The findings link weight gain and diabetes to a number of malignancies, including breast, prostate and colorectal cancer.
Women with diabetes have a 50 percent increased risk of developing colorectal cancer, according to the first study. The group, followed more than 45,000 women enrolled originally in a breast cancer detection program for more than eight years.
The increased incidence of colorectal cancer remained significant after all possibly confounding factors were taken into account. While the reason for the increased risk is not known, it could be due to the elevated levels of insulin seen with diabetes.
High levels of insulin in diabetic women could explain a threefold higher risk of death from breast cancer, said the second study. They measured blood levels of C-peptide, a marker of insulin secretion, in women in a long-term study of breast cancer. Over an eight-year period, the women in the highest third of C-peptide levels had twice the risk of dying from breast cancer, compared to women in the bottom third.
Another study, found that weight gain after a diagnosis of invasive breast cancer could significantly increase a women’s risk of death from the cancer.
The study of more than 4,000 women with breast cancer classified them by body mass index, a ratio of weight to height. For obese women, the risk of dying of breast cancer was 2.4 times greater than for women with a normal body weight, a relationship that persisted when age, menopausal status and smoking were taken into account.
Another study provided a possible explanation for the lower risk of prostate cancer seen in men with diabetes. The researchers matched 264 men diagnosed with the cancer with a group of 264 cancer-free men, measuring C-peptide levels in both groups.
Men with elevated blood levels of C-peptide when the study started were one-third less likely to develop prostate cancer than those with lower levels. Men with higher C-peptide levels had half the risk of developing prostate cancer confined to the prostate.
The protective effect of those high levels could be due to the activity of insulin in relation to the male hormone testosterone. C-peptide derives from the same parent molecule as insulin, and insulin is known to reduce the activity of testosterone, which stimulates the growth of prostate cancer.
The possible protective effect of insulin against prostate cancer could offer a mirror image of the negative effect of estrogen — the female sex hormone — in breast cancer.