Being obese or even overweight may increase a person's risk of developing up to a dozen different types of cancer, European researchers report in a new study.
Doctors have long suspected a link between weight gain and certain cancers, including colon and breast cancers. But the new study, published Friday in the journal Lancet, suggests it could also increase chances for cancer of the esophagus, thyroid, kidney, uterus and gall bladder, among others.
While the study suggests a link, there is no definitive proof that being fat in itself causes cancer.
"To make the link between cause and effect, we need to tick several boxes," said Dr. Andrew Renehan, the study's lead author and senior lecturer at the School of Cancer Studies at the University of Manchester. "This study begins to tick the first two or three boxes, but more research is needed to confirm it."
The researchers compiled data from 141 studies and considered more types of cancers and more diverse populations than had been done previously. The research covered more than 280,000 cases from North America, Europe, Australia and Asia.
The subjects, both overweight and normal weight, were followed for about nine to 15 years, with researchers tracking their body mass index, or BMI — a calculation based on weight and height — and correlating it with incidents of cancer.
In men, an average weight gain of 33 pounds increased the risk of esophageal cancer by 52 percent, thyroid cancer by 33 percent, and colon and kidney cancers each by 24 percent, the research found.
In women, a weight gain of 29 pounds increased the risk of cancer in the uterus and gall bladder by nearly 60 percent, esophagus by 51 percent and kidney by 34 percent, the study said.
The link was weaker for bone and blood cancers, for both men and women.
In Asian populations, there appeared to be a stronger link between increased BMI and breast cancer, the study said.
"This study provides a lot of circumstantial evidence about the dangers of obesity," said Dr. David Robbins, a gastroenterologist at Beth Israel Medical Center in New York, who was not involved in the study. "It also highlights the cancer crisis we face as obesity rates increase worldwide."
Scientists are unsure how being overweight could make people more susceptible to cancer.
"One of the hypotheses is that the presence of excess fat cells could affect the levels of hormones in your body," Renehan said. "At a cellular level, that may favor the development of tumors in humans."
Because many studies have found that fatter people are more likely to get cancer, experts often recommend losing weight to reduce cancer risk.
"The simple message is that, if you manage to keep a healthy body weight, you will have a lower risk of developing cancer," said Ed Yong, of Cancer Research United Kingdom.
The Lancet study was paid for by British Medical Association, the University of Manchester and the University of Bern, Switzerland. Renehan has consulted for several pharmaceutical companies that make hormone replacements.