Researchers found a dose-response relationship for several cancers involved in eating, meaning the higher the weight, the greater risk of cancer.
For more than a decade, being obese was associate with increased risk of certain cancers: colon, esophagus, kidney, breast, and the uterus.
Now, an international panel reports in the New England Journal of Medicine that there’s sufficient evidence to link 8 more cancers with “excess body fatness,” as researchers called it: stomach, liver, gallbladder, pancreas, ovary, thyroid, multiple myeloma, and meningioma.
The special report of the International Agency for Research on Cancer (IARC), which convened in April 2016 in Lyons, France, made its assessments based on more than 1000 epidemiological studies, the vast majority of which were observational. Clinical trials involving weight loss or weight control interventions, they noted, were few.
The IARC’s gauge of cancer risk for those who are overweight or obese is based on body mass index (BMI) of 25 to 29.9, which is considered overweight, and 30 or higher, which is considered obese, relative to those with a BMI of 18.5 to 24.9. The team noted that an estimated 640 million adults and 110 million children were obese in 2014, a number 6 times higher than in 1975. Obesity rates worldwide are 10.8% among men, 14.9% among women, and 5% among children. Today, more people are obese or overweight than underweight, they wrote.
They found that cancers of the organs involved in eating—colon, rectum, stomach, liver, gallbladder, pancreas, kidney, and esophagus—all show a dose-response; this means that the more one weighs, the greater the risk of cancer. Relative risk was 1.2 to 1.5 for those who were overweight and 1.5 to 1.8 for those who were obese for cancers of the colon, liver, stomach, pancreas, and kidney; relative risk for cancer of the esophagus was as high as 4.8 for those with a BMI of 40 or more.
When researchers compared studies that focused on BMI with those that evaluated cancer risk of waist circumference, results were consistent. Links between certain cancers and BMI were consistent across geographic regions, and they were similar for men and women.
Being obese as a child or a teenager carries similar risks as being obese as an adult, the studies show, “despite some differences in magnitude and patterns,” the researchers report. The notable exception is the lack of a positive association with postmenopausal breast cancer. There is some limited evidence that losing weight can reduce cancer risk, or losing that losing it once cancer is diagnosed can reduce the chance it comes back.
How does obesity cause cancer? “Obesity is associated with substantial metabolic and endocrine abnormalities, including alterations in sex hormone metabolism, insulin, and insulin-like growth factor signaling, and adipokines or inflammatory pathways,” the authors write. The role of inflammation in particular is strong, but the good news is that people can reduce their cancer risk by losing weight.
Lauby-Secretan B, Scoccianti C, Loomis D, et al. for IARC Working Group. Body fatness and cancer—viewpoint of the IARC Working Group [published online August 25, 2016]. N Engl J Med.