Women in the study with a BMI higher than 35 had a 58% greater risk of invasive breast cancer than women with a BMI less than 25.
The study, published in JAMA Oncology, was conducted by Dr. Marian L. Neuhouser, of the Fred Hutchison Cancer Research Center in Seattle, WA, and coauthors.
While previous observational studies, meta-analyses and systematic reviews have associated obesity with increased breast cancer risk, researchers remain uncertain about the role of obesity in postmenopausal breast cancer.
Questions remain, write the study authors, "including whether obesity is associated with breast cancer characteristics, such as tumor hormone receptor status and stage at diagnosis, or whether use of postmenopausal hormone therapy (HT) modifies the obesity-breast cancer association, because both obesity and HT alter a woman's hormone profile."
Recently, Medical News Today reported on a review of data from two Women's Health Initiative (WHI) clinical trials that revealed the varying effects of menopausal hormone therapy on the incidence of breast cancer over time, offering new evidence of progesterone's role in the cancer.
In the US, obesity is considered by some to be a public health epidemic. The Centers for Disease Control and Prevention (CDC) report that more than one third of adults in the US - around 78.6 million - are obese.
For the study, the researchers utilized data for 67,142 women participating in the WHI - a 15-year research program designed to investigate cardiovascular disease, cancer and osteoporosis in postmenopausal women.
As part of the WHI, each participant had height, weight and incidence of cancer measured, alongside baseline and either annual or biennial mammograms. The participants were enrolled between 1993-1998 and were followed up for a median of 13 years.
Among the cohort of women selected for the study, the researchers noted a total of 3,388 cases of invasive breast cancer.
Overweight women who lost weight did not reduce their breast cancer risk
Compared with women defined as having a normal weight, with a body mass index (BMI) of less than 25, the researchers found that women who were overweight or obese had an increased risk of invasive breast cancer.
The risk of invasive breast cancer was highest among women with a BMI higher than 35, whose risk was 58% greater than women with a BMI less than 25.
Women with BMIs of 35 and higher were also associated with increased risks of estrogen and progesterone receptor-positive breast cancers - the most common forms of the disease - although not with estrogen receptor-negative breast cancers.
In addition to increasing breast cancer risk, obesity was also associated with indicators of poor cancer prognosis, such as large tumors, poorly differentiated tumors and the involvement of lymph nodes.
The connection between weight and breast cancer risk was further supported by the observation that women who started the study with a BMI of less than 25 but then gained more than 5% of body weight had an increased risk of developing breast cancer.
Unfortunately for participants that were overweight or obese at the start of the study, no association was found between weight change during the study and breast cancer risk. However, the researchers found that the association between BMI and breast cancer risk was also unaffected by HT.
The researchers acknowledge that the study is limited by a shortage of data on longer-term BMI changes and an inability to distinguish between intentional and unintentional weight loss. Despite this, they believe their findings support the need for clinical trials evaluating the role of obesity prevention and treatment on breast cancer risk.
In a related commentary, Dr. Clifford Hudis, of Memorial Sloan Kettering Cancer Center, and Dr. Andrew Dannenburg, of Weill Cornell Medical College - both in New York, NY - write that the study helps refine our understanding of the risk of overweight and obesity:
"Overweight and obesity are a growing global challenge, and the increased burden of malignant disease, to which it contributes, is another one. Their report helps focus our thinking and motivates us to pursue a deeper understanding of why overweight and obesity are a problem so that we can plan more effective and thoughtful responses.
Cancer is also a growing global challenge, according to a recent report. Researchers from around the world state that cancer represents a still growing proportion of all deaths, accounting for over 8 million in 2013.