26% lower risk of borderline ovarian tumors compared with women who did not breastfeed. These associations were consistent across different studies in both tumor types.
Regardless of the number of pregnancies and children, women who breastfed at some point in their lives had a lower risk of developing any type of invasive ovarian cancer, particularly high-grade serous, endometrioid cancer, and clear cell cancer.
“We observed no statistically significant effect modification by age, body mass index at young adulthood, history of endometriosis, and family history of ovarian cancer,” the investigators wrote.
Race and ethnicity appeared to modulate this risk, as breastfeeding only significantly reduced ovarian cancer risk in white women (by 27%). A trend was observed in black and Asian women, but it did not reach statistical significance, likely because the “study population included predominantly [89%] white women,” they wrote.
Statistical analyses also indicated that longer periods of breastfeeding were associated with increasingly lower risks of invasive ovarian cancer. Women who breastfed for one to three months had a 18% lower risk of invasive ovarian cancer, while those who breastfed for more than a year had a 34% lower risk.
“Similar associations were observed in analyses restricted to high-grade serous tumors,” the researchers wrote.
Women who had stopped breastfeeding less than a decade ago were seen to have a 44% lower risk of ovarian cancer. This effect seemed to persist over long periods of time, with investigators reporting that women who had stopped breastfeeding more than 30 years ago still had a 17% lower ovarian cancer risk.
“In conclusion, breastfeeding is associated with a significant decrease in ovarian cancer risk overall and for high-grade serous tumors, the most lethal subtype. The World Health Organization recommends exclusive breastfeeding for at least six months and continued breastfeeding with complementary foods for two or more years,” the researchers wrote.
“Our results support these recommendations, while noting that breastfeeding fewer than three months per child is still associated with significant ovarian cancer risk reduction,” they added.
These findings also indicate that breastfeeding is a modifiable factor that reduces the likelihood of ovarian cancer, potentially by modulating immune and metabolic factors involved in cancer risk.
This association “needs to be investigated in large populations of other races and ethnicities,” the researchers concluded.
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