OBESITY AND CANCER RISK

Being overweight/obese is already known to increase the risk for certain cancers, but this association has just become much wider. Another eight cancers have been added to the list, joining the five already there.

The new findings come from the International Agency for Research on Cancer (IARC), which is part of the World Health Organization (WHO). They are published in the IARC Handbooks of Cancer Prevention, Volume 16: Body Fatness, which provides an update of part of IARC Handbooks of Cancer Prevention Volume 6: Weight Control and Physical Activity, published in 2002.

A summary was published online August 25 in the New England Journal of Medicine.

Rather than saying that overweight/obesity increases the risk for cancer, the IARC has presented its findings another way: "the absence of excess body fatness reduces the risk of cancers."

The IARC confirmed its previous findings (published in 2002) for five cancers ― colorectal, esophageal (adenocarcinoma), renal cell carcinoma, breast cancer in postmenopausal women, and uterine endometrial cancer.

Now, from a new evaluation of published scientific literature, the agency has added eight more cancers to that list: stomach (gastric cardia), liver, gall bladder, pancreas, ovarian, thyroid, meningioma, and multiple myeloma.

There is also limited evidence suggesting a link for three other cancers ― fatal cancer of the prostate, breast cancer in men, and diffuse large B-cell lymphoma.

Several mechanisms linking excess body fat with carcinogenesis were identified, including chronic inflammation and dysregulation of the metabolism of sex hormones, the IARC notes.

"The identification of new obesity-related cancer sites will add to the number of deaths worldwide attributable to obesity," the IACR warns.

In 2013, there were an estimated 4.5 million deaths worldwide attributable to overweight and obesity, it adds.

Worldwide Obesity Epidemic

It is a worldwide problem ― globally, more people are overweight or obese than are underweight, the agency notes. Up to 40% of the population are overweight or obese in some countries or regions.

Worldwide, an estimated 640 million adults were obese in 2014, which is a sixfold increase since 1975. There were 110 million obese children and adolescents in 2013 (a twofold increase since 1975).

What to Do?

Obviously, the best way forward would be to prevent people from becoming overweight (defined as having a body mass index [BMI] ≥ 25 kg/m²) and obese (BMI ≥ 30 kg/m²) in the first place.

"The new evidence emphasizes how important it is to find effective ways, at both the individual and societal level, to implement WHO recommendations on improving diets and physical activity patterns throughout life if the burden of cancer and other noncommunicable disease is to be tackled," commented IARC director Christopher Wild, PhD.

There would need to be a concerted effort across many different groups to achieve this. "Changes in dietary and physical activity patterns are often the result of environmental and societal changes associated with development and lack of supportive policies in sectors such as health, agriculture, transportation, urban planning, environment, food processing, distribution, marketing, and education," the IARC comments in a question-and-answer document released with the latest findings.

But once people have excess body fat, does reducing it also reduce the increased risk for cancer? Here, there is evidence from animal studies, but not yet from studies in humans.

"Caloric or dietary restriction in overweight animals reduces the risk of cancers of the mammary gland, colon, liver, pancreas, skin and pituitary gland," the IACR notes.

Some data from mechanistic studies add to these data from experimental animals. Together, they "suggest a causal cancer preventive effect of intentional weight loss," but the authors add that "the evidence in humans remains to be established."

Coauthor Kurt Straif, MD, PhD, MPH, has received grant support from the Centers for Disease Control and Prevention and the American Cancer Society outside the submitted work. The other authors have disclosed no relevant financial relationships.