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Obesity may be linked to more cancers than doctors thought.

Being obese or even overweight may increase a person's risk of developing up to a dozen types of cancer, European researchers report in a study.

Doctors have long suspected a link between weight gain and certain cancers, including colon and breast cancers. But the new study, published today 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.

The researchers compiled data from 141 studies in 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. "It also highlights the cancer crisis we face as obesity rates increase worldwide."