Being overweight or obese carries even more cancer risk than previously thought, according to a new review. The researchers—a working group convened by the International Agency for Research on Cancer—examined more than 1,000 studies and found that obesity increased risk for 13 types of cancer, 8 more than had been previously linked to weight. In fact, about 9% of all cancers may be directly due to obesity, according to the group’s findings, which were published in the New England Journal of Medicine in August.
The good news in this rather bleak picture, the researchers write, is that weight gain is largely preventable. “If we don’t gain weight, we won’t end up in the high-risk category,” says Graham Colditz, PHD, MD, MPH, chair of the IARC working group and the deputy director at the Institute for Public Health at Washington University in St. Louis.
Of course, sometimes it’s too late for that tactic, and in that case, will losing weight lower risk? Probably—and it certainly shouldn’t hurt to shed excess pounds for many reasons—but believe it or not, there’s not much long-term data available showing these benefits, Colditz says. “Successfully following people who lose weight to show a reduction in cancer diagnoses is largely not done because so many people who lose weight gain it back.”
Previously, the IARC had concluded weight played a role in colon, esophageal, kidney, breast, and uterine cancers. Here are the 8 latest additions and why body weight matters in each case.
Stomach – Excess body fat can lead to chronic inflammation, especially in the GI tract, Colditz says. The link between stomach cancer and obesity wasn’t all that surprising, he says, given the existing link with cancer of the esophagus, since they’re part of the same system. The chronic inflammation that can be sparked by obesity may lead to more irritation from stomach acids that could then lead to cancer, he adds.
Liver – Obesity can cause changes in this organ that look like the effects of alcoholism. “We end up with somewhat similar mechanisms from cirrhosis that lead to cancer,” Colditz says. It’s not drinking that’s doing the damage, though—it’s changes in the fattiness of the liver itself, called non-alcoholic fatty liver disease.
Gallbladder – Obesity ups a person’s risk of gallstones, the small deposits of cholesterol that can cause inflammation in the gallbladder, and having gallstones seems to increase a person’s risk of gallbladder cancer.
Ovary – Fat cells produce estrogen, and extra estrogen, at least among the postmenopausal set, seems to be the culprit behind greater risk of breast and uterine cancer in heavier women, Colditz says. “Ovarian cancer looks like it is also driven by these increased hormone levels as well as chronic inflammation from obesity,” he says.
Meningioma – Many of these tumours, which grow from the meninges, the thin tissue that covers the brain and spinal cord, are actually noncancerous, according to the American Brain Tumour Association. Little is known about what causes them and what makes some cancerous, Colditz says, but extra hormones produced by fat cells may play a role.
Thyroid – Once again, out-of-whack hormone production is likely involved here, according to a 2012 study in the Journal of International Medical Research. But with weight gain, the thyroid itself also increases in size. The bigger the thyroid, the more chances there are for cells to mutate and become cancerous.
Multiple myeloma – Research points to chronic inflammation here. “Obesity probably stimulates the inflammatory process, leading to higher turnover of bone marrow cells,” Colditz says. More cell turnover means more chances for mutations into this type of blood cancer.