Thu Jul 18, 2002
By Ed Edelson
THURSDAY, July 18 (HealthScoutNews) -- Here's one more incentive for younger overweight women to slim down: A study finds obesity almost doubles their risk of colorectal cancer.
"Younger" is the key word here, because the association between obesity and cancer risk is not seen in women after menopause, says a report in the August issue of Gut.
A complex relationship between insulin, an insulin-related molecule and the female sex hormone estrogen appears to be the link, says lead author Peter D. Terry, an epidemiologist at Albert Einstein Medical College in the Bronx.
Excess body fat increases levels of insulin. That increases levels of another molecule, insulin-like growth factor, which has been associated with the development of colorectal cancer, Terry says. The third actor in the scenario is estrogen, which helps prevent colorectal cancer. Estrogen is produced primarily by the ovaries before menopause, but fat tissue becomes an important source of the hormone after menopause. So, the beneficial effects of estrogen produced by fat tissue appear to offset the cancer-causing effect of insulin-like growth factor in older women, Terry says.
A previous study by Terry and his colleagues found an age-associated doubled risk of colorectal cancer in obese women under the age of 55, but not in older women. "Ours is the first to look specifically at menopausal status," he says.
Data for the report comes from the Canadian National Breast Screening Study, whose purpose was to determine the value of mammography in early detection of breast cancer. The Canadian researchers recorded the weights of almost 90,000 women, aged 40 to 59, as well as all cases of cancer over more than a decade. During that time, 527 women got colorectal cancer.
Obesity is measured by the body mass index (BMI) -- weight in kilograms divided by the square of height in meters. Obesity is defined as a BMI of 30 or over. The incidence of colorectal cancer in pre-menopausal women in the Canadian study was 88 percent higher than for thinner pre-menopausal women. There was almost no difference in incidence between thinner and obese women after the menopause.
Terry says that, as an epidemiologist, he can't make recommendations about cancer risk for obese women, but there is some obvious advice: "Obese younger women, and their doctors, should do everything in their power to get on a program of weight reduction."
The report should not affect the current recommendations for colorectal cancer screening for most people, says Dr. Thomas Fogel, a radiation oncologist and a spokesman for the American Cancer Society. Some screening procedure -- preferably a colonoscopy, but fecal occult blood testing or a sigmoidoscopy will do -- is recommended, starting at age 50.
"Certainly, if there is a younger woman at high risk because of a family history or other factors, there might be a change in the screening recommendation for that person," Fogel says.
Another report about colorectal cancer in the same issue of Gut could also be of interest to women. A researcher in Northern Ireland reports in a small study that large doses of folic acid can reduce the risk for people at very high risk of developing the cancer.
Dr. Kourosh Khosraviani, a professor of surgery at Royal Victoria Hospital in Belfast, had six patients with precancerous intestinal polyps take 2 milligrams of folic acid every day for three months, while another five patients who also had polyps took an inactive substance. Periodic biopsies of intestinal tissue found a reduction in the numbers of actively dividing cells, which are potentially cancerous, Khosraviani reports. The effect persisted for six weeks after the folic acid supplementation was stopped.
The recommended daily intake of folic acid for most people is about 400 micrograms, which translated to four-tenths of a milligram, but pregnant women are advised to take up to 5 milligrams to help prevent neural tube birth defects, Khosraviani notes. For patients with precancerous polyps, folic acid works by preventing genetic mutations that could make the growths turn cancerous, he says.
However, it's far too early to talk about folic acid as a cancer preventive, he says.
"Absolutely not," he says. "There is no way that this small pilot study can recommend folic acid supplementation. It needs to be looked at in a much wider, much larger-scale study. If that comes up with the same results, it would be reasonable to talk about it."