Studies Discover That Obesity Is Linked to Numerous Types of Cancer
Taken together, the cancers account for 42 percent of all new cancer diagnoses
Solid evidence that being overweight or obese increases a person's risk for at least 13 types of cancer has been discovered by a review of more than 1,000 studies.
There was already strong evidence linking five types of cancer to being overweight or obese. These were adenocarcinoma of the esophagus, colorectal cancer, breast cancer in postmenopausal women, and uterine and kidney cancers.
The review, which was conducted by a working group of the International Agency for Research on Cancer, part of the World Health Organization, links eight more cancers to the conditions: gastric cardia (cancer of the part of the stomach located closest to the esophagus), liver cancer, gallbladder cancer, pancreatic cancer, thyroid cancer, ovarian cancer, meningioma (a kind of brain tumor that is usually benign), and multiple myeloma, a kind of blood cancer.
Taken together, these 13 cancers account for 42 percent of all new diagnoses of cancer, according to Dr. Graham Colditz, chairman of the working group and professor of medicine and surgery at Washington University in St. Louis.
"Only smoking comes close," as an environmental factor that affects cancer risk, Colditz said. "And that's an important message for nonsmokers. Obesity now goes to the top of the list of things to focus on."
Obesity is linked to significant metabolic and hormone abnormalities as well as to chronic inflammation. These factors may partly explain the condition's connection to cancer.
Elizabeth A. Platz works at the Johns Hopkins Bloomberg School of Public Health as an epidemiology professor, and she is also a widely-published researcher on cancer who did not participate in the study. According to Platz, this report came from a "high-caliber working group of respected epidemiologists and laboratory researchers," and the findings are of particular interest to women.
"The strongest association they found," she said, "is with uterine cancer. And postmenopausal breast cancer is also connected to obesity, especially estrogen receptor positive cancer. These are important messages that women need to hear."
The majority of the studies examined by the researchers were observational and therefore cannot prove cause and effect; however, the researchers considered the evidence to be sufficient if a link could not be explained either by chance, bias, or any other puzzling factors. Most of the studies also compared any risk increases to those of an adult who has a normal weight with a body mass index (BMI) between 18.5 and 24.9.
The review found that in the case of some cancers the risk was greater when the person weighed more. For instance, a woman whose BMI was between 25 and 29.9 had a 50 percent higher relative risk for endometrial cancer than did a woman of healthy weight. The risk more than doubled when the woman's BMI was between 30 and 34.9, and it more than quadrupled when it was between 35 and 39.9. For a woman whose BMI was 40 or higher, the risk for endometrial cancer was seven times greater.
Only limited evidence was found linking obesity and three other types of cancer: male breast cancer, prostate cancer, and diffuse large B-cell lymphoma, the most common kind of non-Hodgkin's lymphoma. The researchers did not find adequate evidence connecting obesity with squamous-cell esophageal cancer, gastric noncardia cancer, cancer of the biliary tract, lung cancer, cutaneous melanoma, testicular cancer, urinary tract cancer, or glioma of the brain or spinal cord.
Although studies of animals suggest that losing weight reduces the risk of developing cancer, Colditz said that this hypothesis is "hard to study in humans because so few people lose weight and keep it off. But the priority of avoiding weight gain is the first thing we need to address."