By Louise D. Metz M.D.
Earlier this month, I was bombarded with multiple medical news emails, a radio news story, and multiple online news articles about a study purporting to show rising cancer rates in young people due to “obesity”. Here are a couple of the many headlines: “Obesity linked to higher cancer rates in young people” (from the AMA), and “For millenials, cancers fueled by obesity are on the rise, study says” (CNN). Not surprisingly, the conclusions being drawn in the study and news articles are rooted in weight bias. The study by Sung et al. in The Lancet simply looked at incidence of multiple types of cancers over a 20 year period in different age groups, and did not at all assess for causes of these cancers. They found that rates of half (6 out of 12) of the cancers that have been shown in previous studies to be associated with “obesity” have increased in young people between 1995 and 2014 (multiple myeloma, colorectal, uterine, gallbladder, kidney, and pancreatic cancer). The headlines don’t tell us that the authors found that rates of 2 types of cancer that have not been found to be associated with “obesity” have also increased (gastric and leukemia), and that they found no increase in incidence of the other 6 cancers that have been associated with larger body size (breast, ovarian, esophageal adenocarcinoma, gastric cardia, liver and intrahepatic bile duct, and thyroid cancer). In addition, the study shows us that the absolute rates of these cancers in young people are still extremely low, with incidence rates of 0.1-10 per 100,000 people (0.0001%- 0.01%), and that the absolute rates of increase for these cancers were very small.
If we take a closer look at the assumptions made in this study and associated news articles, we can find other ways to look at this data. Though this study only looked at incidence of cancers by age, their conclusions were based on previous research showing correlation of body size with certain cancers. While there are studies showing this correlation, the association of body size with cancer or any other medical condition does not mean that the physical trait of body size caused the health conditions. Two variables can be associated, or correlated, but one does not necessarily cause the other. There may be a third factor (or factors) that is the cause of the health outcome. When body size is found to be associated with certain cancers or other health conditions, it may be a confounder for other causal factors (such as genetics, environmental factors, weight stigma, and weight cycling). Therefore, stating that cancers are “fueled” by obesity is not an accurate statement based on this study.
Similar to the association between weight and some cancers, increasing height has been found to be associated with certain cancers, including colon, kidney, thyroid, ovarian, endometrial, melanoma, and lymphoma. We know intuitively that the physical trait of height is correlated with but not the cause of cancer, and we do not attempt to shrink people’s height in order to prevent or treat cancer. We also don’t hear headlines about tall people having rising rates of cancer since being tall is a desirable trait in our society and we know that this trait cannot be changed.
In addition, there are actually multiple cancers that occur at lower rates in people with larger bodies, including lung, stomach, and melanoma. Researchers call this the “Obesity paradox” because it challenges their belief that weight is always an indicator of poor health. These findings can be so surprising to researchers that they must be deemed a perplexing finding to be explained rather than just another objective scientific finding. These “paradoxical” findings also don’t make the headlines since they do not fit the weight-normative paradigm of our healthcare system and society.
If a causal relationship does exist between body size and certain cancers, it is important to acknowledge that our body size is not within our control and that changing our body size and maintaining weight loss is not possible for the majority of people. In addition, we do not have evidence that successfully changing body size would decrease risk of cancer and other health conditions. We need to look at this evidence from other angles and without bias with the goal of improving health outcomes.
Despite the alarm in the headlines about body size and cancer, this has not translated to increased concern for the health of people of larger bodies in clinical practice. People in larger bodies with some types of cancer (cervical, colon, kidney, liver, pancreas among others) have higher mortality rates than those in smaller bodies. There are likely complex reasons for these higher mortality rates, including a decreased rate of cancer screening tests, delayed diagnosis due to weight bias in medical care, and inadequacy of cancer treatments that were not designed for people in larger bodies (radiation and chemotherapy). In fact, we know that women in larger bodies are less likely to receive screening tests for cervical cancer with pap smears or for breast cancer with mammograms. This may be due to a decreased rate of test ordering by providers, and avoidance of medical care by people in larger bodies due to past experiences of weight stigma in medical settings. If we had true concern about the possibility of an association between cancer risk and body size, we should also be addressing the lower rates of cancer screening and higher cancer mortality rates for those in larger bodies rather than placing blame on individuals for their disease.
Here are some alternative headlines that could be drawn from this study and other literature on body size and cancer:
“Only half of cancers previously associated with obesity have seen increasing rates in young people, suggesting other causes for these cancers”.
“Multiple types of cancers found to occur at lower rates in people of size. More evidence to dispel the myth that higher weight determines poor health.”
“Weight stigma is leading to inadequate medical care: Women in larger bodies are less likely to be screened for cervical cancer and breast cancer.”
“People in larger bodies with cancer have a lower life expectancy than those in smaller bodies. Why is our medical system failing people of size?”
Weight bias and weight stigma exist in our research and clinical practice. It’s time to change the headlines.
Sung et al. Emerging cancer trends among young adults in the USA: analysis of a population-based cancer registry. Lancet online. February 3, 2019.
Green et al. Lancet Oncology. 2011 Aug;12(8):785-94.
Calle et al. N Engl J Med 2003; April 2003. 348:1625-1638
Poorolajal et al. European Journal of Cancer Prevention:May 2016- Vol 25-Issue 3: 232-238.
Frumovitz et al. Obstetrics & Gynecology: December 2014- Vol 124-Issue 6: 1098-1104.
Nunney. 285 Proceedings of the Royal Society B: Biological Sciences 2018.