Cancer researchers have long studied the link between obesity and cancer, but there is still a lot of uncertainty about what drives that link and for which cancers the link is the strongest. A new study has investigated the strength of the research supporting that connection between being overweight and developing or dying from cancer, finding a significant amount of evidence is low quality.
The new umbrella review analyzed 204 meta-analyses in total and found that 77 percent reported a statistically significant link between obesity and the incidence of or mortality from cancer (BMJ 2017;356:j477).
Yet, of 95 meta-analyses that investigated the link between continuous obesity measures and the risk of developing or dying from 36 different cancers (or cancer subtypes), only 12 of those analyses (13%) were supported by strong evidence. The other studies were found to either have highly suggestive (18%), suggestive (25%), weak (20%), or no (25%) evidence of an association between the obesity measures and cancer.
"This potentially makes excess body fat the second most important modifiable cancer risk factor after tobacco use," noted Yikyung Park, ScD, Associate Professor of Surgery, and Graham A. Colditz, DrPH, MD, MPH, Deputy Director of the Institute for Public Health, Chief of the Division of Public Health, and Professor of Surgery, both at Division of Public Health Sciences at Washington University School of Medicine (BMJ 2017;356:j908).
The evidence suggests that better data on the link between being overweight and cancer risk is needed, the authors of the umbrella review concluded.
"As obesity becomes one of the greatest public health problems worldwide, evidence of the strength of the association between obesity and cancer may allow finer selection of people at high risk, who could be selected for personalized primary and secondary prevention strategies," the researchers noted in the original paper.
Study co-author Kostas Tsilidis, PhD, Senior Lecturer in Cancer Epidemiology in the Department of Epidemiology and Biostatistics within the School of Public Health on the Faculty of Medicine of Imperial College, London, explained more about what the data showed and what additional research in this area is needed.
1 Cancer researchers have been talking about the link between cancer and obesity for a long time. What does this new review you and your colleagues published in BMJ add to what is known about this link?
"These findings are complementary to what we already know about obesity and cancer risk. Obesity leads to lots of disruption of hormonal and metabolic pathways. Excess fat has been linked to higher estrogen levels, higher insulin levels, and increased inflammation, all of which can affect cell division, but different cancers may be more or less susceptible to the aforementioned mechanisms.
"We found that strong evidence supports the association between obesity and 11 cancers (esophagus, stomach, colon, rectum, biliary tract system, pancreas, breast, endometrial, ovary, kidney, and bone marrow). There could be associations between obesity and other cancers, but substantial uncertainty remains in the biomedical literature. These findings are important because cancer is a leading cause of death worldwide, and the prevalence of obesity has more than doubled over the past 40 years."
2 Where are the most urgent gaps-and what additional research is most important in terms of better understanding the link between obesity and cancer? And what are the challenges standing in the way of that research?
"Future large prospective observational studies and consortia thereof with better assessment of the dynamic nature of body fatness could assist in complementing the current literature evidence. But, since we need to start acting to reduce the obesity epidemic, we especially need better research to evaluate potential effective guidelines and public health interventions for weight management in children and adults.
"Randomized controlled trials of potential weight management interventions will be expensive. It also would be difficult to identify effective interventions with weight management results that are sustainable. Working with children and their parents is also challenging, but it is paramount to evaluate weight management programs at young ages."
3 There are a few conclusions that can be drawn from this research. What is most important for oncologists and oncology care providers (and everyone) to know about this work and the findings from this large-scale analysis?
"Strong evidence supported the association between obesity and 11 cancers. We should probably start to seriously act to reverse the obesity epidemic."