Approximately 100,500 cancers occurring in the US every year can be attributed to excess body fat, according to estimates released today by the American Institute for Cancer Research (AICR). The figure underscores the central role that overweight and obesity are now understood to play both in the development of cancer and in cancer survivorship, said researchers.
The new estimate was calculated by combining the latest US cancer incidence data with the conclusions of the AICR/WCRF report, Policy and Action for Cancer Prevention, released earlier in 2009. That report estimated the percentage of various kinds of cancer that are attributable to such risk factors as poor diet, lack of physical activity and excess body fat.
According to AICR, the estimated number of cancers that are linked to excess body fat include:
49% of endometrial cancers1 = 20,700 cases/year2
35% of esophageal cancers = 5,800 cases/year
28% of pancreatic cancers = 11,900 cases/year
24% of kidney cancers = 13, 900 cases/year
21% of gallbladder cancers = 2,000 cases/year
17 % of breast cancers = 33,000 cases/year
9% of colorectal cancers = 13,200 cases/year
TOTAL: 100,500 cases/year
1 Percentages from AICR/WCRF's Policy and Action for Cancer Prevention, 2009
2 Calculated from total estimated incidence, Cancer Facts and Figures 2009, ACS
Excess Body Fat Now Seen as Major Cause of Cancer
Laurence Kolonel, MD, PhD, Deputy Director of the Cancer Research Center of Hawaii and AICR/WCRF expert panel member, presented the new preventability estimates and reviewed the evidence linking obesity to cancer risk.
"We now know that carrying excess body fat plays a central role in many of the most common cancers," he said. "And it's clearer than ever that obesity's impact is felt before, during and after cancer - it increases risk, makes treatment more difficult and shortens survival."
Dr. Kolonel also highlighted the conclusions of a recent AICR/WCRF expert report which found cancers of the endometrium, esophagus, pancreas, kidney, breast (post-menopausal) and colorectum to be convincingly linked to excess body fat, while the link between body fatness and gallbladder cancer was judged probable. This report is continuously updated; the new evidence on breast cancer was released in September, with updates on colorectal cancer and prostate cancer due in the coming months.
Dr. Kolonel also highlighted the emerging evidence that is revealing the nature of the link between excess body fat and cancer. Some of the strongest evidence, he noted, suggests that excess body fat increases the body's level of sex steroids and other hormones that are linked to cancer growth. For example, fat tissue produces estrogen; studies have shown that estrogen promotes cell proliferation in breast tumors that contain receptors for the hormone, the so-called ER positive tumors.
Recent findings also suggest that excess body fat lowers immune function and increases oxidative stress, which can lead to DNA damage.
Emerging Evidence Implicates Obesity and Overweight in Poor Cancer Survivorship
Melinda Irwin, PhD, MPH, Associate Professor of Epidemiology at Yale University, presented evidence that overweight and obesity continue to play a negative role after cancer has been successfully treated. Dr. Irwin stressed, however, that there are two ways of looking at data linking higher Body Mass Index (BMI) to poorer survival, and she prefers the one that encourages survivors to get active.
"It's true that higher BMI is associated with poorer outcomes. Now we need to ask why this is the case," she said. "An increasing number of studies suggest that regular physical activity improves cancer survival, even among survivors who are overweight or obese. That's really the take-home message here."
Dr. Irwin reviewed data showing that although obesity increases risk of death among cancer survivors, some factors that influence obesity, such as physical activity, have also been shown to have an impact on survival that is independent of BMI. A growing number of publications, for example, have reported that increasing physical activity after diagnosis, especially among those who were inactive before they were diagnosed, is associated with an improvement in survival.
One reason for the association between physical activity and better outcome, said Irwin, may have to do the fact that regular physical activity helps regulate hormone levels. Studies have shown that women with high insulin levels at diagnosis have a 2 to 3-fold increased risk of death than is seen among women with low insulin levels. (High insulin levels are associated with inactivity and overweight).
Dr. Irwin presented recent findings from her own Yale Exercise and Survivorship (YES) Study intervention trial, which showed that within 6 months, regular exercise decreased insulin levels by about 8% among 933 breast cancer survivors.
She concluded her remarks by encouraging clinicians to recommend exercise to their survivor patients, and praising the growing number of exercise programs available to cancer survivors.
The researchers spoke at a press conference coinciding with AICR's Annual Research Conference on Food, Nutrition, Physical Activity and Cancer in Washington, DC.
Released November 5, 2009