British scientists looked at the link between obesity and cancer in 300,000 Americans, who were monitored between the ages of 18 and 65.
During that time, some people gained little weight, while others became “morbidly” obese — meaning they were so fat it endangered their health.
The population was then followed for an average of 15 more years, while cancer rates were recorded.
Men whose body mass index (BMI) rose from around 22 to 27 had a 50% increased risk of developing obesity-related cancer, compared with men who stayed within a healthy weight range.
Women who went from a BMI of 23 to around 32 experienced a 17% increase in risk.
BMI is a measurement that relates to height and weight.
Being overweight or obese is linked to a wide range of cancers, including of the bowel, breast, and pancreas.
Several obesity-related cancers, such as womb and ovarian cancer, only affect women.
Lead scientist, Dr Hannah Lennon, from the University of Manchester, said: “This research shows how important it is to look at weight gain over a person’s lifetime, to give a clearer picture of cancer risk through life, compared to assessing someone’s BMI at a single point.
“This study could also be really useful in public health.
“It could help identify people who would benefit the most from taking action to control their weight, before any health problems arise, including a cancer diagnosis.”
The findings were presented at the National Cancer Research Institute’s (NCRI) Cancer Conference, in Liverpool.
Harpal Kumar, the chief executive of Cancer Research UK, which funded the study, said: “This is a really interesting way to look at lifetime risk of obesity-related cancers and helps us understand the effects of weight gain over time.
“It’s important that people are informed about ways to reduce their risk of cancer. And while there are no guarantees against the disease, keeping a healthy weight can help you stack the odds in your favour and has lots of other benefits, too.
“Making small changes in eating, drinking, and taking exercise, that you can stick with in the long-term, is a good way to get to a healthy weight, and stay there.”