No link seen between high-carb diet, colon cancer

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Chinese women who eat a traditional diet rich in white rice and other starchy foods that spur a surge in blood sugar do not seem to have an elevated risk of colon cancer, a new study suggests.

The findings, reported in the American Journal of Clinical Nutrition, add to the conflicting body of evidence as to whether foods with a high “glycemic index” are related to an increased risk of colon cancer.

Glycemic index, or GI, refers to how rapidly a carbohydrate causes blood sugar to rise. High-GI foods, like white bread, white rice and potatoes, tend to make blood sugar levels rise quickly. With low-GI foods, such as lentils, soybeans, yogurt and many high-fiber grains, blood sugar levels also rise, but not as fast and not as high.

A related concept, called the glycemic load, refers to both the GI and the amount of carbohydrates in a given food: a low-calorie piece of fruit, for instance, may have a relatively high GI, but still provide only a small glycemic load.

The idea that a diet with a high glycemic load might contribute to colon cancer risk is based on human physiology: High blood sugar levels trigger release of the blood-sugar-controlling hormone insulin, which—along with a related hormone called insulin-like growth factor 1—may stimulate the growth and spread of cancer cells.

In line with that, a number of studies have found that people with type 2 diabetes, who have abnormally high blood sugar and insulin levels, get colon cancer more often than do people without diabetes.

So in theory, a diet heavy in high-GI foods could be a risk factor for colon cancer. But studies on the question have so far come to conflicting conclusions.

For the new study, researchers followed 73,000 middle-aged and older Chinese women over a decade, looking at the association between reported diet habits and the risk of developing colon cancer.

The women, who were cancer-free and between the ages of 40 and 70 at the study’s start, completed detailed diet questionnaires that allowed the researchers to estimate the total glycemic load of their diets.

Overall, 475 women were diagnosed with colon or rectal cancer during the 10 years the researchers studied them. When the researchers divided the study participants into five groups based on dietary glycemic load, they found no evidence that the women’s risk of colorectal cancer increased along with glycemic load.

Because rice was by far the greatest contributor to the women’s glycemic load, the researchers also looked at the relationship between the number of rice servings a woman ate per day and her risk of colorectal cancer.

Again, there was no apparent link, according to the investigators, led by Dr. Hong-Lan Li of the Shanghai Cancer Institute in China.

The study has its limitations. Like any study that relies on diet questionnaires, it is prone to erroneous measurements. And while there was no association between glycemic load and colorectal cancer in this study group, the findings cannot disprove a possible role of high-GI diets in colon cancer development.

However, the researchers point out that many of the studies that have found a GI-colon cancer link have been so-called case-control studies, where patients with colon cancer reported on past diet habits and were compared with a group of healthy individuals.

Only limited conclusions can be drawn from that type of research. Prospective studies such as the current one, which follow initially healthy people over time, are considered a stronger design.

And, Li’s team notes, most similar studies from the U.S. and Europe have also found no association between high-GI diets and colon cancer. A majority of studies focused on women, but a couple included men as well.

Still, the researchers write, further studies are warranted, including ones focusing on men and different types of carbohydrates.

Some of the established risk factors for colorectal cancer include older age (most cases arise after age 50), having first-degree relatives who developed the cancer, and a personal history of ulcerative colitis or Crohn’s disease. Studies have also consistently linked smoking, obesity and diets high in red and processed meats to an increased risk.

SOURCE: American Journal of Clinical Nutrition, online October 20, 2010.

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