Obesity is at an epidemic level in the United States, as well as globally. More than two-thirds of American adults are overweight or obese. It contributes to avoidable and premature deaths from the top two killers: cardiovascular disease and cancer.1 People who are overweight or obese have a higher risk of other serious health conditions, including type 2 diabetes and high blood pressure.

The most recent statistics reported by the US National Institutes of Health (NIH) is that approximately two-thirds of adults age 20 years or older are overweight or obese, with a body mass index (BMI) greater than 25, and nearly one-third have a BMI greater than 30.1,2 Less than one-third of Americans are at a healthy weight with a BMI of 18.5 to 24.9. In 1995, the economic cost of overweight and obesity in the United States alone was estimated to be $117 billion per year.

Obesity is a risk factor for several types of cancer. Although a higher weight may not necessarily directly cause cancer, maintaining a healthy weight is associated with a lower risk of many chronic diseases, including cancer. Factors that cause people to become overweight or obese include genetic, hormonal, environmental, emotional, and cultural factors; however, the main causes are controllable.

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Scientists and physicians have been trying to understand weight gain and cancer risk. Several studies have explored why being overweight or obese may increase cancer risk and growth.3 People who are obese have more fat tissue that can produce hormones such as insulin, testosterone, and estrogen, which may cause cancer cells to grow.3 Fat cells also produce many other chemical messengers that affect how the body works. The fat cells are active, producing hormones and proteins that are released into the bloodstream and carried throughout the body. These chemical messengers can increase the risk of several different cancers.

Most recent investigations have focused on excess insulin. The hormone insulin is a very important part of how the body uses energy from food. Overweight or obese people have much more insulin in their bodies. How this leads to cancer is not clear, but high insulin levels are a common feature of many cancers.

Even how the fat is distributed has been shown to correlate with the risk of certain cancers. Too much fat carried around the belly can do more damage. This so-called apple shape is linked to colorectal, kidney, esophageal, pancreatic, breast, and uterine cancers.1,3

Oncologists have not typically been involved in weight reduction management, which is not by any fault of their own as weight loss instruction has not been integrated into medical school, residency, and fellowship programs. For now, oncologists should start by talking to cancer patients and survivors about calculating body mass index and assessing weight status, and how the role of healthy eating and exercise can reduce fatigue, improve body image, and lower incidence of recurrent cancer.3,4

Most studies suggest that obesity and a cancer diagnosis may be linked to increased recurrence and mortality. Obesity can also affect the appropriate dosing of cancer therapy and increase the risks of treatment-related side effects, as much as it may reduce the efficacy. Calculating optimal chemo-biotherapy doses is very difficult when the patient has an excessive amount of body fat per body mass. In addition past studies have demonstrated that activity included in a healthier lifestyle post cancer diagnosis helps reduce the chances of both breast and colorectal cancer spread and recurrence, to the same degree as adjuvant chemotherapy.4,5 Also, it’s free!

In order to promote long-term behavior change, it’s important for oncologists to partner with other members of the cancer care team, including primary care physicians and other professionals such as dietitians and physical therapists. Oncologists should also attempt to model and promote healthy lifestyles in the community.

As with smoking, obesity is a controllable risk for cancer. Many resources are available to help people maintain a healthy weight, including doctors and dietitians. Talk with your patients about developing an appropriate weight control plan for them.

Donald Fleming is an oncologist/hematologist at the Cancer Care Center, Davis Medical Center, Elkins, West Virginia. 


1. Polednak AP. Estimating the number of U.S. incident cancers attributable to obesity and the impact on temporal trends in incidence rates for obesity-related cancers. Cancer Detect Prev. 2008;32(3):190-199.

2. Ballard-Barbash R, Berrigan D, Potischman N, Dowling E. Obesity and cancer epidemiology. In: Berger NA, ed. Cancer and Energy Balance, Epidemiology and Overview. New York, NY: Springer; 2010.

3. Roberts DL, Dive C, Renehan AG. Biological mechanisms linking obesity and cancer risk: new perspectives. Annu Rev Med. 2010;61:301-316.

4. Demark-Wahnefried W, Platz EA, Ligibel JA, et al. The role of obesity in cancer survival and recurrence. Cancer Epidemiol Biomarkers Prev. 2012;21(8):1244-1259.

5. Schmid D, Leitzmann MF. Association between physical activity and mortality among breast cancer and colorectal cancer survivors: a systematic review and meta-analysis. Ann Oncol. 2014;25(7):1293-1311.