Being overweight doesn't mean you'll get cancer.
But Americans who are overweight or obese are at heightened risk for 13 types of cancer, and these types of cancer are increasing in the U.S. even as other types of cancer decline, the Centers for Disease Control and Prevention said Tuesday.
In their analysis of U.S. cancer diagnoses between 2004 and 2014, researchers found that 4 out of 10 cases of cancer diagnosed in 2014 were cancers associated with obesity or being overweight.
Among women and older adults, the rate was even higher. Fifty-five percent of cancers diagnosed in women — and two-thirds of cancers diagnosed in people 50 or older — were those commonly associated with excess weight.
Researchers aren't sure why obesity is associated with cancer, but many believe that excess weight impairs the body's immune system, causes chronic inflammation and interferes with hormone levels and cell growth.
The new study, however, cannot prove this, and in analyzing cancer statistics, the researchers did not have information on the cancer patients' weight. But even without evidence of causation, the researchers believe it's important for Americans to be aware of the association.
"CDC found the incidence of cancers associated with overweight and obesity, excluding colorectal cancer, increased significantly during 2005-2014 among persons aged 20-74 years, mirroring increases of obesity observed since 1960," lead author Dr. Brooke Steele told The Deseret News. "CDC would like to get the word out that excess weight is associated with some cancers."
Half of Americans, however, aren't aware of the connection between weight and cancer. If you're one of them, here's what you should know.
Researchers have known for years that overweight people are more likely to develop some types of cancer, which are sometimes called "lifestyle cancers." So what's new about the latest report?
For one thing, the numbers are greater than previously known. Last year, the World Cancer Research Fund was saying that 20 percent of cancers in the U.S. were associated with body fat, inactivity, alcohol consumption and poor nutrition.
This week's report doubles that number.
What also troubles public health officials is the fact that other types of cancer are declining, while of those associated with obesity, all but one — colorectal cancer — are increasing. A 23 percent decline in colorectal cancer is attributed to early screening since precancerous polyps can be removed during a colonoscopy.
Even so, the report said that people who are overweight are about 30 percent more likely to develop colorectal cancer than normal-weight people. And other types of cancer associated with excess weight rose 7 percent between 2005 and 2014, while the rates of cancer not associated with obesity declined by 13 percent, Julie Steenhuysen of Reuters reported.
• Persons who are overweight are nearly twice as likely to develop cancers of the esophagus, stomach, liver and kidney.
• Women who are overweight or obese are two to four times more likely to develop endometrial cancer.
• The cancers that are believed to be associated with excess weight include meningioma (tumors in the membranes surrounding the brain and spinal cord), multiple myeloma (a cancer of the blood), adenocarcinoma of the esophagus, and cancer of the thyroid, Newsweek reported.
Other cancers include postmenopausal breast cancer, gallbladder, stomach, liver, pancreas, kidney, ovaries, uterus, colon and rectal cancers.
An important caveat
According to federal statistics, 35 percent of adult men and 40.4 percent of women are obese, and more than 70 percent of Americans 20 or older are either obese or overweight.
Many health officials believe that excess weight and inactivity is responsible for a third or more of disease and chronic illness in the U.S., and obesity is also responsible for a decline in life expectancy reported last year.
But while two-thirds of Americans are overweight or obese, two-thirds of us don't get cancer. About 4 in 10 of us do, the National Cancer Institute says.
And some health experts cautioned that it would be wrong to believe that someone developed cancer simply because they were overweight. It adds to our risk, but other factors are at play, including genetic mutations, chronic infections and alcohol and tobacco use.
Dr. Farhad Islami, strategic director of cancer surveillance research for the American Cancer Society, told CBS News that "only a fraction of the cancers included in the calculation in this report are actually caused by excess body weight."
Cancer can arise from a combination of factors, and in some people, the cause is never known, Islami said.
Also, the authors of the CDC report acknowledged what seems an important limitation of the study: Its conclusions came from statistics that said nothing about the cancer patients’ weight.
But the authors point out in their report that you don't have to be significantly overweight for health risks to accumulate.
"Observational studies have provided evidence that even a 5-kg (11 pound) increase in weight since early adulthood is associated with increased risk of overweight- and obesity-related cancers,” they wrote.
Steele, the lead author, told The Deseret News that "it will take everyone — communities, health care providers, states and the federal government — working together to reduce and prevent these cancers and other health conditions that are associated with excess weight."
It remains unclear, however, how much losing weight mitigates cancer risk. The report said that effects of weight loss on cancer risk might not be observable for 10 years, and large-scale studies that might prove this have not been conducted.
Some studies, however, have shown a reduced risk for endometrial and breast cancer among postmenopausal women who lost weight, and Dr. Anne McTiernan, who conducts research on the effects of nutrition and weight loss, believes that it helps.
"It’s never too late to make health-improving changes, including decreasing your risk for cancer. In our studies, people aged 60 years and older often had greater effects from weight loss on their cancer risk factors. And people who had previously lost and regained pounds did just as well as those who had never lost weight in the past," McTiernan, the author of "Starved: A Nutrition Doctor's Journey from Empty to Full," wrote earlier this year for the American Institute for Cancer Research.