Fiber and whole grains may cut cancer risk

Nutrition Health Review, Summer, 2004

Using a new method, researchers have discovered that whole grains like corn, whole wheat, oats, and brown rice exhibit a level of anti-cancer activity that is equal to, and sometimes greater than, the level known to occur in vegetables and fruits.

In findings reported at the American Institute for Cancer Research (AICR) International Conference on Food, Nutrition and Cancer, researcher Rui Hai Liu, M.D., Ph.D., and his colleagues at Cornell University found that whole grains contain many potent antioxidants that have gone undocumented for years because researchers have not known how to look for them,

The finding may begin to clear up one of the most controversial and confusing questions in contemporary diet-cancer science: the role of high-fiber diets in lowering colon cancer risk. For years, scientists have wondered why populations that consume diets high in fiber-rich whole grains consistently have lower risk for colon cancer, yet short-term clinical trials involving small groups of subjects yield inconsistent results.

The answer may lie in the fact that clinical trials have tended to overlook a possible role for whole grains in favor of focusing on the role of fiber alone in lowering colon cancer risk, often to the point of giving subjects isolated fiber supplements. Dr. Liu's revelation about whole grains' protective activity suggests that future clinical studies would benefit from a broader perspective--one that accounts for the collective, interactive effects of whole grains themselves, the fiber they contain, and the various protective substances Dr. Liu's work has revealed.

"This research also serves to underscore the need for Americans to opt for whole-grain choices whenever they can," said Melanie Polk, RD, AICR Director of Nutrition Education. "Whether it's boiling up some whole wheat pasta, asking for brown rice at an Asian restaurant, trying unusual grains like millet and kasha, or simply making sure that your bread is 100% whole wheat, we have more reasons than ever to make room for whole grains."

Previous Research Looked for Antioxidants in the Wrong Place

For years, scientists have been measuring the antioxidant power of many natural plant substances (which are collectively called phytochemicals). But researchers tended to measure only the "free" forms of these substances--the kind that dissolve quickly and are thus immediately absorbed into the bloodstream. They did not concern themselves with the "bound" forms of these substances--the kind that are attached to the walls of plant cells and do not get absorbed into the bloodstream until they are released by intestinal bacteria during digestion.

"In vegetables and fruits, antioxidants mostly occur in the free, soluble form. But we discovered that the majority of antioxidants found in whole grains occur in the bound form," said Dr. Liu. "And previously, researchers examined whole grains with the same process used to measure antioxidants in vegetables and fruits. As a result, the amount and activity of antioxidants in whole grains has been vastly underestimated for years."

Dr. Liu believes his findings may partially explain why diets high in whole grains can help reduce the incidence of colon cancer, breast cancer, prostate cancer, heart disease and diabetes.

Comparison Reveals a Substantial Difference

Dr. Liu's research shows dramatic differences between the antioxidant composition of vegetables and fruits and whole grains.

Phenolics are one major class of phytochemicals that have been widely studied. Included in this broad category are such compounds as quercetin, curcumin, ellagic acid, catechins, and many others that make frequent appearances in the health news. "Phenolics are powerful antioxidants that work in multiple ways to prevent disease," said Dr. Liu.

Dr. Liu and his colleagues measured the relative amounts of phenolics--and the forms in which they occur--in common vegetables like broccoli and spinach. They found that, on average, phenolics in the "free" form made up 76 percent of the total number of phenolics present. In common fruit like apples and red grapes, "free" phenolics again averaged about 76 percent of the total.

In whole grains, on the other hand, "free" phenolics accounted for less than 1 percent of the total. Ninety-nine percent of the phenolics occured in the "bound" form.

Yet despite these relative differences in phenolic make-up, the total antioxidant activity in produce and in whole grains is similar, according to Dr. Liu's research. The researchers measured the antioxidant activity of different foods, and assigned each a number based on a formula (micromoles of vitamin C equivalent per gram.) Broccoli and spinach measured 81 and 80, respectively, in total antioxidant activity, while apples and bananas measured 98 and 65.

Of the whole grains tested, corn had the highest total antioxidant activity at 181, followed by whole wheat at 77, oats at 75, and brown rice at 56.

Refined Wheat Flour Loses Most of Its Protective Phytochemicals

Dr. Liu believes that the key to whole grain's enormous cancer-fighting potential lies in its very wholeness. A grain of whole wheat is composed of three parts: endosperm, bran, and germ. When wheat, or any grain, is refined, the bran and germ are removed. "These two parts make up 15 to 17 percent of the weight," Dr. Liu said, "but most of the protective phytochemicals are in them, as well as the fiber. Eighty-three percent of the phenolics occur in the bran and germ."