FRIDAY, Dec. 7, 2007 (HealthDay News) -- Obesity and diabetes -- risk factors so often linked to heart disease -- can also affect the incidence and severity of cancer, a collection of four new studies suggests.
The findings, presented Friday at the American Association for Cancer Research's Sixth Annual International Conference on the Frontiers of Cancer Prevention Research in Philadelphia, link weight gain and diabetes to a number of malignancies, including breast, prostate and colorectal cancer.
"All of these are consistent with what we would expect with the occurrence of each of these cancers or cancer survival," said Elizabeth Platz, associate professor of epidemiology at the Johns Hopkins Bloomberg School of Public Health. "Metabolic perturbations enhance certain cancers. Insulin and other hormonal factors influence cell growth and make cells multiply."
Women with diabetes have a 50 percent increased risk of developing colorectal cancer, according to the first study, by researchers at the University of Minnesota. The group, led by Andrew Flood, assistant professor of epidemiology and public health, 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, Flood said 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, by researchers at Yale University. 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, the researchers said.
Another study, by researchers at the Johns Hopkins Bloomberg School of Public Health, 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 Johns Hopkins 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, Plantz said. 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, she said.
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, said Dr. Rexford Ahima, professor of medicine at the University of Pennsylvania.
"We have known for years that women who are obese are at high risk of breast cancer," Ahima said. "Fat tissue makes estrogen, which promotes breast cancer. The frightening thing is that the more obese you are, the greater the risk you have of dying of cancer. For every increase of 10 kilograms, 14 pounds, there is a 14 percent increased risk of breast cancer death."
So, instead of thinking of obesity just as a risk factor for heart disease, its effects on cancer must also be taken into account, Platz said. "In general, it is a good thing to do to avoid obesity," she said. "That is a message consistent with what we know about good health."
To learn more about obesity and the associated health risks, visit the U.S. National Library of Medicine.