Nearly half a million new cancer cases per year can be attributed to high body mass index, according to a new analysis conducted by researchers from the International Agency for Research on Cancer and published in The Lancet Oncology.
High body mass index (BMI) is a known risk factor for cancer; earlier this year, another study published in The Lancet revealed that high BMI is associated with the incidence of at least 22 cancers, including those affecting the esophagus, colon, rectum, kidneys, pancreas, gallbladder, breasts, ovaries and endometrium.
The new analysis, which looked at data from 2012, found that 3.6 percent of the total global cancer burden is linked with high BMI, and that cancer due to overweight and obesity is far more common in developed countries than in less developed countries.
“Overall, we see that while the number of cancer cases associated with overweight and obesity remains highest in richer countries, similar effects are already visible in parts of the developing world,” explains co-lead author Dr. Isabelle Soerjomataram.
The most affected area remains North America, where an estimated 111,000 new obesity-related cancer cases emerged in 2012 – around 23 percent of the total global BMI-related cancer burden.
Europe also has a large cancer burden linked with overweight and obesity, with Eastern Europe accounting for 6.5 percent (65,000 cases) of all new European cancer cases.
The proportion of cancers associated with overweight and obesity in Asian countries is not as large, but due to population size, the authors note that “it still translates into a considerable absolute number of cases.”
For instance, although only 1.6 percent of China’s new cancer cases were found to be attributable to overweight and obesity, this still comprised 50,000 cancer cases in total.
The researchers compare this prevalence to rates in Africa, where 1.5 percent of all new cancer cases were related to overweight and obesity, but the total number of cases attributable to obesity in the entire continent was just 7,300 during 2012.
Significant gender differences of BMI-related cancer cases
In the analysis, the incidence of BMI-related cancer varied across countries according to gender.
The countries with highest cancer burden attributable to overweight and obesity in men were:
- Czech Republic (5.5 percent of the country’s new cancer cases)
- Jordan (4.5 percent )
- UK (4.4 percent )
- Malta (4.4 percent ).
For women, the countries with the highest cancer burden attributable to overweight and obesity were:
- Barbados (12.7 percent of the country’s new cancer cases)
- Czech Republic (12.0 percent )
- Puerto Rico (11.6 percent ).
The proportion of cancers related to obesity was found to be significantly higher in women than in men: the population-attributable proportion of BMI-linked cancers in new cancer cases among women was 5.3 percent , compared with just 1.9 percent among men.
“Women are disproportionately affected by obesity-related cancers,” says Dr. Melina Arnold, one of the study’s lead authors. “For example, for postmenopausal breast cancer, the most common cancer in women worldwide, the study suggests that 10 percent of these cancers could have been prevented by having a healthy body weight.”
Indeed, in a recent study of over 92,000 postmenopausal women over age 50, researchers found that going up just one skirt size over a period of 10 years after age 25 was linked with a 33 percent increase in the risk of breast cancer after menopause. Going up two skirt sizes was linked with a 77 percent increase in risk.
In this latest study, cancers of the endometrium, colon and breast accounted for 73 percent of the cancers linked to high BMI in women. Kidney and colon cancers accounted for 66 percent of all cancers associated with high BMI in men.
Unraveling the BMI-cancer link
Several possible mechanisms have been suggested to explain the association of obesity with increased risk of certain cancers:
- Fat tissue produces excess amounts of estrogen, high levels of which have been associated with the risk of breast, endometrial, and some other cancers.
- Obese people often have increased levels of insulin and insulin-like growth factor-1 (IGF-1) in their blood (a condition known as hyperinsulinemia or insulin resistance), which may promote the development of certain tumors.
- Fat cells produce hormones, called adipokines, that may stimulate or inhibit cell growth. For example, leptin, which is more abundant in obese people, seems to promote cell proliferation, whereas adiponectin, which is less abundant in obese people, may have antiproliferative effects.
- Fat cells may also have direct and indirect effects on other tumor growth regulators, including mammalian target of rapamycin (mTOR) and AMP-activated protein kinase.
- Obese people often have chronic low-level, or “subacute,” inflammation, which has been associated with increased cancer risk.
Other possible mechanisms include altered immune responses, effects on the nuclear factor kappa beta system, and oxidative stress.
“The number of cancers linked to obesity and overweight is expected to rise globally along with economic development,” Dr. Christopher Wild, director of the International Agency for Research on Cancer.
“This study stresses the importance of putting in place efficient weight control measures, to curb the high number of cancers associated with excess body weight and to avoid the problems faced by rich countries being repeated in those now undergoing rapid development,” adds Dr. Wild.
The good news? Past studies have shown that even a small weight loss (10 percent of one’s current body weight) lowers the risk of several diseases. Besides cancer, people who are overweight or obese also have a greater chance of developing conditions such as high blood pressure, high blood cholesterol and other lipid disorders, diabetes, heart disease, and stroke — all of which can be prevented with lifestyle changes.