Diabetes found to protect against prostate cancer

Orlando, FL-In a surprise finding, a diagnosis of diabetes or cardiometabolic disorders was not associated with an increased risk of incident prostate cancer, but rather a decreased incidence of the disease, according to data presented at the American Society of Clinical Oncology annual meeting.

In a separate study from the National Cancer Institute, Bethesda, MD, researchers found that elevated fasting levels of serum insulin (but not glucose) that are still within the normal range are associated with a higher risk of prostate cancer (J Natl Cancer Inst 2009; 101:1272-9).

Diabetes and cardiometabolic disorders are associated with a dysfunctional growth hormone/insulin-like growth factor (IGF1) axis. Many of the hormones and factors within the IGF1 axis promote tumor growth. Diabetes is associated with an increased risk of developing many cancers.

"We hypothesized that diabetes would encourage the development of prostate cancer, but we found the reverse," said lead investigator Yaacov Richard Lawrence, MD, assistant professor of radiation oncology at the Kimmel Cancer Center at Thomas Jefferson University.

"We followed over 11,500 men, all of whom had coronary artery disease, for 12 years and found that patients with diabetes had about a 40% relative decreased incidence of prostate cancer compared to those patients with no diabetes or metabolic syndrome. Patients with metabolic syndrome fell somewhere in between normal patients and those with diabetes, with a slight preventive effect."

At the initial screening (1990-1992), clinical, demographic, and metabolic characteristics of the patients were recorded and a physical examination performed. The Israel Cancer Registry and the Ministry of the Interior were used to identify those men who had had a cancer diagnosis or who had died up through the end of 2004. Slightly more than half (53%) of the men did not have diabetes or cardiometabolic disorders at baseline.

The relative risk of developing prostate cancer was 34% lower (p=.007) among the men with a baseline diagnosis of diabetes and 16% lower among those with a baseline diagnosis of metabolic syndrome (p=.083).

"Having found these results, which surprised us, we went back to the literature and found a few studies with similar findings [ie, Am J Epidemiol 2004; 159:1160–7; Int J Cancer 2009; 124:1398–403; Diabetologia 2004; 47:1071–8]. That reassured us that our findings are not a statistical quirk, but are true," Dr. Lawrence told Urology Times.

"Since testosterone is an essential element in the development of prostate cancer, a low testosterone level is a possible biological explanation for our results."

The study investigators also considered the possibility that the protection against prostate cancer afforded by diabetes was a result of increased mortality among diabetics.

"We adjusted the data to take account of differing mortality rates between the cohorts, but it did not influence the results," Dr. Lawrence noted.

In the National Cancer Institute study, Demetrius Albanes, MD, and colleagues investigated the relationship of serum insulin, glucose, and surrogate indices of insulin resistance. Researchers studied 100 case subjects with incident prostate cancer and 400 noncase subjects without prostate cancer.

Insulin concentrations in fasting serum collected a mean of 9.2 years before diagnosis among case subjects were 8% higher than among noncase subjects, and the molar ratio of insulin to glucose and homeostasis model assessment of insulin resistance (HOMA-IR) were 10% and 6% higher, respectively, but these differences were not statistically significant. Among men in the second through fourth insulin quartiles, versus those in the first quartile, increased insulin levels were associated with statistically significantly increased risks of prostate cancer.