An extensive review by the World Health Organization (WHO) indicates that obesity may leave us vulnerable to many more types of cancer than previously thought.

A team organized by the International Agency for Research on Cancer (IARC) — the WHO’s cancer research branch — analyzed more than 1,000 epidemiological studies that had looked at the possible connection between excess body fat (as defined by body mass index) and different types of cancer. Their assessment, an update of a similar one they conducted in 2002, found evidence for a heightened risk of 8 more cancer types, including liver, ovarian, and thyroid cancer, from excess body fat. Generally, the increased risk was highest among people who were morbidly obese, adding more support for a direct link.

The researchers noted that obesity leads to a state of chronic inflammation throughout the body and disturbs the normal production of hormones like insulin, testosterone, and estrogen — all factors that may contribute to cancer.

"The burden of cancer due to being overweight or obese is more extensive than what has been assumed," said Dr. Graham Colditz, a chronic disease epidemiologist at the Washington University School of Medicine in St. Louis and chair of the IARC Working Group, in a statement. "Many of the newly identified cancers linked to excess weight haven't been on people's radar screens as having a weight component."

Coupled with the earlier analysis conducted by the IARC, excess body fat may be a contributing factor for at least 13 different types of cancer, the researchers concluded. The scope of this increased risk varied, depending on the type of cancer, and the highest added risks were seen with cancers of the esophagus and uterus (also called endometrial cancer). Morbidly obese people were five times more likely to develop esophagus cancer, and seven times more likely to develop uterus cancer compared to someone with a healthy weight.

For greater context, it’s estimated that 0.5 percent of all men and women will develop esophageal cancer in their lifetimes, and 2.8 percent of women will do the same for endometrial cancer. With most of the other types, such as breast and pancreatic cancer, the increased risk for morbidly obese people ranged from 10 to 80 percent higher than their normal BMI counterparts.

The IARC group also found an increased risk of stomach and gallbladder cancer as well as multiple myeloma (cancerous plasma cells) and meningioma (an often benign but still potentially harmful form of brain tumor) in their updated assessment. The group found limited evidence for obesity’s role in 11 other cancer types. Overall, the IARC’s findings align with earlier research published elsewhere, such as a 2014 study in The Lancet that connected obesity to 10 different types of cancer.

The researchers are confident that extensive lifestyle changes could reduce these added risks in overweight and obese individuals, despite a lack of research in this area.

"Lifestyle factors such as eating a healthy diet, maintaining a healthy weight and exercising, in addition to not smoking, can have a significant impact on reducing cancer risk," Colditz said. "Public health efforts to combat cancer should focus on these things that people have some control over."

Colditz did acknowledge the difficulty with long-lasting weight loss, and suggested patients embrace realistic expectations. "Rather than getting discouraged and giving up, those struggling to take off weight could instead focus on avoiding more weight gain," he said.

It’s currently estimated that 640 million adults worldwide are obese. In the U.S., 38 percent of women and 34 percent of men are considered obese, according to the latest report by the Centers for Disease Control and Prevention.

Source: Lauby‑Secretan B, Scoccianti C, Loomis D, et al. Body Fatness and Cancer — Viewpoint of the IARC Working Group. 2016.

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