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Men battling prostate cancer are more than twice as likely to die from the disease if they are obese, according to a new study that suggests soaring insulin levels related to excess weight are to blame.

“If we were detectives, we'd say we have a very serious suspect,” said Michael Pollak, an oncology expert at McGill University who co-authored the study with Harvard University researchers.

The study found that obese men with prostate cancer are 2.5 times more likely to die from the disease than patients of a healthy weight. But obese men with abnormally high insulin levels – a condition linked to excess weight – have an even bleaker prognosis: They are four times more likely to die than men with normal insulin levels.

The results, published Sunday in the journal Lancet Oncology, should inspire men to eat their greens and exercise more, Dr. Pollak said.

But a possible link between insulin secretion and risk of dying from prostate cancer also paves the way for more research into new strategies for the prevention and treatment of prostate cancer, he said.

“It really opens a new field of investigation,” Dr. Pollak said. “If we understand in more detail the basis of this obesity relationship … we might be able to do a lot more for our prostate cancer patients.”

Previous studies have suggested there is a link between excess weight and risk of mortality from prostate cancer, but results were varied, he said. This study not only helps to confirm those results, it shows that the link is even stronger than previously thought. The study may also explain why this relationship exists, he said.

The Harvard-McGill study, led by Harvard researcher Jing Ma, looked at 2,546 men diagnosed with prostate cancer. After controlling for factors such as age and whether they smoked, the researchers assessed blood samples taken from the men, who were all participants in the Physicians' Health Study, a larger, long-term study involving more than 22,000 doctors.

The researchers looked at each subject's body mass index (BMI), C-peptide concentrations – a marker of insulin secretion – and whether or not they died of prostate cancer.

Men who were overweight (a BMI of 25 to 29.9) or obese (a BMI of greater than 30) before diagnosis were significantly more likely to die from their prostate cancer than men of normal weight, the researchers found.

They also found that those with the highest C-peptide concentrations also had a significantly greater risk of prostate-cancer mortality compared with men with the lowest concentrations.

The study “emphasizes the value of reaching out to the Canadian public about the importance of maintaining a healthy body weight,” said Heather Chappell, senior manager of cancer control policy with the Canadian Cancer Society, which helped fund the study along with the National Institutes of Health in the United States.

Dr. Pollak warned that more research is needed to confirm whether insulin levels are linked to prostate cancer survivor rates. But if so, this research could provide a basis for investigation into new therapeutic and prevention strategies, such as the use of insulin-lowering or anti-diabetic drugs as therapy for prostate cancer, he said.

“I wouldn't say that this is the last word, but it strongly [implies] that insulin is likely to be one of the mediators of this relationship between obesity and a poor outcome,” he said. “It's kind of like a wake-up call.”