Obesity is defined as the excessive accumulation of fat and/or adipose tissue within the body, and those with a BMI ≥30 kg/m(2) are considered to be in this group. Incidence of Obesity has been increasing globally and in the U.S. for the last 50 years and is now considered to be at epidemic levels. Within the U.S., approximately 93 million individuals are obese, which amounts to 40% of the population.¹ The presence of obesity is associated with an increased risk of all-cause mortality and a number of conditions including Type-2 diabetes and cardiovascular disease.¹ The medical costs related to treating obesity among adults are also increasing, and amount to approximately $342 billion per year.² Health care costs for those living with obesity are also higher when compared with individuals who maintain a normal weight. 

A recent public health campaign in the United Kingdom by Cancer Research aimed to highlight the associations between obesity and cancer and compared the impact to the results of smoking and subsequent diseases. At the same time, the American Cancer Society estimates that there will be over one million new cases of cancer, and more than 600,000 deaths from the disease in 2019.³ Findings ways of reducing the number of cancers, and understanding the underlying causes of the disease are important issues of public health concern. Here we discuss the current scientific evidence detailing the links between obesity and cancer.

Real-World Research Evidence

As early as 2002, The International Agency for Research on Cancer (IARC) reported that there was sufficient evidence pointing to a consistent association between obesity and cancer.⁴ Within more recent studies, it has been estimated that between 10-20% of all cancers are attributable to obesity.⁵ A 2013 study in the U.S. estimated that 85,000 cases of cancer annually are caused by obesity.⁶ Importantly, obesity appears to impact site-specific cancers differently and also impacts the risk of developing cancer directly related to various demographic factors. For example, amongst post-menopausal women, it is estimated that half of all cancers are caused by being obese.⁷ There is strong evidence that obesity contributes to the incidence of numerous site-specific cancers. The strongest evidence supports a link between obesity and the following cancers:

• Endometrial 
• Esophageal renal
• Pancreatic adenocarcinomas
• Liver 
• Gastric cardia 
• Meningioma (brain and spinal cord membrane)
• Multiple myeloma (plasma cells)
• Colorectal
• Leukemia
• Post-menopausal breast
• Ovarian
• Gallbladder
• Non-Hodgkins lymphoma
• Prostate
• Gall bladder
• Thyroid cancers.^5,7,8

Biology

There are several pathways that potentially explain the way that obesity impacts Cancer Risk, and these relate to changes to healthy insulin levels, alterations in hormones and systemic inflammation.⁸ The potential lack of physical activity, and corresponding sedentary behavior, among those who are obese may also explain some of the increase in cancer risk. A lack of physical activity has been found to be associated with approximately 10 site-specific cancers, including colorectal and breast cancer.⁹ Weight loss among those who are obese also appears to have a limited impact on increased cancer risk, perhaps because of the difficulty inherent in maintaining weight loss in the long term.⁸ At the same time, more research is needed to fully understand how reduced weight loss changes an individual’s cancer risk.

Conclusion

Obesity is associated with an increased risk of a diverse array of conditions and diseases. At the moment, it appears that efforts to slow rates of obesity are unsuccessful, as are population-level prevention efforts. From the current evidence, it appears that obesity does infer an increased risk of several site-specific cancers. Although there is still much that we do not know about the biological backdrop which creates the association between obesity and cancer, public health campaigns should aim to enable individuals to maintain a healthy weight. 

Author: Dr. Vanessa Gordon-Dseagu gained her Ph.D. from University College London in 2014 and subsequently moved from the UK to Washington, D.C. to take up a postdoc fellowship within the National Cancer Institute’s (NCI) Metabolic and Epidemiology Branch. Since leaving NCI she has been working as a freelance epidemiologist, research and policy consultant. She has written and published numerous peer-reviewed papers, as well as several white papers and blog posts, and presented her work at internationally-renowned conferences. Currently, her work focuses upon a diverse array of epidemiological, public health and policy issues – including pancreatic cancer survival rates, the provision of end of life care in different countries, hypertension and healthcare costs, and the use of mobile phone applications to support the work of health professionals.

References

1. CDC. Adult Obesity Facts. (2018).
2. Biener, A., Cawley, J. & Meyerhoefer, C. The High and Rising Costs of Obesity to the US Health Care System. J Gen Intern Med 32, 6–8 (2017).
3. Siegel, R. L., Miller, K. D. & Jemal, A. Cancer statistics, 2019. CA Cancer J Clin 69, 7–34 (2019).
4. Vainio, H., Kaaks, R. & Bianchini, F. Weight control and physical activity in cancer prevention: international evaluation of the evidence. Eur. J. Cancer Prev. 11 Suppl 2, S94-100 (2002).
5. Avgerinos, K. I., Spyrou, N., Mantzoros, C. S. & Dalamaga, M. Obesity and cancer risk: Emerging biological mechanisms and perspectives. Metab. Clin. Exp. 92, 121–135 (2019).
6. Basen-Engquist, K. & Chang, M. Obesity and Cancer Risk: Recent Review and Evidence. Curr Oncol Rep 13, 71–76 (2013).
7. Reeves, G. K. et al. Cancer incidence and mortality in relation to body mass index in the Million Women Study: cohort study. BMJ 335, 1134 (2007).
8. Pischon, T. & Nimptsch, K. Obesity and Risk of Cancer: An Introductory Overview. Recent Results Cancer Res. 208, 1–15 (2016).
9. Kerr, J., Anderson, C. & Lippman, S. M. Physical activity, sedentary behaviour, diet, and cancer: an update and emerging new evidence. Lancet Oncol. 18, e457–e471 (2017).