Obesity and Cancer, Part 4

by National Cancer Institute

(Page 4 of 4)

16. What biological mechanisms are thought to be involved in explaining the link between obesity and cancer?

The biological mechanism that explains how obesity increases cancer risk may be different for different cancers. (See Questions 6-11.) The exact mechanisms are not known for any of the cancers. However, possible mechanisms include alterations in sex hormones (e.g., estrogen, progesterone, and androgens), and insulin and IGF-1 in obese people that may account for their increased risk for cancers of the breast, endometrium, and colon. Sex-hormone binding globulin, the major carrier protein for certain sex hormones in the plasma, may also be involved in the altered risk for these cancers in obese people.

17. What are current research needs?

Although there has been extensive research with large populations looking at the possible link between obesity and cancer, few clinical trials have studied the effect of weight control, physical activity, and energy balance (the calories consumed compared with those burned) on cancer. For some cancers, such as colon and breast, it is not clear whether the increased cancer risk in obese people is due to the extra weight; a high-fat, high-calorie diet; a lack of physical activity; or a combination of these factors.

The 2002 International Agency for Research on Cancer (IARC) report on weight control, physical activity, and cancer made several recommendations for future trials:

• Conduct long-term intervention studies on the effect of dietary changes on weight gain and cancer risk;

• Conduct long-term intervention studies on the effect of patterns of physical activity (the intensity, frequency, and duration of various sorts of physical activity) in relation to weight gain and cancer risk;

• Conduct long-term intervention studies on the combined effects of changes in diet and physical activity on obesity and cancer risk; and

• Conduct community intervention studies to prevent weight gain and promote physical activity.

Several international reports have concluded that controlling the obesity epidemic will require substantial investments by many segments of society. Efforts to increase physical activity and promote healthy eating are needed in families, day care centers, schools, and work sites. The efforts of community services such as health care and public education are needed, as well as transportation systems that encourage walking and the use of bicycles.

18. Is the National Cancer Institute (NCI) currently studying the possible link between obesity and cancer?

Several NCI-funded studies are investigating the relationship between obesity and cancer, including breast, ovarian, endometrial, prostate, colorectal, and esophageal cancers. Some of the studies with women include the following:

• The Four Corners Breast and Endometrial Cancer Study is focusing on the effects of obesity and weight changes on breast and endometrial cancer risk among Hispanic, Native American, and non-Hispanic white women.

• A study of white, African American, and Latina women is investigating whether phytoestrogen consumption can modify the risk of endometrial cancer associated with obesity.

• The Black Women's Health Study is considering the effect of risk factors, including obesity, on breast cancer risk.

• The Health, Eating, Activity and Lifestyle (HEAL) Breast Cancer Prognosis Study is examining the interrelationships between diet, weight, physical activity, hormones, breast cancer prognosis, and quality of life in a multi-ethnic cohort of 1,200 breast cancer survivors.

The Division of Cancer Epidemiology and Genetics (DCEG), one of NCI's intramural research divisions, is conducting a series of large-scale epidemiologic studies on the influence of obesity and physical inactivity on several major cancers. These include cohort studies within clinical trials, such as the Alpha-Tocopherol Beta-Carotene Study; the Prostate, Lung, Colorectal, and Ovarian Cancer study; and the Polyp Prevention Trial. In addition, DCEG is studying energy balance in cohort and case-control studies in Sweden, China, and the United States; these include the NIH-AARP (National Institutes of Health-American Association of Retired Persons) Diet and Health Study, a prospective cohort study of nutrition in relation to major cancers among over half a million American men and women, and the Cohort Consortium, a new effort that combines several prospective cohort studies from around the world, gathering information on energy balance-related factors from each cohort.

NCI is also developing and supporting research initiatives to improve the measurement of diet and physical activity in the population, understand health professionals' knowledge and practices about obesity treatment, and support the establishment of centers in nutrition, energetics, and physical activity and cancer outcomes. Some of these initiatives are cosponsored with other parts of the NIH.

Other studies being conducted or funded by NCI use animal models to study obesity and cancer. One such study is examining the role of body fat mass on cancer progression in a mouse model of prostate cancer. The Center for Cancer Research, one of NCI's intramural research divisions, is conducting studies examining mechanisms of energy modulation, specifically caloric restriction, fasting, physical activity, diet-induced obesity, and genetically induced obesity, frequently using animal models of cancer.

NCI is supporting the training of new scientists through programs such as the Nutrition and Obesity Training Program at the University of California, Los Angeles and grants to individual new scientists, who are studying topics such as Culturally Proficient Smoking and Weight Control Treatment, Exploration and Intervention in Weight Gain Associated With Adjuvant Chemotherapy for Breast Cancer, and Visceral Adipose Tissue and Colorectal Neoplasia.

Because the delivery of research results is important, NCI has ongoing programs to move obesity-related research into practice. For example, NCI's Division of Cancer Control and Population Sciences is supporting the integration of diet and physical activity science into comprehensive cancer control planning. Cancer Control PLANET (Plan, Link, Act, Network With Evidence-Based Tools) provides access to data and resources that can help planners, program staff, and researchers design, implement, and evaluate science-based cancer control programs. The modules on Cancer Control PLANET include science-based information on interventions related to diet, physical activity, and NCI's 5 A Day For Better Health Program (http://www.5aday.gov/), which has incorporated messages on the importance of maintaining a healthy weight and a physically active lifestyle in its public awareness efforts. These messages are a key component of a recent NCI campaign, "9 A Day Campaign for African American Men," to increase African American men's consumption of fruits and vegetables. As the national health authority for 5 A Day, the NCI provides leadership through the implementation of a national media campaign, support of state 5 A Day programs, coordination of national partnership efforts and activities, and funding of nutrition behavior change research.

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