The medical community has long known that being overweight or obese can significantly increase your risk of cancer. Now, a study suggests that risk may actually be twice as high as previously thought, the University of Bristol reports.

The study, published in the International Journal of Epidemiology, was led by the International Agency for Research on Cancer (IARC). Researchers at the University of Bristol Medical School also contributed, conducting genetic analyses on eight common obesity-related cancers.

Specifically, researchers looked at how body mass index (BMI) related to cancer risk in the estimates of a number of classical cohort studies. Researchers then applied the Mendelian randomization research method to determine cause-and-effect relationships while accounting for genetic variations in people.

BMI, a simple calculation of adult body fat based on height and weight, is one of the most common tools for measuring obesity, a condition of excess body fat. People who have a BMI of 25 to 30 are considered overweight. People with a BMI of 30 or higher are considered obese.

According to the results of this new analysis, the proportion of cancers attributable to being overweight or obese could be as much as double what was previously calculated. That’s huge, said researchers, since current estimates suggest excess body fat accounts for just 6% of all cancers in high-income countries.

“The importance of these analyses is that they suggest that the effect of being overweight on cancer risk has been underestimated in the past and that obesity plays an even more important role in cancer than previously suggested,” said Richard Martin, professor of clinical epidemiology at the University of Bristol Medical School.

According to this research, being overweight puts people at greater risk for cancer because fat cells send out signals that encourage other cells in the body to divide more often. Doctors in the study said the best way to cut obesity-related cancer risk is to lose weight.

Read the University of Bristol press release here.

Read the study abstract here.

See also, “Obesity Is Driving Rise in Cancer Among Young People