WEDNESDAY, March 1, 2017 (HealthDay News) -- Carrying extra weight increases the risk of a number of cancers, a new review reports.
Additional pounds appear to particularly influence the risk of cancers related to the digestive organs or those driven by hormonal abnormalities, according to the review's European authors.
The evidence is so strong at this point that important organizations such as the International Agency for Research on Cancer describe "excess body weight as an important cause of cancers," said Susan Gapstur. She's vice president of epidemiology at the American Cancer Society.
The new evidence review was led by Maria Kyrgiou, of Imperial College London's Department of Surgery and Cancer. The review found that a jump in a person's body mass index (BMI) of 5 was associated with a higher cancer risk in the esophagus, bone marrow, biliary tract system, pancreas and kidneys.
BMI is a rough estimate of a person's body fat based on height and weight. A BMI of 18.5 to 24.9 is considered normal weight. A BMI from 25 to 29.9 is overweight, and a BMI of 30 or higher is considered obese.
The new study also found that a higher BMI increased the risk of colon and rectal cancer in men, and endometrial cancer in women.
The evidence review also linked obesity with a higher risk of gall bladder, stomach and ovarian cancers.
The researchers based their evaluation on 204 previous evidence reviews of studies investigating whether excess weight influenced the risk of developing 36 primary cancers.
They found that for every increase of 5 in BMI, the risk of developing certain cancers rises. The increases range from 9 percent for colorectal cancer among men to 56 percent for cancer of the biliary tract system, which aids in digestion, the researchers said.
Risk of postmenopausal breast cancer among women increased 11 percent for each 11 pounds of weight gain, if they'd never used hormone replacement therapy.
Findings from the review were published Feb. 28 in the BMJ. Because the research is a review of past studies, not all of the measures of outcomes were the same. Some looked at weight gain; others looked at BMI.
Excess weight likely influences the risk of gastrointestinal cancers by altering insulin levels and promoting inflammation, said Dr. Graham Colditz. He's deputy director of the Institute for Public Health and chief of Public Health Sciences at Washington University School of Medicine in St. Louis.
Overweight and obesity also can alter levels of sex hormones, which could explain the increased risk of breast and endometrial cancer in women, Colditz added.
"There's no easy way to gain weight and not gain an increase in risk of many cancers," said Colditz, who wrote an editorial accompanying the new review.
Obesity worldwide has more than doubled among women and tripled among men during the past four decades, the study authors said in background notes. The overall number of overweight and obese people increased from about 857 million in 1980 to 2.1 billion in 2013.
The next step is to determine whether losing weight would decrease cancer risk, Colditz and Gapstur said.
Gapstur said people who undergo bariatric (weight-loss) surgery can experience a reduced risk of some types of cancer.
"For people who don't opt to reduce their weight that way, we're still learning what the impact of intentional weight loss is on cancer risk," Gapstur said. "There is some suggestive evidence that intentional weight loss even in middle and older adulthood can potentially reduce risk."
Dr. Paolo Boffetta is associate director for cancer prevention with the Tisch Cancer Institute at Mount Sinai in New York City. He thinks research will ultimately prove that it's never too late to lose weight if one wants to moderate their cancer risk.
He pointed to other studies that have shown that quitting smoking reduces cancer risk no matter how long someone's been smoking.
"Quitting is good at any age," Boffetta said. "This is probably also true for obesity."
For more about obesity and cancer, visit the U.S. National Cancer Institute.