More than 100,000 cancers in the US each year are linked to excess body fat, according to data released Thursday by the American Institute for Cancer Research (AICR). Researchers have known for some time that being overweight puts people at an increased risk of some types of cancer – AICR's data highlights the extent of the problem.

The estimates show that excess body fat is linked 49% of endometrial cancers (20,700 cases per year), 35% of esophageal cancers (5,800 cases per year), 28% of pancreatic cancers (11,900 cases per year), 24% of kidney cancers (13,900 cases per year), 21% of gallbladder cancers (2,000 cases per year), 17% of breast cancers (33,000 cases per year), and 9% of colorectal cancers (13,200 cases per year).

"The evidence is clear: If people sustain a normal body weight and remain physically active throughout life, it will have a major impact on cancer incidence," said Laurence N. Kolonel, MD, PhD, Deputy Director of the Cancer Research Center of Hawaii and a member of the expert panel who crunched the numbers.

AICR's figures are based on combining US cancer incidence data with conclusions from a February 2009 AICR and World Cancer Research Fund (WCRF) report that looked at the relationship between cancer and risk factors such as poor diet, lack of physical activity, and excess weight.

AICR researchers will continue to update these numbers as more data becomes available.

Nature of the link

American Cancer Society (ACS) researchers were among the first to hone in on the issue of excess weight and cancer risk. A 2003 ACS study of more than 900,000 men and women found the heaviest men had death rates from all cancers combined that were 52% higher than the rates among normal-weight men. The heaviest women had cancer death rates 62% higher than normal-weight women, the study found.

Why does being overweight make a person more susceptible to cancer? Several mechanisms are thought to be at work, says Kolonel. Excess body fat appears to increase the amount of hormones like estrogen circulating in the body, and it can also disrupt how the body processes insulin. Both of those factors have been linked to an increased cancer risk.

In addition, Kolonel says, "being overweight creates low-grade inflammation in the body, and there's a lot of research going on right now that links chronic inflammation to cancer."

Weight gain a problem after diagnosis, as well

At a press conference Thursday to discuss the new results, Melinda L. Irwin, PhD, MPH, Associate Professor of Epidemiology at Yale School of Medicine, presented data showing that weight gain is associated with poorer outcomes both during and after cancer treatment. Irwin has studied the effects of exercise on breast and ovarian cancer survival.

“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.”

Reducing your risk

Half of all Americans aren't aware of the role obesity plays in cancer risk, according to an August 2009 risk assessment study by AICR also discussed at the press event.

"Public awareness of the link between obesity and cancer risk is alarmingly low," said Alice Bender, MS, RD, Nutrition Communications Manager at AICR. "We are working towards a day when obesity is right up there with tobacco in the public eye."

To reduce your risk, the American Cancer Society recommends balancing calorie intake with physical activity. Eat at least 5 servings of fruit and vegetables every day, choose whole grains over processed grains, and limit red meat. Adults should engage in at least 30 minutes of moderate or vigorous physical activity 5 days a week or more.

"The bottom line for people concerned about this issue is to try to balance the calories you take in with those your body expends every day," says Michael J. Thun, MD, MS, American Cancer Society Vice President Emeritus, Epidemiology and Surveillance Research.

Bender also discussed the importance of schools, the workplace, and other environments in promoting a healthy lifestyle.

"The American Cancer Society stresses the importance of a maintaining a healthy body weight and avoiding weight gain during adulthood," says Marji McCullough, ScD, RD, American Cancer Society, Strategic Director, Nutritional Epidemiology. "Our recommendations also underscore the important role of the community in providing easy access to safe opportunities for exercise and ready access to healthy foods."

For more information, see The American Cancer Society Guide to Nutrition and Physical Activity.

Reviewed by: Members of the ACS Medical Content Staff