Decimation of Gut Bacteria and Chemotherapy-Induced Weight Gain in Patients With Breast Cancer

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Researchers have uncovered a potential link between chemotherapy-induced changes to gut bacteria and weight gain seen in patients with breast cancer, according to a recent study published by Walker et al in BMC Medicine. The findings may help to identify approaches to avoid obesity-related diseases later in life for breast cancer survivors. 


Previous studies have linked obesity to several types of cancer, including breast cancer. Nonetheless, oncologists have long observed that cancer treatments seem to exacerbate obesity. Many patients lose weight after a cancer diagnosis, but that trend is reversed in patients with breast cancer who undergo chemotherapy. The studies have shown that although 50% of patients with breast cancer have overweight or obesity prior to diagnosis, the rate increases to 67% after treatment. 

“[Patients] tend to be a little less active during and after chemotherapy, but they also tend to significantly reduce their caloric intake,” explained John Walker, PhD, Professor in the Department of Medicine and Dentistry at the University of Alberta and Head of Medical Oncology at the Cross Cancer Institute.

Study Methods and Results

In the recent study, the researchers enrolled 40 patients with breast cancer. They found that those who were treated with chemotherapy lost muscle mass and gained abdominal fat—which has been linked to cardiovascular disease, diabetes, and cancer recurrence. The patients who received chemotherapy also exhibited signs of inflammation and significant changes to the number and variety of gut bacteria.

“Changes in the bacterial populations within the gut directly correlate with unhealthy weight gain and increased body fat composition in [patients with] breast cancer who were treated with chemotherapy,” emphasized Dr. Walker. ”There’s something unique about this modulation of the gut microbiome for [patients with] breast cancer who receive chemotherapy,” he stated.


The researchers noted that it wasn’t surprising to find that the chemotherapy agents had bacteria-killing effects, since some of them are derived from antibiotics, and all are metabolized through the liver and then the gut.

“Breast cancer is an unparalleled success story in medicine. We see cure rates over 90% today, so survivorship is now equally important. We want to ensure that in survival, our patients aren’t then dealing with the metabolic consequences of weight gain during treatment,” Dr. Walker concluded.

Disclosure: For full disclosures of the study authors, visit

The content in this post has not been reviewed by the American Society of Clinical Oncology, Inc. (ASCO®) and does not necessarily reflect the ideas and opinions of ASCO®.