Body fat is a risk factor for 13 different cancers, according to the International Agency for Research on Cancer (IARC), which coordinates and conducts research for the World Health Organization on the causes of human cancers.
Fat doesn’t necessarily cause these cancers. The relationship between fat and cancer is not that simple and straightforward. Rather, the IARC, which is based in Lyon, France, has determined that the ABSENCE of excess body fatness lowers the risk of these 13 cancers occurring.
In 2002, a scientific working group of the IARC concluded that sufficient evidence existed to support a “cancer-preventive effect of avoidance of weight gain,” for cancers of the:
2) Esophagus (adenocarcinoma)
3) Kidney (renal-cell)
4) Breast (only postmenopausal)
5) Corpus uteri (endometrial cancer, which develops in the inner lining of the uterus)
Translation: People of a healthy weight were sufficiently less likely than overweight and obese people to develop these cancers. Their absence of body fat served as a cancer preventive.
Last year, a new working group of the IARC added the following types of cancer to the list:
6) Gastric cardia (a stomach cancer)
7) Liver (see my Facebook post of 3/24/17 about fatty livers)
12) Multiple myeloma (a blood cancer)
13) Meningioma (a type of brain tumor)
For its 2016 report, the IARC purportedly reviewed more than 1,000 relevant epidemiological studies, most of which were observational studies on cancer risk and excess body fatness. The scientists in the working group presented their results in a special report for The New England Journal of Medicine (Aug. 25, 2016) 375: 794-98.
The study evaluations that the IARC considered were based primarily on increased risks associated with excess body fatness rather than on reduced risks associated with weight-loss or weight-control interventions. Current data on body-weight loss and cancer risk are too sparse to reach any conclusions, it says, although they do suggest that intentional weight loss may reduce cancer risk, most notably for breast and endometrial cancers.
In the NEJM article, the IARC defines “overweight” and “obesity” as an “abnormal or excessive accumulation of body fat that presents a risk to health.” Body-mass index, which is a person’s weight in kilograms divided by the square of his/her height in meters, is customarily used as a “good proxy,” it says, for assessing overall body fatness.
Among adults, the IARC explains, a BMI of 25.0 to 29.9 constitutes being “overweight,” and obesity is a BMI of 30 or more. The agency further divides obesity into three classes:
Class 1: BMI of 30.0 to 34.9
Class 2: BMI of 35.0 to 39.9
Class 3: BMI of 40.0 or more
According to the IARC, numerous observational studies show positive associations between adult BMI and postmenopausal breast cancer (particularly for estrogen-receptor-positive tumors), the relative risk of the cancer—which is a comparison of the risk a person with a normal BMI has vis-à-vis a person with an overweight or obese BMI—increasing with the BMI category.
The association between BMI and endometrial cancer was particularly pronounced for type 1 endometrial cancer, the agency notes. It documents the risk of type 1 endometrial cancer for overweight women at 50 percent higher than the risk for women of a healthy weight. This risk more than doubles for women who are obese and is seven times greater for women who have class 3 obesity.
Other examples from the IARC’s report: Obese people are nearly five times as likely to develop esophageal adenocarcinoma than people of normal weight, and excessive body weight almost doubles the risk of cancers of the colon, kidneys, liver, and pancreas.
For more about obesity-associated relative risk of the 13 cancers, see the IARC’s article, “Body Fatness and Cancer—Viewpoint of the IARC Working Group,” in the NEJM at http://www.nejm.org/doi/full/10.1056/NEJMsr1606602. (Be sure to check out the risk chart.)
Why excess body weight contributes to cancer risk is not well understood, the IARC says, but several plausible explanations have been suggested. According to an article about the IARC’s 2016 report in the March 2017 issue of “Health After 50,” they include:
–“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.”
–“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.”
“Health After 50” quotes the National Cancer Institute as asserting that “if every American adult lowered BMI by only 1 percent—just over two pounds on average—obesity-related cancer cases would drop dramatically.
That’s a stunning claim and hard for me to believe, but perhaps I expect more drama in my “dramatic” loss.
What’s irrefutable is that being obese is associated with a long list of health hazards, including diabetes, high blood pressure, heart disease, stroke, osteoarthritis, sleep apnea, liver malfunction, cancer, and more. Having heart disease and/or diabetes also makes treating cancer more challenging.