Feb. 3, 2004 -- A diet rich in foods that trigger a quick and drastic jump in blood sugar levels can do more than just boost risk of type 2 diabetes and contribute to obesity. A new study indicates they may also lead to colon cancer.
Researchers at Harvard and UCLA find that the future risk of colorectal cancers is nearly three times higher in women who eat the most high glycemic-load foods compared with those who eat lesser amounts. These foods include breads, pastas, pancakes, and other carbohydrates made from refined "white" grains, as well as other processed or sugary foods such as cakes, cookies, and other snacks.
What Is Glycemic Index?
A food's glycemic index is a number that tells how much and how quickly blood sugar increases after eating a food that has carbs.
"We find a very straightforward and clear association between high-glycemic foods and the risk of colorectal cancers," researcher Simin Liu, MD, ScD, tells WebMD. "It's because these foods seem to trigger a greater tendency toward insulin resistance."
Insulin resistance, already linked to type 2 diabetes, is believed to create an environment in the colon that is conducive to tumor growth, says Liu, director of nutrition research at Harvard's Brigham and Women's Hospital and associate professor of medicine and epidemiology at Harvard Medical School.
His study, published in the Feb. 4 issue of the Journal of the National Cancer Institute, is now the fourth in recent years to find a link between high-glycemic foods and colon cancers, and comes as people with diabetes -- and those at risks of developing diabetes -- are increasingly urged to pay closer attention to the glycemic index of foods they eat.
With insulin resistance, cells' ability to respond to the action of insulin is hampered and blood sugars are not decreased to a normal range. To compensate, the pancreas secretes more insulin to help maintain a normal blood glucose level. Since high-glycemic foods are quickly digested, they provide a sudden rush of blood sugars that are not easily metabolized, prompting obesity as well as diabetes. Conversely, foods with a low-glycemic load -- typically those rich in fiber and slower to be digested -- raise blood sugar more gradually.
"This finding doesn't surprise me at all," says Marji McCullough, ScD, RD, nutritional epidemiologist and a senior researcher for the American Cancer Society. "The idea that insulin and its associated hormones elevate cancer risk is very plausible, and this mechanism is being actively studied in cancer research."
Already, she says, there is evidence that a diet rich in high-glycemic load foods can boost risk of pancreatic cancer, and there is some research -- which hasn't been verified -- that indicates these foods may also boost breast cancer risk.
As a general rule, low-fat, high-fiber foods -- fruits, vegetables, and whole grains and legumes -- often prescribed to manage weight and help prevent diabetes and other health conditions, have a low-glycemic index. Conversely, starchy and processed foods such as potatoes, breads, and cereals usually have a high-glycemic index.
Problem Hard to Pinpoint
"It's hard to pinpoint the real impact of a high-glycemic load diet, because it varies on a number of things, such as how much the food is processed, how big is the meal, and what other foods are in the meal," she tells WebMD. "If you have large portions, for instance, that also raises blood sugars very quickly."
These other factors may explain why the largest and longest study to date -- tracking nearly 50,000 Canadian women for 16 years -- found no link between intake of high-glycemic foods and future risk of colon cancers. That study was published last June.
"Because there have been very few studies of this issue to date, it is too early to make a final decision regarding an association with glycemic load to colon cancer," says Paul Terry, PhD, of the National Institute of Environmental Health Sciences in North Carolina, who led that study. "But the results of this study are interesting."
In Liu's study, some 40,000 American women filled out detailed questionnaires about their eating habits and their rates of colorectal cancers were tracked for nearly eight years. Overall, women consuming the most high-glycemic foods were three times more likely to develop colon cancers, but some women observed had a sixfold risk.
Although there are exceptions -- potatoes, All-Bran cereal, and bananas, for instance, have a high-glycemic load. But most "whole" foods rich in fiber have a low-glycemic load and since they take longer to digest, are less likely to trigger insulin resistance.
"If one was to make recommendations, I'd say you should replace refined grains with whole grains, replace potatoes with other vegetables, and eat more nuts, which have a low-glycemic load," Liu tells WebMD. "It's hard to pinpoint whether it's the fiber per se, or the fact that foods that contain high fiber also have phytochemcials that also help in preventing colon cancer."