The link between obesity and cancer has increasingly been emphasized in public health messages, but is the current message correct?
"Being overweight or having obesity increases your risk of getting cancer," warns the US Centers for Disease Control and Prevention. It warns that overweight/obesity is "linked with a higher risk of getting 13 types of cancer...[which] make up 40% of all cancers diagnosed in the United States each year."
But that message, which is also promulgated by many cancer organizations, is based on data from observational studies, which have many limitations.
A new study based on Mendelian randomization studies has come to a slightly different conclusion and has found a potential causal association with just six cancers.
In addition, it found an inverse relationship for breast cancer, in which early-life obesity was associated with a reduced risk of breast cancer, and the relationship with obesity was "complicated" for lung and prostate cancer.
The study, headed by Zhe Fang, MBBS, Harvard T. H. Chan School of Public Health, Boston, Massachusetts, was published in the Journal of the National Cancer Institute on March 1.
"For a seemingly straightforward question of whether excessive body fatness causes cancer, the answer may not be straightforward after all," writes Song Yao, PhD, professor of oncology, Roswell Park Comprehensive Cancer Center, Buffalo, New York, in an accompanying editorial.
"How to craft a simple public health message to convey the complexity and nuances of the relationships may be a challenge to be grappled with going forward," he added.
In an interview with Medscape Medical News, Yao said that it "really depends on what kind of message you want to get out.
"If you want to talk about cancer overall, as one disease, we all know that a clear association with obesity does not exist," he said: "it's not that simple.
"You really cannot say that obesity increases cancer risk overall," he said.
For some cancers included in the study, Yao continued, it was "very clear that obesity increased the risk...but for some other cancer types, we either don't have enough data yet or the association is not as consistent."
This, he said, is especially the case for prostate and lung cancer.
All of this indicates that there is a complex relationship between obesity and cancer risk, he maintains.
"We always think obesity is bad, not only for cancer but also for more common conditions, like hypertension, diabetes, and cardiovascular disease," Yao noted. This points to the link between obesity and chronic inflammation, he added.
However, there are also other hypotheses, including synthesis of estrogen in adipose tissue, which may explain the link between obesity and breast cancer risk in older women.
However, in younger woman, obesity protects against breast cancer, and "we really don't know why," Yao said.
The new study used Mendelian randomization to examine these relationships. This is a "new tool that we have developed over the past 20 years or so, largely because there is so much data coming from genome-wide association studies," Yao explained.
It has "advantages" over other methods, including observational studies. One of its strengths is that it is "not impacted by reverse causality," because genetic risk does not change over time.
However, he said, it is "quite straightforward to think that the genetics do not change, but at the same time, the environment we live in throughout our life course changes," and the impact of genetic variants may be "washed out."
How genetics influences cancer risk may therefore change over time, and it is a "dynamic process," Yao commented.
In addition, this approach has its own limitations, he said, because it depends on how much of the variation in a given measure can be attributed to genetic factors.
In their study, Fang and colleagues reviewed 204 meta-analyses of 2179 individual estimates from 507 cohort or case-control studies. They found "strong evidence" that supports the association between obesity and 11 cancers.
These are esophageal adenocarcinoma, multiple myeloma, and cancers of the gastric cardia, colon, rectum, biliary tract system, pancreas, breast, endometrium, ovary, and kidney.
They note, however, that the associations "may be causal for some malignancies" but that the co-occurrence of obesity with various cancer risk factors means that others may be "susceptible to potential confounding bias."
To overcome some of these limitations, the team looked to Mendelian randomization studies that examined the association between genetic variants linked to body mass index (BMI), indicating lifetime risk of high BMI, and cancer risk for a range of cancer types.
These Mendelian randomization studies were then compared with the results of large-scale conventional observational studies, as well as with evidence in reports from the International Agency for Research on Cancer and the World Cancer Research Fund–American Institute of Cancer Research, which also include experimental studies.
The researchers say that, overall, the Mendelian randomization studies "further establish the causality of obesity" with six cancer types: colorectal, endometrial, ovarian, kidney, and pancreatic cancer, and esophageal adenocarcinoma.
In addition, these studies further establish the inverse relationship of early-life obesity with breast cancer.
However, the approach could not confirm a positive association between obesity and gallbladder and gastric cardia cancer, as well as multiple myeloma.
"This could be due to low power," the team suggests, "and larger studies are required."
With respect to lung cancer, the Mendelian randomization identified a positive association with obesity that supports the inverse association identified in observational studies, ie, that obesity may reduce the risk for lung cancer.
The researchers suggest this may reflect reverse causality ralated to the loss of lean body mass before diagnosis, as well as confounding by smoking.
For prostate cancer, the evidence was "conflicting" and "implies a complicated role of obesity," Zhang and colleagues comment.
The link between obesity and lower prostate-specific antigen levels, they suggest, may result in a detection bias by masking the presence of prostate cancer, or it "could be biological" in origin, owing to reduced androgen levels.
For six cancer types for which a causal relationship with obesity could be established, the effect estimates from the Mendelian randomization studies were stronger than those seen in conventional studies, with the magnitude of risk ranging from 1.14-fold for early-life obesity and breast cancer to 1.37-fold for adult obesity and esophageal adenocarcinoma.
In another editorial accompanying the new study, Graham A. Colditz, MD, DrPH, from Washington University School of Medicine, St. Louis, Missouri, underlined that childhood and adolescent obesity and their contribution to cancer risk need further attention.
"To reap the reward from past research, we must act to implement effective strategies to reduce childhood and adolescent adiposity, reduce excess weight gain in adult years, and maintain a healthy weight," he writes.
"This will require us to change the way we live, but COVID-19 has shown we can make changes to how we live and work. Let us keep the changes we have already made, or take on new ones, that will cut our collective cancer toll," he implores.
No funding for the study was described. Colditz is supported by the Breast Cancer Research Foundation. No other relevant financial relationships were described.
J Natl Cancer Inst. 2022 Mar 8;114:361-371, Fang Z et al, Abstract; 331-332, Yao S, Full text; 333-334. Colditz GA, Full text
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Cite this: Liam Davenport. Obesity Increasing the Risk for Cancer: It's Complicated - Medscape - Mar 30, 2022.