The Emerging Evidence about the Role of Obesity in Cancer
Dr. Rachel Ballard-Barbash
Concern about the public-health consequences of obesity has risen as its prevalence has increased worldwide. Obesity rates have more than doubled since 1980, according to the World Health Organization. In the United States alone, the 2007–2008 National Health and Nutrition Examination Survey results show that 34.2 percent of adults 20 years of age or older are overweight, 33.8 percent are obese, and 5.7 percent are extremely obese. In 1988–1994, in contrast, only 22.9 percent of adults were obese.
A recent NIH research initiative, based on simulation modeling, estimated the public health and economic consequences of the continued rise in obesity among the aging populations of the United States and the United Kingdom. The researchers found that, by 2030, 65 million more U.S. residents will be obese, and that this increase will carry associated costs of $48 to $66 billion per year for treating obesity-related diseases. Clearly, the costs of obesity are substantial and increasing rapidly.
Many people are familiar with the evidence that obesity increases the burden of common chronic diseases such as diabetes, cardiovascular disease, asthma, and arthritis. Surprisingly, despite decades of research indicating a strong association between obesity and cancer incidence and prognosis, obesity's contribution to cancer has been widely recognized only recently.
Before effective cancer screening and treatments were commonly available, many people were not diagnosed until their cancer was advanced, when they may have already experienced weight loss and cachexia. In addition, patients undergoing cancer treatment often experienced significant nausea and vomiting, which led to further weight loss. Cancer was thus considered to be associated with weight loss, rather than with obesity.
To measure obesity, researchers commonly use a scale known as the body mass index (BMI). BMI is calculated by dividing a person's weight in kilograms by his or her height in meters squared. BMI provides a more accurate measure of obesity than weight alone.
NIH guidelines place adults 20 years of age and older into the following categories based on their BMI:
|18.5 to 24.9||healthy|
|25.0 to 29.9||overweight|
|30.0 and above||obese|
For children and adolescents under 20 years of age, the definitions of overweight and obesity are based on the Centers for Disease Control and Prevention's BMI-for-age growth charts.
|BMI-for-age at or above sex-specific 85th percentile, but less than 95th percentile||Overweight|
|BMI-for-age at or above sex-specific 95th percentile||Obese|
Research during the 1970s in animal models and epidemiologic studies examining factors influencing breast cancer began to suggest, however, that higher body mass index (BMI) ratings increased the risk of breast cancer. Since then, extensive research at the basic, clinical, and population levels by investigators around the world has shown that obesity is associated with an increased risk of cancers of the endometrium, postmenopausal breast, gastrointestinal tract (colon, pancreas, adenocarcinoma of the esophagus, and gallbladder), kidney, and thyroid, as well as aggressive forms of prostate cancer. Adult weight gain and increased amounts of abdominal body fat have also been associated with increased risk for several cancers.
During the last two decades, an extensive body of research has begun to identify an association between obesity and worse prognosis and outcomes among some cancer patients, particularly those with breast, prostate, and colon cancer. In interpreting the research on cancer risk and prognosis, it is important to understand that obesity is associated with physical inactivity and poor dietary practices that may also increase the risk for cancer.
Researchers are exploring the many potential mechanisms by which obesity may influence cancer risk and prognosis. Early research focused on the effect of obesity on adverse changes in sex hormones such as estrogens and androgens, particularly during puberty, pregnancy, and menopause.
More recent research has examined mechanisms related to insulin and related growth factors, adipokines (cytokines secreted by fat tissue), other metabolic and growth factors, inflammatory factors, altered immune response, and oxidative stress, relative to all phases of cellular growth and cell death. Researchers are also looking at the effects of obesity and of energy expenditure and intake—at the cellular and whole-body level—on many other mechanisms that may influence cancer. Other research indicates that sleep, alterations in circadian rhythms, and changes in the microbiome may also influence obesity and cancer.
Although important findings have already been made about the links between obesity and cancer, much research remains to be done in a number of areas. For example, relatively few studies of obesity and cancer risk have adjusted for the potential effects of physical activity; more have adjusted for dietary factors that may influence cancer risk, such as total calories or amount and types of dietary fat consumed. In addition, no clinical research to date has examined the effect of weight loss on the initial development of cancer; nor have clinical trials been funded to test the effect of weight loss on the likelihood of dying from cancer once diagnosed.
This special issue of the NCI Cancer Bulletin explores how NCI is supporting extensive research at the cellular, animal, and clinical levels to address these gaps in our knowledge about the role of obesity in cancer. NCI's initiatives include partnerships with institutes across NIH, that seek to advance research to understand the environmental, policy, and social forces that may be contributing to the worldwide obesity epidemic.
For example, given the substantial evidence showing how difficult it is to reverse obesity once it occurs, much research is focused on obesity prevention in children, families, and the communities in which they live, play, and work. NCI is working with its partners at NIH, the U.S. Department of Agriculture, the Centers for Disease Control and Prevention, and the Robert Wood Johnson Foundation on the National Collaborative on Childhood Obesity Research. This initiative seeks to enhance the implementation and effectiveness of research to identify multilevel individual, social, environmental, and policy factors that may help to reverse the rising trends in childhood obesity, particularly among populations that are at the greatest risk of obesity and its adverse consequences.
Obesity prevention efforts, such as the Let's Move campaign, are also important, not just because they seek to help control childhood obesity, but because they may also reduce cancer-related morbidity and mortality in the United States.
Dr. Rachel Ballard-Barbash
Associate Director, Applied Research Program
NCI Division of Cancer Control and Population Science