According to the International Agency for Research on Cancer (IARC), having lower overall body fat lowers the risk of developing eight tumor types: cancers of the gastric cardia, liver, gallbladder, pancreas, ovaries, and thyroid, in addition to multiple myeloma and meningioma—a type of brain tumor.
IARC’s report reaffirmed the conclusion of the agency’s 2002 paper that found evidence that the risk of being diagnosed with five cancer types—colorectal cancer, adenocarcinoma of the esophagus, renal cell carcinoma, postmenopausal breast cancer, and uterine cancer—is lower among people who are not overweight or obese. This new report, published in the New England Journal of Medicine in August 2016 (doi: 10.1056/NEJMsr1606602), now brings the total number of tumor types linked to being overweight or obese to 13.
The basic message from the IARC’s literature review: Limiting weight gain in the long term can reduce the risk of many cancers.
“Since 2000, there have been many, many more studies that have examined the link between obesity and cancer,” said Susan Gapstur, Ph.D., M.P.H., a cancer epidemiologist at the American Cancer Society. “There is more scientific evidence showing that excess body weight increases the risk for several cancer types.”
“The association between obesity and cancer is still not common knowledge. When people think of obesity, they think of diseases such as diabetes and cardiovascular issues.”
Worldwide, about 640 million adults in 2014 are obese—six times more than in 1975 ( Lancet 2016; 387:1377–96). The world now has more overweight and obese people than those who are underweight.
Because obesity is a modifiable risk factor for cancer, researchers see these data as a tool to educate people on the benefits of weight control and a healthy, active lifestyle.
“It would be really good if physicians would have conversations with their patients and patients’ families about the link between excess body weight and elevated cancer risk,” Gapstur said. “It’s not clear whether people appreciate that the increased risk for cancer occurs not just in the obese but also in the overweight category.”
“The association between obesity and cancer is still not common knowledge. When people think of obesity, they think of diseases such as diabetes and cardiovascular issues”, said study author Beatrice Lauby-Secretan, Ph.D., an IARC scientist responsible for the Handbooks of Cancer Prevention series.
Keeping It On
Although obesity appears to contribute to the development of several cancers, it’s unclear how the duration of time that someone carries excess body fat contributes to the risk.
“There is no clear picture emerging yet: There are not many studies, and it is difficult to measure accurately,” Lauby-Secretan said.
Studies that track people’s weight histories are challenging to perform because recalling past weight and its fluctuations is difficult for people and tracking a population is a long (and costly) endeavor. To overcome the challenge, Melina Arnold, Ph.D., a research scientist at the IARC, focused on estimating the cancer burden attributable to excess weight. Meanwhile, her colleagues used data from the observational Women’s Health Initiative (WHI) study, which tracked the health of 73,913 postmenopausal women and measured their height and weight for an average of 12.6 years. About two-thirds of the women in the study were either overweight or obese. During the study, 6,301 obesity-related cancers were diagnosed.
“What we don’t know is whether there is a dose-dependent relationship between the time an individual carries excess body fat and the risk of cancer,” said study author Hoda Anton-Culver Ph.D., professor and chair of the department of epidemiology at the University of California, Irvine.
Arnold, Anton-Culver, and their colleagues used available data from the large cohort and modeled body mass index (BMI, expressed in kilograms per square meter of body surface area) across all ages of adulthood for the women. The researchers found that being overweight for a longer period was associated with obesity-related cancers, including those of the kidneys, ovaries, liver, and pancreas. The risk of developing an obesity-associated cancer increases by 10% for every 10 years someone remains obese (BMI > 30 kg/m 2 ). The absolute increase in risk depends on the absolute risk of a particular cancer.
The most statistically significant increase was for postmenopausal breast and endometrial cancers, whose risks increased 5% and 17%, respectively, for every 10-year overweight period.
When the researchers analyzed other obesity-related cancers, they found no statistically significant association for rectal, liver, gallbladder, pancreatic, ovarian, and thyroid cancer, probably because of the small numbers of those cancers within the cohort.
“This should not be taken as negative results because we need further data with sufficient power to obtain conclusive results [on these tumor types],” Anton-Culver said.
Still, “the study’s conclusions cannot be directly translated to the individual level,” said Arnold. “Not everyone who has been overweight for some time will develop cancer. The etiology of cancer has many genetic and environmental components.”
“The question the authors are asking is really important,” said Graham Colditz, M.D., D.Ph., an epidemiologist who studies cancer prevention at Washington University in St. Louis. “If an individual is obese in the early adult years, does that have a bigger adverse effect on cancer risk than staying lean until age 50 or older? This is among the first studies to address the question of whether more years in an overweight state results in a greater risk of developing cancer.”
A 2015 study used a different subset of the WHI cohort—the randomized clinical trials testing how dietary modification and postmenopausal hormone therapy affected postmenopausal breast cancer risk. Women who were overweight or obese had an increased risk of invasive breast cancer compared with normal-weight women ( JAMA Oncol. 2015; 5:611–21). The risk was strongest for estrogen receptor–positive cancers.
Moreover, women with a BMI of 25.0 kg/m 2 or less who gained more than 5% of their body weight during the study’s follow-up period had a 36% higher relative risk of breast cancer than that of women whose weight was stable. Yet the risk of the overweight or obese women in the trial who lost or gained weight during the follow-up period remained steady. The strength of this study, said Marian Neuhouser, Ph.D., R.D., of the Public Health Sciences Division of the Fred Hutchinson Cancer Center in Seattle, was that she and her coauthors could rely on investigator-generated clinical trial height and weight measurements instead of relying on self-reported data. Because an annual or semiannual mammogram was part of the WHI protocol, detection bias was minimal, according to Neuhouser.
“The best follow-up to the work now is to identify ways for women to remain at a healthy weight in their postmenopausal years,” Neuhouser said. “We are currently working on these studies and believe that a comprehensive approach of diet, exercise, and a living environment conducive to maintaining a healthy weight will be key.”
“The bottom line is that avoiding weight gain looks like a priority for cancer prevention,” Colditz said. “Working on how to make this happen for the general population is now a major priority [for researchers].”
