What your body fat says about your cancer risk

If you’re concerned about your cancer risk, experts say it’s worth asking your doctor about your body fat. That advice comes from a new review examining the links between obesity and cancer.

About 10% of new cancer diagnoses each year are tied to overweight and obesity, and losing more than 10% of body weight may reduce that risk, according to the analysis led by Dr. Neil Iyengar, an oncologist and director of Emory University’s Winship Cancer Institute.

“The more fat tissue (someone has) and the more obese a person is, the more profound the negative or harmful effects,” Iyengar said. The review was published earlier this month in the Journal of the American Medical Association.

“The cancer-promoting effects are also present for overweight individuals as well, and perhaps most alarming, we also find that these effects are present for people who have normal body weight and a high body fat level.”

Overweight is defined as a body mass index — calculated as weight in kilograms divided by height in meters squared — of 25 to 29, while obesity is a BMI of 30 or higher. A body fat percentage of more than 30% is also associated with a higher risk of cancer, Iyengar said.

Obesity-related cancers, such as breast and colorectal cancers, are increasing at faster rates than others among adults 25-49, according to the JAMA review. With March being Colorectal Cancer Awareness Month, the American Cancer Society recently released a study showing colorectal cancer is now the leading cause of cancer death in adults younger than 50 in the country.

Iyengar said the scientific review was needed because obesity-related cancer rates continue to rise worldwide despite the increased use and popularity of glucagon-like peptide 1 (GLP-1) receptor agonists and bariatric surgery.

“We have a better scientific understanding of how obesity drives cancer development, and we now have effective medical weight loss therapies with the GLP-1 receptor agonists and also bariatric surgery,” he said.

In observational studies cited in the JAMA analysis, patients who lost more than 10% of body weight through those treatments had modest reductions in their incidence of obesity-related cancer. Iyengar also explained obesity causes inflammation, which fuels cancer cell growth.

The longer a person has obesity, the more difficult it is to reduce the effects. “Obesity can be thought of as a carcinogen, such that longer exposure to being obese translates to increasing cancer risk.”

The U.S. Food and Drug Administration has approved weight loss drugs for diabetes and obesity, but not cancer prevention, Iyengar said.

“To me, that’s very alarming when we’re seeing that obesity-related cancers are the ones that are going up in incidence, particularly in our young patients.”

He said the main purpose of the scientific review is to educate practicing clinicians about the biology of obesity so they can better judge available treatments for their patients.

“The field of obesity-related cancer has really been advancing rapidly in the last few years,” Iyengar said. “We want to educate our clinicians to understand that if we’re treating obesity, we’re also treating cancer risk.”

The JAMA analysis also calls for more research to develop prevention strategies and serves as a wake-up call to the public health community about the cancer risks from obesity and how they should respond to it, he said. For instance, BMI has been the traditional way of defining risk but there could be other options, such as measuring body fat percentage, glucose or insulin levels.

“We do a lot of this already. People go to the doctor and they get their blood sugar and lipid levels measured to screen for diabetes and heart disease, but we need to start thinking about how we can use those screening tools to also predict cancer risk.”

Iyengar encourages patients to discuss cancer risk with their doctors when making lifestyle changes for diabetes or heart disease, because many of the same steps can help lower cancer risk.

Dr. Alicia Shelly, an obesity medicine specialist at Wellstar Douglasville Medical Center, said such scientific reviews highlight a subject not often discussed by patients or doctors.

“It helps me convince a hesitant patient that losing weight is in their best interest,” Shelly said. Evidence that weight loss can reduce disease risk may motivate patients to improve their diet and exercise habits.

Doctors on the fence about the benefits of bariatric surgery or GLP-1 weight loss medicine would also have further evidence those treatments can help reduce incidence of obesity-associated cancer, Shelly said.

“Review articles like these play an important role in medicine,” she said. “Every few years, researchers synthesize findings from many individual studies into one comprehensive paper so clinicians and the public can better understand the overall evidence. Many people are still unaware of the strong connection between obesity and cancer risk, and highlighting this relationship can help raise awareness and encourage individuals to take their health more seriously.”

Shelly said she agreed with the researchers in the JAMA review that more randomized clinical trials are needed to validate the observational findings, including the long-term effect of weight-loss medications on cancer risk.