Excess body fat increases the risk of many cancers, and losing weight may prevent obesity-related cancers, according to a special report released Thursday by the World Health Organization (WHO), Washington, DC.

The WHO’s International Agency for Research on Cancer (IARC) published the report in the New England Journal of Medicine.

"One clear message from this IARC initiative is that the cancer research community needs to pivot from the question of whether obesity is an important risk factor for many cancers – it is – to the question of how we can reduce the impact of obesity on cancer," said Stephen D. Hursting of the University of North Carolina at Chapel Hill, a member of the IARC report’s working group and an expert on AICR/WCRF’s Continuous Update Project reports.

"Given that the AICR/WCRF reports and other sources estimate that 20 percent of cancers are attributed to excess weight, we urgently need to address this question."

AICR links excess body fat to increased risk of 11 cancers, including postmenopausal breast, colorectal and esophageal. Aside from not smoking, staying a healthy weight is the single largest step individuals can take to lower their risk. If everyone in the United States were a healthy weight, AICR estimates that 130,600 cases of U.S. cancer cases could be prevented each year.

For those already overweight, evidence appears that intentional weight loss, regardless of the means, has a cancer preventive effect if significant weight loss is achieved, Hursting said.

"We know that almost half of Americans are not aware that obesity can increase their risk of cancer," said Alice Bender, AICR’s head of nutrition programs. "We see this as an empowering message. Rather than throwing up your hands, there are lifestyle choices everyone can make. Take that first step whatever it is: Go for a 10 minute walk, start adding more vegetables and plant foods to your plate, and pay attention to portion sizes."

The new IARC findings identify 13 cancers in which the absence of excess body fat lowers cancer risk. Eight cancers have been added since the group's last report in 2002. Since that time, a large number of new scientific studies on the effects of weight or weight change on cancer risk have been published. In this report, the authors evaluated more than 1,000 population studies, along with animal and cell studies investigating mechanisms.

The updated evidence is published in IARC Handbooks of Cancer Prevention Volume 16: Body Fatness.

In population studies, body mass index (BMI) is how studies classify individuals as overweight and obese. A BMI of 25 or more is categorized as overweight; a BMI of 30 kg/m2 or greater is obese. BMI is only one indicator of having excess body fat and those classifications would not apply to each individual.

Worldwide, an estimated 640 million adults in 2014 were obese, an increase of six-fold since 1975. In the United States, approximately two-thirds of adults and one-third of children are overweight or obese.

Obesity may increase cancer risk by a number of pathways, according to Hursting. Excess body fat is associated with substantial metabolic and endocrine changes that favor several hallmarks of cancer, including increased cell proliferation, invasiveness and decreased cell death. Established mechanisms include alterations in hormones and inflammatory factors that play a role in cancer development. Emerging mechanisms that are likely contributors but currently have limited evidence regarding the obesity-cancer link include the gut microbiome and antitumor immune responses.