Diabetes, insulin levels, and weight gain increase various risks associated with some cancers in women, while decreasing the risk of prostate cancer in men, according to findings presented in December at the American Association for Cancer Research’s Sixth Annual International Conference on Frontiers in Cancer Prevention Research.

Women with diabetes are one-and-a-half times more likely to develop colorectal cancer than women who do not have diabetes, researchers announced. One common factor in both colorectal cancer and diabetes is obesity, and diet and lifestyle have been shown to play a large role in the risk of developing both. Also, the increased insulin levels in people with type 2 diabetes could be the direct cause of the increased cancer risk, researchers say.

And while the studies presented did not examine development of breast cancer in women with diabetes, they did draw conclusions about mortality in women with breast cancer. Among women with invasive breast cancer, the risk of death in women with high levels of C-peptide (a marker for insulin secretion) was three times the risk of death in women with normal levels of C-peptide. The results, researchers said, demonstrate that C-peptide, and most likely insulin itself, are markers for breast cancer prognosis. Women in their 40s and women with invasive breast cancer that had spread to other tissues showed the greatest risk for increased mortality. However, the relationship between increased C-peptide and higher mortality rates existed among nearly all the women studied.

For the study, researchers followed 689 women from 1995 to 1998. The women had been diagnosed with breast cancer but did not have type 2 diabetes. Researchers said that the link between insulin and breast cancer prognosis should prompt breast cancer survivors to talk to their oncologists about the risk for diabetes and hypertension (high blood pressure). Breast cancer patients should eat a low-fat diet and exercise to lower insulin levels, they said.

Another AACS study found that among women who were diagnosed with invasive breast cancer and who subsequently gained weight, risk of death increased by more than half. Researchers leading the Collaborative Women’s Longevity Study concluded that the weight gain increased risk for death due to all causes, not just breast cancer.

Finally, men with diabetes demonstrated a decreased risk for prostate cancer, when researchers at Johns Hopkins University compared a group of 264 men with prostate cancer to a group of 264 men without prostate cancer. Men with increased levels of C-peptide—some with diabetes and some without—at the start of the study were one-third less likely to develop prostate cancer. Researchers think that the decreased testosterone levels that occur over time in people with diabetes could explain the lower risk for prostate cancer.

These various study results demonstrate the differing mechanisms for different types of cancer—but all emphasize the significance of weight and metabolism in risk for cancer and related mortality.