Obesity increases the likelihood of developing many forms of cancer
The link between obesity and risk for several types of cancer is now well established by more than a decade of scientific research. Epidemiologists from the American Cancer Society were the first to raise the alarm about the risks of obesity-associated cancer in a series of papers that began in 2003. Examples of those papers can be found here and here.
According to the most recent 2014 Cancer Progress Report from the American Association for Cancer Research (AACR), overweight/obesity is responsible for nearly 25% of the relative contribution to cancer incidence, which ranks second only to tobacco use. Quite alarmingly, when obesity is combined with other related behaviors, including a lack of physical activity and poor diet, the relative contribution rises to 33% of newly diagnosed cancer cases in the U.S. Obesity is strongly tied to many of the most common types of cancer, including post-menopausal breast cancer, colorectal cancer and cancers of the endometrium, kidney, thyroid and gallbladder. There is also evidence to suggest a link to cancers of the liver, cervix, multiple myeloma and Non-Hodgkin's lymphoma. However, evidence is less convincing and somewhat controversial for prostate cancer, lung cancer and pre-menopausal breast cancer; several studies report an inverse relationship between obesity and these particular cancers.
We call attention to a November 2014 position statement by the American Society of Clinical Oncology (ASCO), which identified obesity as a health risk that is "quickly overtaking tobacco as the leading preventable cause of cancer." (Ligibel et al. J. Clin. Oncol. 2014;32:3568-3574)
The ASCO position statement brings that important organization into alignment with and reinforces the well known American Institute of Cancer Research and World Cancer Research Foundation statement of concern of 2007. (World Cancer Research Fund/American Institute for Cancer Research (2007) Food, Nutrition, Physical Activity and the Prevention of Cancer: a Global Perspective. AICR: Washington, DC). AICR/WCRF was the first organization to note that, after smoking, obesity is the most important preventable cause of cancer.
Obesity worsens survival after a cancer diagnosis
While obesity is clearly linked to risk for developing many forms of cancer, the deleterious impact of obesity on prognosis has proven even more compelling in several studies. Indeed, in a high-profile study published more than a decade ago by a team at the American Cancer Society, obesity was linked to increased mortality risk from the most common forms of cancer in men (prostate 34%, kidney 70%, colorectal 84%, esophagus 91%, stomach 94%, pancreas 2.5-fold, liver 4.5-fold) and women (colorectal 46%, ovary 51%, breast 2-fold, cervical 3-fold, kidney 5-fold, uterine 6-fold). Interestingly, this relationship holds true even for cancer sites where the link between obesity and cancer risk is less clear, including a 75% increased in mortality in premenopausal women diagnosed with breast cancer, and evidence that obese men are more likely to developed more aggressive prostate cancer and to have more advanced disease at the time of diagnosis.
Mechanisms linking Obesity to Cancer
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.
- People with obesity 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, which 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. Thus, the leptin-adiponectin ratio in the microenvironment may be important for tumor cell aggressiveness.
- 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.
- People with obesity often have chronic low-level, or "subacute" unresolved inflammation, which has been associated with increased cancer risk.
- Other possible mechanisms include altered immune responses, effects on the nuclear factor kappa B (NF-κB) system and oxidative stress.
Overall, whereas the mechanisms underlying the obesity-carcinogenesis relationship are not fully understood, sufficient evidence exists to support recommendations that adults and children maintain reasonable weight for their height and ages for multiple health benefits, including decreasing their risk of cancer.
American Association for Cancer Research (AACR) 2014 Progress Report
American Cancer Society (ACS)
American Institute for Cancer Research (AICR)
Centers for Disease Control and Prevention (CDC)
National Cancer Institute (NCI)
Prevent Cancer Foundation
Last updated January 2015.