The relationship between obesity and overweight and breast cancer has some elements of mystery. But this is not one of them: in metastatic breast cancer (MBC), excess body weight does not negatively influence outcomes.
Multiple small studies have demonstrated this point, and now, for the first time, a large multicenter cohort analysis indicates the same.
Using medical records from 18 French comprehensive cancer centers, investigators reviewed body mass index (BMI) and overall survival (OS) data for nearly 13,000 women. The median OS was 47.4 months, and the median follow-up was about the same length of time. The team reports that obesity and overweight “were clearly not associated with prognosis.”
However, underweight was independently associated with worse OS (median, 33 months; hazard ratio, 1.14; 95% confidence interval, 1.02-1.27), report Khalil Saleh, MD, of Gustave Roussy in Villejuif, France, and colleagues.
In short, obesity or overweight had no effect on the primary outcome of OS, but underweight did.
“Underweight should be the subject of clinical attention at the time of diagnosis of MBC, and specific management should be implemented,” said study author Elise Deluche, MD, of CHU de Limoges, in an email to this news organization.
The study was published online Dec. 1 in The Breast.
“It’s really wonderful to have such a large cohort to look at this question,” said Jennifer Ligibel, MD, of the Dana-Farber Cancer Institute, Boston, who was asked for comment.
Is this another case of obesity paradox in cancer (as in renal cell carcinoma and melanoma, where excess weight is tied to better cancer-specific survival)?
No, said Dr. Ligibel: “There’s no hint at all [in this study] that people with obesity and overweight did better. … They just didn’t have worse outcomes.”
The study authors point out that the opposite is true in early-stage breast cancer. In this patient population, excess weight is associated with worse outcomes.
For example, in a 2014 meta-analysis of 82 follow-up studies in early-stage disease, obesity was associated with higher total mortality (relative risk, 1.41) and breast cancer–specific mortality (RR, 1.35) as compared to normal weight.
Why is there such a contrast between early- and late-stage disease?
“I don’t think we know exactly,” answered Dr. Ligibel. “It may be that, with breast cancer, as disease progresses, the pathways through which lifestyle may impact breast cancer may become less important.
“Obesity and overweight are associated with cancer risk in general,” said Dr. Ligibel, citing more than a dozen malignancies, including breast cancer.
But there is also an age element. Overweight or obesity is an independent predictor of breast cancer risk in postmenopausal women, but in premenopausal women, it appears to be protective. “Historically, there has been a lower risk of hormone receptor–positive breast cancer in women with obesity at younger ages that we don’t completely understand,” Dr. Ligibel noted.
That age-based difference is a conundrum, said Dr. Ligibel: “People have been trying to figure that out for a long time.”
Dr. Ligibel summarized as follows:
“There is a clear relationship between obesity and the risk of developing breast cancer; there is a clear relationship in early breast cancer that obesity is related to an increased risk of occurrence and mortality. What we are seeing from this study is that, by the time you get to metastatic breast cancer, body weight does not seem to play as important a role.”