A chemical produced by fat cells makes colon cancers grow faster, a US study has suggested.
Leptin is stored in fat tissue
The British Journal of Surgery study could help explain why severely overweight people appear to be at far greater risk of the disease.
A team at the University of California, San Diego found that the hormone leptin triggered increased growth in human colon cancer cells.
Obese people are up to three times more likely to develop colorectal cancer.
Other researchers have already found that some colon cancer cells appear to be set up to respond to leptin, with "receptors" for the chemical on their surfaces.
The more fat cells a person has, the more leptin will be in their bloodstream.
The San Diego team wanted to find further evidence of the link by watching what happened to human cancer cells exposed to the hormone.
In a laboratory, they added the hormone to different varieties of cancer cell.
Growth was stimulated in all the cell lines - and in two out of three tested, the hormone also hampered the usual process of programmed death that allows the body to replace normal cells, but which often malfunctions in cancers.
Dr Kim Barrett, who led the research, said: "These results may explain why obesity increases a person's risk of colonic cancer.
"The fact we have shown how leptin stimulates these cells means that drug companies may be in a better position to develop new treatments against the disease."
In the UK it is currently estimated that one in five men and a quarter of women are obese, with as many as 30,000 people a year dying prematurely from obesity-related conditions, including cancers.
Britain now has the most obese population in Europe.
A study published this week suggested that severely obese men doubled their risk of dying from prostate cancer, and obesity is also believed to increase the chance of developing breast cancer.
Body Mass Index
Underweight: less than 18.5
Normal weight: 18.5-24.9
Obese: over 30
Morbidly obese: over 40
Professor Alan Clarke said: "This research builds on previous work suggesting that leptin, a hormone released from fat cells, may play a role in the link between obesity and cancer.
"The researchers have shown that leptin is involved in two molecular pathways known to play a role in bowel cancer development."
"Although at an early stage, the findings shed light both on the development of bowel cancer and the link between obesity and cancer."