Are insulin-resistant men less prone to prostate cancer?

Obese men may be less likely to develop prostate cancer, but are more likely to die of the disease if they do develop it, reported The Guardian . These men “have a greater risk of developing one of the most aggressive and life-threatening forms of prostate cancer,” the newspaper explained.

The story is based on a well-conducted study of insulin resistance in men with prostate cancer. Insulin resistance has been shown in many studies to be strongly associated with obesity; however, this study did not investigate obesity, only insulin resistance, and the authors acknowledge that some of their findings are speculative.

The researchers are quoted by the BBC as saying, “Obese men… are less likely to develop prostate cancer in the first place.”  The BBC explained that “the [prostate] cancer was much less likely to develop in people who are insulin resistant – a pre-diabetes condition linked to obesity.”

The design of this study makes it impossible to say that insulin levels are responsible for the different prostate cancer risks observed between groups. The study has highlighted a link between insulin resistance and risk of prostate cancer that should form the basis for further research.

Where did the story come from?

Tanja Stocks and colleagues from the Department of Surgical and Perioperative Sciences, University Hospital, in Sweden carried out this research. The study was funded by the World Cancer Research Fund and was published in the peer-reviewed medical journal International Journal of Cancer .

What kind of scientific study was this?

The research behind this story is a nested case-control study. The researchers identified 392 men with prostate cancer from the regional cancer register. Measurements were available for these men on their blood glucose, cholesterol, triglycerides (fat levels in the blood), and height, weight and blood pressure. These had been collected as part of their enrolment in a larger prospective study, the Vasterbotten Intervention Project (VIP).

At the same time, 392 men who were free from cancer and who had a blood sample available through the VIP project were used as controls. These controls were matched with cases of the same age and date of recruitment to the project. In a nested study the control group can be selected from a group of similar men in the original long-term study. This means they’re more likely to be similar to the studied men than if the controls were chosen in a different way.

What were the results of the study?

The researchers found that high levels of leptin (a hormone), C-peptide (a substance involved in processing of insulin), insulin resistance (HOMA-IR), and HbA1c (blood carrying glucose) were associated with a reduced risk of prostate cancer. When they analysed the results by age, they found that this relationship was only significant in men less than 59 years of age at blood sampling, not in older men.

What interpretations did the researchers draw from these results?

The researchers concluded that, “Factors related to insulin resistance are associated with the development of prostate cancer.”

What does the NHS Knowledge Service make of this study?

There are some concerns about the interpretation of the results of this study.

• Most importantly, studies that employ a case-control design are not able to establish whether one factor causes a disease; the study has identified factors that require further study.
• When the results were adjusted for the concentration of leptin (a hormone) in the blood, most of them were no longer significant. This means that leptin is playing some part in the relationship between other substances and the risk for prostate cancer; this needs further examination. 
• It is difficult to draw confident conclusions from the researcher’s analysis of risk of non-aggressive versus aggressive cancers. The trends in the aggressive cancer subgroup were all non-significant and results were not adjusted for other factors, such as leptin or age, which seem to be affecting the relationships between insulin resistance and cancer risk.

This study has attempted to untangle the complex relationship between sex hormones, hormones related to insulin resistance or obesity and prostate cancer. The authors acknowledge that some of the links between these hormonal changes and the development of prostate cancer is speculation. The finding that the suggestive relationship may be different for young men with early disease compared with those with more aggressive presentations of prostate cancer is intriguing and will need testing in other studies.