Whole Grain, Not Fiber, Reduces Risk of Colorectal Cancer

The AICR experts said the study is to be commended because, unlike many earlier investigations, its authors took care to analyze the role of dietary fiber from different food sources.

22/06/07 Experts at the American Institute for Cancer Research (AICR) offered praise for a recently published study which showed that whole grain fiber, and not fiber from other food sources, is associated with lower risk of colorectal cancer.

The AICR experts said the study is to be commended because, unlike many earlier investigations, its authors took care to analyze the role of dietary fiber from different food sources. By acknowledging that the fiber one gets from whole grains is different than the fiber one gets from "starchy" foods like white bread and processed cereal, the study represents an important step toward a more precise understanding of a long-standing scientific controversy.

Findings from the Study

The study in question, published this month in the American Journal of Clinical Nutrition, found that consumption of whole grains was associated with lower risk for colorectal cancer. The same study found no significant link between consumption of fiber from other food sources and colorectal cancer risk.

Researchers at the National Institutes of Health (NIH) analyzed survey results from a large prospective cohort study called the NIH-AARP Study, which involves more than 291,000 men and 197,000 women aged 50 to 71.

The scientists analyzed the participant's intake of fiber from many different food sources, but only fiber from whole grains was associated with lower risk of colorectal cancer. In the study, those subjects who ate the most whole grains had a 20 percent lower risk of colorectal cancer than those who ate the least.

The observed protective effect of whole grain consumption was stronger for rectal cancer (35 percent lower risk).

The Fiber Controversy

AICR nutrition experts have been following the conflicting and often contradictory findings on fiber and colorectal cancer for years.

Much of the previous research simply measured the particpants' total fiber intake. But when so much of the American diet is made up of heavily processed grains (in pasta, cereals and breads), it is helpful to distinguish between different food sources of fiber.

Why? Because scientists are learning that there's something different about whole grain fiber.

The "Whole" Story

All grains, from the familiar (wheat, oats, rye, corn) to the less well-known (barley, bulgur, millet, quinoa) start out as kernels. The bran (the outermost layer of the kernel) is where most of the fiber is found. The germ (the kernel's center) is where most of the vitamins, minerals and fatty acids reside. In between lies the endosperm, which contains a few vitamins and minerals and most of the starch.

Because the refining process removes the bran and germ, the main component of white bread and other products made from refined grains or white flour is starch. The reason whole grain products are darker and chewier than refined grain products is because all three layers of the kernel are ground together to make whole grain flour.

This provides the kernel's full complement of protein, antioxidants, fatty acids and a host of phytochemicals. Most importantly, perhaps, the fiber content of whole grains can be as much as four times that of refined grains.

Why Seek Them Out?

The evidence connecting consumption of whole grains to reduced risk for cancer, cardiovascular disease, stroke and diabetes comes chiefly from population studies and laboratory work. Only recently have researchers begun to identify specific ways a diet high in whole grains promotes health.

In February 2006, for example, a study published in the American Journal of Clinical Nutrition found that a high whole-grain intake had an observable, across-the-board effect on a variety of physiological indicators (or markers) associated with both diabetes and cardiovascular disease.

The evidence for whole grains specifically lowering cancer risk is less strong, although a large 2003 European study with over half a million participants found that high consumption of fiber (from fruits, vegetables and whole grains) reduced risk for colon cancer by 25 percent.

Recently, a Cornell University researcher discovered that whole grains are packed with more antioxidants than was previously expected. These potent health-promoting substances bind directly to the two layers (the germ and the bran) that are discarded in the refining process.