Though they are not required to do so, some food manufacturers are voluntarily listing the amount of dietary fiber on their package labels. But until a single method for measuring food fiber is in common use, it may be difficult for consumers to compare the fiber content of various foods. And, the numbers can be meaningless unless consumers first understand approximately how much fiber they should eat each day.

Until last year, no expert body would venture to say how much fiber Americans should consume each day, because the research had been so inconclusive. But last spring, the National Cancer Institute recommended that Americans approximately double their fiber intake, from the current average of 10 to 20 grams per day, to 25 to 35 grams, depending on an individual's size. The NCI advice is based on the observation that populations which consume twice as much fiber as Americans have significantly lower colorectal cancer rates.

However, the problem with determining how much fiber a food has is that the values are often rough estimates of the actual amount of dietary fiber in that food. That's because there has been no good, single standardized method for measuring fiber in food. The reason: Fiber is really a collective term for a series of substances in a plant, none of which can be digested in the human intestine. Because scientists had to agree on the definition of fiber, and then develop a method that could measure all these components, fiber measurements of the same foods often have differed greatly.

If you're a label reader, you may have noticed that some food packages list either "crude fiber" or "dietary fiber" or both. These terms are based on the methods by which the fiber is measured.

Crude fiber measurement is derived from a century-old method, and is considered inaccurate by today's standards because the process actually destroys much of the fiber, according to June Kelsay of Carbohydrate Nutrition Laboratory at the U.S. Department of Agriculture's Human Nutrition Research Center in Beltsville. Kelsay says the crude fiber method ends up measuring only about one-fifth to one-half of the actual fiber content of the food.

When used on the label, the term "dietary fiber" usually indicates a more up-to-date method of measurement, such as the "neutral detergent fiber method," which is considered to be accurate in measuring the water-insoluble fibers found in cereals and breads. It does not pick up the water-soluble fibers, like the pectins and gums found in oatmeal, beans and many fruits and vegetables.

The newest method for fiber measurement has just won approval from the Association of Official Analytical Chemists (AOAC), the standard-setting group for analytical methodology. Developed by Food and Drug Administration scientists, working in collaboration with a group of international chemists, the method is capable of measuring all dietary fiber components and thus is referred to as the "total dietary fiber" method.

By using a series of enzymes to simulate the human digestive process, it strips away all the digestible parts of a food, leaving only the dietary fiber to be weighed by chemists. Leon Prosky, an FDA biochemist instrumental in developing the new method, says some food companies are already using the method for labeling purposes.

David Hurt, director of nutrition at Quaker Oats, Co., says his firm plans to convert to the new method and predicts many other companies will switch to it in the next six months or year. Currently, however, Quaker Oats' Mother's Oat Bran cereal is labeled with a crude fiber value, which fails to measure the water-soluble fibers found in oats. "It is confusing right now," Hurt acknowledges about the lack of standaridized fiber labeling. But he says the reason Quaker Oats decided to put crude fiber values on Mother's Oat Bran cereal was because "we were unsure" of the other methods that were available to measure soluble fibers, none of which were "officially recognized."

Many food manufacturers have continued to list crude fiber values because it has been the only method of measurement officially recognized by FDA for fiber measurement. However, that may change, according to Dr. Allan Forbes, Associate Director for Nutrition and Food Sciences at FDA, who says the agency is giving "serious thought" to finalizing a regulation that would make it clear to companies that the AOAC-approved fiber method "is the appropriate approach to measuring dietary fiber at this time."

"Companies are perfectly free to use whatever method they please," Forbes explained, but by sanctioning a specific method, FDA "puts them on notice" that it intends to use that method to determine whether a company's labeling values are correct.

In some cases, it may be to a manufacturer's advantage to stick to the outdated method. The Center for Science in the Public Interest, a consumer advocacy group, recently attacked several bread manufacturers for claiming that their reduced-calorie breads contain 400 percent more natural food fiber than whole-wheat bread. Much of the fiber in these breads are derived from wood pulp, which is listed as "alpha cellulose" in the ingredient statement.

To make the claim, the companies rely on the crude fiber method, which measures primarily cellulose and washes away most of the other fiber components. When a newer method is used, the actual fiber content of the reduced calorie breads and whole wheat bread is the same, according to CSPI Nutrition Director Bonnie Leibman. "It's foolish to allow companies to use crude fiber," says Liebman, adding. "You would think FDA would have closed this loophole."

Lew Ort, president of Ort's Inc., which makes the claim that its "Less" bread contains four times as much fiber as whole wheat, says FDA approved its label "a long time back." If the agency changes its position, "we'll certainly go along with it," Ort says, but in the meantime, "we regard fiber as pure cellulose fiber."

The availability of a reliable, commonly accepted method might also bring pressure on FDA to require fiber content labeling as part of nutritional labeling. Forbes sees fiber values on labels as an addendum to nutrition labeling, but as for requiring that dietary fiber be listed as part of the nutrition label, he responds, "Good heavens, no."

"Given the scientific base, we'd have a hard time justifying it. It's an enormously controversial area," Forbes says, contending that nutrition labeling is required only for nutrients for which there exists a "pretty firm consensus."

Since the addition of fiber to foods lowers caloric content -- it takes up space but is not digested -- there is impetus by food companies to add it to foods, according to Prosky of FDA. He predicts that companies will be working to "purify" certain fibers that can then be included as ingredients in many foods. Unfortunately, scientists don't yet know if the components of fiber, by themselves, will act in the same way in the body as fiber found naturally in foods. One of Prosky's next assignments is to find a method for measuring each component of fiber individually.

Liebman says she has no doubt that "even as we speak, food scientists are busily finding new methods of incorporating all sorts of fiber into all sorts of foods.

"I'm concerned they're going to add fiber to junky foods," she says, in the same way manufacturers have fortified foods such as powdered drinks with vitamin C, leaving consumers to infer that they have increased health value.

And when it comes to fiber, is the sky the limit? Liebman is concerned about the fiber "horserace," as manufacturers attempt to pack more and more fiber into one serving of food. General Mills, for example, recently introduced its "Fiber One" cereal, with a hefty 12 grams of fiber per one-ounce serving. At that rate, hearty eaters could be packing away 25 grams at one meal.

Liebman says populations that consume a high-fiber diet eat moderate amounts throughout the day. Little is known about whether such large amounts at one sitting could irritate the colon or cause other harmful effects. Too much fiber may also inhibit the body's ability to absorb certain minerals, such as calcium, magnesium and zinc.