Breast Cancer Implications of Insulin, Activity, and Diet

What is the problem and what is known about it so far?
Past research has suggested that obesity, and low levels of physical activity are associated with increased risk of breast cancer in women after menopause, and that caloric restriction is linked to lower rates of breast cancer. One suggested cause for the increased risk of certain cancers in obese and inactive women is that they have higher fasting insulin levels. Previous studies have shown an association between high insulin levels and higher rates of chronic diseases and cancers, including breast cancer.

Why did the authors do this study?
The authors attempted to determine if there was an association between fasting insulin levels and caloric intake, physical activity, and body mass index (BMI; Body mass index, a measurement calculated from height and weight). The hope is that appropriate changes in BMI, caloric intake, and physical activity could reduce an individual’s risk of cancer.

Who was studied?
The study population was an ethnically diverse group of approximately 3000 women selected from 160,000 participants in the Women’s Health Initiative (WHI). WHI is an organization in which women participate in large, longitudinal, multi-center studies investigating women’s health through clinical trials and observational studies. Women in the WHI are postmenopausal age 50-79 and unlikely to move or die in the next three years, and for this study women had to have no personal history of cancer, or currently have diabetes.

How was the study done?
The majority of the information for this study came from questionnaires, which were sent to each of the study participants. Information was gathered on diet, physical activity, and BMI. Dietary intake was evaluated using a food frequency questionnaire specifically asking about dietary intake over a three-month period. Physical activity was determined by a questionnaire, which asked about the frequency, duration, and intensity of physical activity each week. From these findings total energy expenditure per week per kilogram was calculated. Medication information was collected by an interviewer-administered questionnaire, which included information about current or previous hormone therapy. Women’s fasting insulin levels were measured in blood samples taken from each participant after at least 8 hours without eating or drinking. The information was analyzed to look for associations between physical activity, BMI, caloric intake and insulin levels,

What did the researchers find?
Insulin levels were lower in women with a lower BMI, high physical activity, and in women with lower caloric intake. High physical activity was associated with lower insulin level regardless of whether women were at the high or the low end for total caloric intake. In addition, regardless of BMI lower insulin levels were seen when caloric intake was low and physical activity high.

What were the limitations of the study?
One of the limitations of the study is that people were asked to fill out questionnaires in order to obtain most of the data. In many cases it is difficult to meaure things such as physical activity and intensity of exercise, even when presented with specific questions. Additionally, it is difficult to remember food intake over a long period. Women who were selected for this study are participants in the WHI, and are interested in participating in research about women’s health, and therefore may not represent "average" women. Another limitation is that the association between high insulin levels and cancer may not represent cause and effect.

What are the implications of the study?
Lower insulin levels were seen in women with high physical activity regardless of BMI, and regardless of caloric intake. This suggests that changes in physical activity may help to decrease insulin levels without the need to make changes to other aspects of women’s lives. Increased physical activity, rather than extensive changes to diet and weight, may be the simplest way to prevent increased insulin levels, and perhaps reduce women’s risk for breast cancer.

Summarized by Beth Gibbons, MD, General Surgery Residency, Fletcher Allen Health Care.
Summarized from "Insulin, Physical Activity, and Caloric Intake in Postmenopausal Women: Breast Cancer Implications". Chlebowski R. et al., Journal of Clinical Oncology, November 2004, Volume 22, Number 22, pages 4507-4513.