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 U.S. 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.
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, Mass., was published in the Journal of the National Cancer Institute
“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, N.Y., 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, Dr. 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, Dr. 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,” Dr. 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 women, obesity protects against breast cancer, and “we really don’t know why,” Dr. 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,” Dr. 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,” Dr. 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.