TORONTO - Men with prostate cancer who are overweight or obese are more than twice as likely as men of normal weight to die of the disease -- and increased levels of insulin may be a major reason, researchers say.
"Previous studies have suggested a relationship between prostate cancer progression and being overweight," says senior investigator Dr. Michael Pollak, a cancer researcher at McGill University.
"This study strongly confirms those earlier findings and, in fact, points to a leading culprit, which appears to be an excess secretion of insulin in these patients."
Earlier research has shown that prostate cancer cells have insulin receptors on their surface, meaning they are sensitive to the hormone that regulates the amount of glucose, or sugar, in the body. Obese people tend to have elevated insulin levels.
Pollak said doctors have long thought that a man's level of testosterone was the most important factor in determining how aggressive -- and deadly -- his prostate cancer would be.
"But this research now suggests that we have to turn our attention to a whole new family of hormones related to insulin that may also impact prostate cancer behaviour," he said in an interview from Montreal.
"So that may lead us to new drugs or new therapeutic approaches if indeed insulin is stimulating the prostate cancer cells to grow."
The study, a collaboration between researchers at McGill and Harvard University in Massachusetts, looked at 2,546 men diagnosed with prostate cancer between 1982 and 2007. The subjects were part of the Physicians Health Study, which involved more than 22,000 male doctors.
Men who were overweight (BMI 25 to 29) or obese (BMI 30 or higher) before diagnosis were significantly more likely to die from their prostate cancer than men of normal weight (BMI under 25), the study found.
Those with high concentrations in the blood of C-peptide, an indicator of excess insulin, also had an increased risk of prostate cancer death compared to men with the lowest concentrations.
In fact, said Pollak, those men with an increased C-peptide concentration and a high BMI prior to their cancer diagnosis were almost four times more likely to die of the disease.
"So the take-home message is that it's better to be of normal weight," he said. "The cardiologists have already been telling people that, for their reasons, it's good not to be overweight and not to eat too much and to exercise."
"And now we think that that same message is more true than ever because it also is relevant to cancer outcomes."
Heather Chappell, senior manager of cancer control policy with the Canadian Cancer Society, said the findings emphasize the importance of maintaining a healthy body weight.
"We need to work to identify barriers that keep Canadians from eating as well as possible and being as physically active as possible," she said in a statement. "It appears that having a healthy body weight can help you survive a prostate cancer diagnosis in the future."
Pollak said future research will look at whether overweight men with prostate cancer can lessen their risk of dying from the disease by losing weight and whether drugs that lower insulin levels could also improve the prognosis.
Such research could also have implications for those with poorly controlled Type 2 diabetes, who have excess levels of the hormone due to insulin resistance by the body.
While that remains to be proven, the implications from the current study are clear, stressed Pollak.
"We now can tell an obese man that we now know for sure if you stay obese and you're unlucky and you get cancer, you're going to be more likely to die of your cancer."
The study, published online in the journal Lancet Oncology, was funded by the Canadian Cancer Society and the U.S. National Institutes of Health.
