Being obese or even overweight may increase a person's risk of developing up to a dozen different types of cancer, European researchers reported in a study published Friday, February 15.

Doctors had long suspected a link between weight gain and certain cancers, including colon and breast cancers. But the new study, published in The Lancet, suggests it could also increase chances for cancer of the esophagus, thyroid, kidney, uterus, and gall bladder, among others.

While the study suggested a link, there was no definitive proof that being overweight 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 latest study looked at more types of cancers and more diverse populations than previous research on the subject. Renehan and colleagues analyzed data from 141 articles from North America, Europe, Australia and Asia, covering more than 280,000 cancer cases in men and women.

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 of body fat based on people's weight divided by the square of their height) and correlating it with incidents of cancer.

In men, an average 15 kilogram (33 pounds) weight gain 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, an approximate 13 kilogram (29 pounds) weight gain increased the risk of cancer in the uterus and gall bladder by nearly 60 percent, in the esophagus by 51 percent and in the kidney by 34 percent, the study said.

The link was weaker in terms of 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, an Assistant Professor of Medicine at the Continuum 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 still unsure about 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.”

But because many studies have found that overweight 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.