For more than a decade, studies have linked excess body weight to several forms of cancer, such as breast and kidney cancers. Some researchers estimate that obesity is responsible for 20 percent of all cancers. New findings in The New England Journal of Medicine in November 2016 identify 13 cancers associated with being overweight or obese.
The latest report, from the International Agency for Research on Cancer, part of the World Health Organization, reviewed evidence from more than 1,000 studies. The researchers avoided pointing to overweight or obesity as the direct cause of the cancers, but they suggested that the “absence of excess body fatness” reduces the risk of the following cancers:
• Colorectal (colon and rectum) cancer
• Endometrial cancer (cancer that develops in the inner lining of the uterus)
• Breast cancer (after menopause)
• Pancreatic cancer
• Kidney cancer
• Liver cancer
• Gallbladder cancer
• Gastric cardia (a stomach cancer)
• Ovarian cancer
• Esophageal adenocarcinoma (cancer in the tube that runs from the throat to the stomach)
• Thyroid cancer
• Multiple myeloma (a blood cancer)
• Meningioma (a type of brain tumor)
Some limited evidence potentially linked excess weight to three more cancers: male breast cancer, fatal prostate cancer, and a type of lymph system disease called diffuse large B-cell lymphoma.
“Being extremely overweight or obese is associated with a long list of health hazards, such as diabetes, high blood pressure, heart disease, and osteoarthritis,” says William Dale, M.D., Ph.D., associate professor of medicine and section chief of Geriatrics & Palliative Medicine at the University of Chicago Medicine.
“But when most people think of cancer, they don’t typically associate it with obesity, perhaps instead imagining a thin person wasting away from cancer. Or, they think of more publicized relationships from public health campaigns, such as those between smoking and lung cancer or sun exposure and skin cancer. Patients need to be educated about the cancer threat posed by obesity and further encouraged to maintain a healthy weight.”
As BMI rises, cancer risk climbs
The higher a person’s body mass index (BMI), the greater the risk of certain cancers, the new report shows. Consider type 1 endometrial cancer. The risk for overweight women is 50 percent higher than for women at a healthy weight. The risk more than doubles for women who are obese—and it’s seven times greater for women who are very obese.
The obesity-cancer link is stronger in certain cancer types. People who are obese are nearly five times as likely to develop esophageal adenocarcinoma than people of normal weight. Excessive body weight almost doubles the risk of cancers of the colon, kidneys, pancreas, and liver. For other forms of cancer, such as breast, ovarian, and prostate, the increased risk is less dramatic but still significant.
Weight gain in childhood or young adulthood appears to be especially dangerous. The reason: The longer people are overweight or obese, the greater the risk of certain cancers. In a paper published in August 2016 in PLOS Medicine, researchers reviewed data from almost 74,000 postmenopausal women enrolled in the national Women’s Health Initiative study. For every 10 years that women reported being overweight or obese, the risk of obesity-related cancers climbed. Each 10-year period of being too heavy added an 8 percent increase to breast cancer risk when compared with that of normal weight women. Being overweight or obese for a stretch of 10 years increased endometrial cancer risk by 37 percent.
The link between fat and cancer
Why excess body weight contributes to cancer risk isn’t well understood. Still, research suggests several plausible explanations. Among them:
• Excess body fat has been shown to raise levels of hormones such as estrogen, which can promote the growth of breast and ovarian cancer cells. After menopause, when women’s ovaries stop producing hormones, fat tissue becomes the most significant source of estrogen.
• Other hormones may also play a role. Fat cells churn out leptin, a hormone associated with body fat and obesity, which may cause cancer cells to multiply. Also, people who are obese typically have low levels of a hormone called adiponectin, which normally puts the brakes on runaway cell growth.
• People who are obese often have high levels of insulin and insulin-like growth factor-1, which may spur the development of certain tumors.
• Being overweight or obese tends to cause low-level inflammation throughout the body, which in turn may increase the risk of certain kinds of malignancies.
More reasons to shed pounds
“The best way to lower your odds of cancer and other serious diseases, including heart disease and diabetes, is to avoid becoming overweight or obese in the first place,” Dale says. “Should one get cancer, having these other obesity-associated conditions can make treating it even more challenging. Better to do everything you can to prevent both. Unfortunately, a growing number of Americans are already dangerously heavy, and this number is growing.”
Seventy percent of adults in the United States are overweight (a BMI of 25 or greater). Some 36 percent are obese (a BMI of 30 or greater). Evidence suggests that obesity at the time of a cancer diagnosis is associated with a worse prognosis.
If you’re overweight or obese, will losing weight lower your risk of cancer? No definitive study to answer this has been done, and researchers say it would be almost impossible to address the question in a randomized trial. But there’s some evidence that losing weight—even a few pounds—can alter hormone levels, which may help reduce breast cancer risk. Small studies also suggest that people who have bariatric, or weight-loss, surgery and lose significant amounts of weight reduce their risk of obesity-related cancers.
According to the National Cancer Institute, if every American adult lowered their BMI by only 1 percent—just over two pounds on average—obesity-related cancer cases would drop dramatically.