Some foods, such as whole wheat bread, are naturally high in fiber. A growing number of products, however, proudly proclaim their high-fiber content, such as Arnold's Double Fiber Bread and Yoplait's Fiber One yogurt, getting some or all of their fiber from so-called isolated or functional fibers — ingredients with names like inulin, maltodextrin and polydextrose — that manufacturers intentionally add to foods to boost total fiber content.

Whether these isolated fibers have all of the same health benefits as the naturally occurring ones remains to be seen.

"We just don't know if they all act the same," says Jennifer Nelson, director of clinical dietetics and nutrition at the Mayo Clinic in Rochester, Minn. "They have not necessarily been studied to see if they're beneficial."

Studies have shown that naturally occurring fiber, which nutrition experts call dietary fiber, can help reduce blood cholesterol levels, prevent constipation and reduce the risk of hemorrhoids and diverticulosis, which causes pouches to form in the large intestine.

This type of fiber comes from the parts of plants that are resistant to human digestive enzymes and may help people feel full, thereby aiding in weight loss. Some studies have also linked a high dietary fiber intake to a reduced risk of colon cancer.

Dietary fiber comes in two forms — one form dissolves in water, the other doesn't — and both are found in fruits, vegetables, legumes, nuts and grains. But these are foods that Americans just don't eat enough of anymore, says Mian Riaz, director of the Food Protein Research and Development Center at Texas A&M University in College Station.

Women younger than 50 need about 25 grams of fiber per day, and men younger than 50 need 38 grams (the daily values are a few grams lower for adults older than 50). But on average, American women get about 13 grams and men 17 grams, according to a 2005 report by the Food and Nutrition Board of the Institute of Medicine.

Food manufacturers have striven to help consumers fill that gap, identifying and developing a variety of fiber sources to add to everyday foods.

Some of these fiber sources are manufactured in the lab; maltodextrin and polydextrose, for example, are long chains of glucose and other molecules that are strung together. They're considered fiber because, like naturally occurring fiber, they're resistant to digestion, says Mary Ann Johnson, professor of foods and nutrition at the University of Georgia in Athens and a spokeswoman for the American Society for Nutrition.

Other types of added fiber are called isolated fiber because they're undigestible material extracted from plants. Inulin, commonly extracted from chicory root, is an example. Other examples include soy hulls, oat fibers and sorghum fibers.

These isolated fibers perform some of the same functions as dietary fibers; they can help prevent constipation and can make people feel more full after eating. Because of this, they are sometimes called "functional" fibers, says Riaz, who is also a spokesman for the Institute of Food Technologists.

But they're not the perfect equivalent of fiber that's naturally found in foods, Riaz says. "They help, but not that much. They don't have the same functionality of a whole grain."

That's because isolated or functional fibers lack the array of vitamins, nutrients, antioxidants and plant chemicals found in whole grains, fruits and vegetables and that are known to benefit health, says Jennifer Anderson, professor of food science and human nutrition at Colorado State University in Fort Collins.

According to Food and Drug Administration guidelines, a food can be labeled a "good" source of fiber if a serving contains at least 2.5 grams of fiber and "high" in fiber if a serving contains at least 5 grams. Some breads and cereals billed as high in fiber get their fiber not from isolated or functional fibers but from whole grains, such as wheat bran, corn bran or whole grain oats.To get a fiber source with the benefits of a whole grain, Nelson recommends looking for the word "bran" on the ingredient list. She also recommends looking for a product with familiar-sounding ingredients, as opposed to hard-to-interpret chemical names.

"Don't just look at the number (of fiber grams) or the health claims," Nelson says. "Dig down into the ingredients."

Better yet, says Johnson, get as much dietary fiber as possible from whole foods.

"People may want something more convenient," Johnson says, but "getting back to whole foods is really the best way to go."