March 28, 2012 -- Obesity and inactivity are emerging as new public health threats in the fight against cancer.

Although a new CDC report shows that the overall cancer death rate dropped among adults and children in the United States, some of those cancers associated with obesity are on the rise.

The joint report from the CDC and some of the nation's leading cancer groups marks the first time they have highlighted the relationship between obesity and cancer. A review of more than 7,000 studies supports a link between obesity and increased risk for colorectal and postmenopausal breast cancers, as well as cancers of the esophagus, kidney, pancreas, and uterus. Lack of physical activity was linked to increased risk for colon cancer and "probable" increased risk for postmenopausal breast and uterine cancers.

"Everybody knows obesity and inactivity can put people at risk for diabetes, heart disease, and many other chronic diseases, but for the most part people don't know that they can cause cancer," says Marcus Plescia, MD, MPH, who directs the Division of Cancer Prevention and Control at the CDC.

Lung, Colon Cancer Rates Decline

The newly published report was a collaborative effort between the CDC, the National Cancer Institute, the American Cancer Society, and the North American Association of Central Cancer Registries, based on comprehensive, nationwide registry data on newly diagnosed cancers and cancer deaths.

It found that cancer death rates in the U.S. decreased, on average, by about 1.6% per year between 2004 and 2008, while cancer incidence rates dropped slightly among men and stabilized among women.

The overall downward trends reflect progress in prevention, screening, early detection, and the treatment of cancer.

Among the trends for some of the most common cancers:

• Lung cancer incidence and deaths continued to decline among men and women, largely because fewer people smoke. Lung cancer rates have dropped the most in states with the strongest tobacco cessation programs, the report noted.
• Breast cancer incidence rates have been relatively stable for the past decade, after steep declines in the early 2000s. The decreased use of hormone therapy by postmenopausal women was credited with this decline.
• Incidence and death rates for colorectal cancer continue to decline, largely due to more people using colonoscopy and other forms of screening.
• The prostate cancer death rate has decreased since the early 1990s, but the contribution of prostate-specific antigen (PSA) testing to this decline is not known, according to the report.
• The rate of childhood cancers has increased by 0.6% per year since the early 1990s, but the dramatic decline in cancer deaths that began in the mid 1970s continues.
• The death rate among children and teens dropped by 2.7% annually between 1975 and 1996, and continued to decline by 1.3% per year between 1996 and 2008.
• Cancer death rates have dropped for all ethnic groups, except American Indians and Alaskan Natives, since 1999.

Melanoma an 'Emerging Epidemic'

Plescia says one particularly concerning trend is the increased incidence of the deadly skin cancer melanoma.

He notes that since 1999, melanoma rates have risen by about 2.5% annually in women and 2.3% in men.

"We see this as an emerging epidemic," he says, adding that increased efforts to get people to use sunscreen and avoid tanning beds could have a major impact on the cancer.

Obesity, Inactivity, and Cancer

For more than three decades, obesity, inactivity, and poor diet have been second only to tobacco as preventable causes of disease and death in the U.S., but tobacco use has declined dramatically since the 1960s, while obesity rates have doubled.

"If we don't do anything about this, we can expect to see large increases in cancer incidence and cancer deaths in the near future," says American Cancer Society vice president of surveillance research Ahmedin Jemal, PhD.

Two out of three adults in the U.S. and one in three children are overweight or obese.

The death rates for two of the cancers associated with obesity -- pancreatic and cancer of the uterus -- have gone up. In addition, the incidence of several cancers associated with obesity, including pancreatic, kidney, esophageal, and uterine, is on the rise.

Rachel Ballard-Barbash, MD, MPH, of the National Cancer Institute, agrees that obesity and inactivity are among the most significant modifiable risk factors for cancer among people who do not smoke.

A report from the International Agency for Research on Cancer found that between a quarter and a third of common cancers in the U.S. and other industrialized countries are caused by the joint effects of obesity and sedentary lifestyle.

Ballard-Barbash says just as public health efforts aimed at tobacco use focused on changing the environment to help people stop smoking, efforts to address obesity should include promoting changes in communities to help people make the right food choices and stay active.