Body mass index (BMI) is not as important to cancer outcomes as previously thought, according to the ABCPP (After Breast Cancer Pooling Project), which analyzed the effects of BMI, comorbidities and physical activity on mortality. Previous studies showed that being overweight increases breast cancer recurrence, breast cancer–specific mortality and overall mortality.
The investigators used data from three patient cohorts (the Women’s Healthy Eating and Living Study, the Life After Cancer Epidemiology Study and the Nurses’ Health Study) that were harmonized for the ABCPP (N=9,513). Four statistical models were used, all of which controlled for various demographic, lifestyle and cancer-related factors. The first model analyzed the effect of BMI alone using the five body weight categories of the World Health Organization. The second looked at hypertension and diabetes or the combined diagnosis. The third examined the effects of four levels of physical activity. The final one modeled all of the variables together.
The first three models found various relationships. BMI was not associated with breast cancer–specific mortality but, predictably, was associated with all-cause mortality. Patients categorized as obese I or II had 23% and 37% increases in all-cause mortality risk; in contrast, being underweight was associated with a 240% increase in all-cause mortality. Physical activity was significantly associated with a 19% decrease in the risk for breast cancer mortality among women in the highest quartile of physical activity (MET [metabolic equivalent] h/wk >20.8), compared with the lowest quartile (MET h/wk <2.5). Physical activity had a significant effect on all-cause mortality, with the hazard ratio decreasing by 20% to 40% across quartiles (Q2, 0.80; Q4, 0.62). Having both hypertension and diabetes increased the risk for breast cancer mortality by 40%, and increased the risk for all-cause mortality 230%.
In the model that examined all factors together, the association previously seen between obesity and all-cause mortality was attenuated. “It was the comorbidities and the physical activity that had a remaining association with all-cause mortality,” said Sandahl Nelson, MS, a doctoral student researcher at the Moores Cancer Center, University of California, San Diego, who presented the study. “Our findings suggest that after active treatment, it is physical activity and comorbidities that are possibly driving the association we see with BMI in these after-treatment obesity studies.” She said the standard of care for breast cancer survivors should include physical activity interventions and treatment for comorbidities, with less focus on weight.
“This study shows we really need to look at the mechanisms of everything that may be contributing to survival,” commented Norah Lynn Henry, MD, an associate professor at the University of Michigan Comprehensive Cancer Center, in Ann Arbor. “That also includes trying to look at the effects of disparities and how that affects the various mechanisms.”