What is clear, however, is that participants only benefited when fiber came from grains, like oatmeal, cornmeal, and brown rice. Fiber from fruits, vegetables, and beans had no impact on death risk. "Whole grains are rich sources of fiber, but also good sources of vitamins, minerals, and other phytochemicals that may provide health benefits," Park says. And grains have powerful antioxidant and anti-inflammatory properties--another reason researchers say grain fiber is beneficial.
Clearly, "all fibers are not created equal," cautions James Anderson, an endocrinologist at the University of Kentucky-Lexington. "Different fibers have different properties." And while the latest study emphasizes grain fiber, past studies have found that fiber from fruits and vegetables can benefit heart health.
Here's a field guide to fiber sources:
Soluble fiber: Foods high in soluble fiber, so called because of its ability to dissolve readily in liquids--include oat bran, oatmeal, beans, peas, rice bran, barley, citrus fruits, strawberries, and apple pulp, according to the American Heart Association. Soluble fibers have been linked to lower levels of "bad" cholesterol. Viscous fibers found in foods like oat bran and beans seem to work particularly well because they form a gel in the gut that slows down fat formation and absorption, says Joanne Slavin, a professor of food science and nutrition at the University of Minnesota and author of the American Dietetic Association's 2008 position paper on dietary fiber. (The Food and Drug Administration allows heart disease health claims for oats, barley, and psyllium, the fiber found in Metamucil.) Soluble fibers also regulate blood glucose levels, says Anderson. But most soluble fibers, except psyllium, don't have the laxative effects that many people associate with fiber, so don't rely on them for that purpose.
Insoluble fiber: High levels of insoluble fiber, too, have been associated with a lower risk of heart disease--perhaps through other mechanisms. So while there's an ongoing debate over which types of fiber confer which heart-protective benefits, the take-home is that no one should rely solely on soluble fibers to get the maximal heart benefit. The AHA lists whole-wheat breads, wheat cereals, wheat bran, cabbage, beets, carrots, brussels sprouts, turnips, cauliflower, and apple skin as foods high in this type of fiber. Unlike soluble fiber, it doesn't dissolve in liquids or form a gel but instead passes through the digestive tract pretty much unchanged. Because insoluble fiber hustles things along in the digestive tract, it's also a good source of relief if you're constipated. In addition, insoluble fiber (and to some extent, the soluble kind) may help you feel fuller and possibly help weight control.
Resistant starch and others: The starch products not digested in the small intestine "fit the newer definitions of fiber," says Slavin. They're found in legumes as well as starches like potatoes, pasta, and rice that have been cooked and cooled (as in potato or pasta salad, or sushi), and barely ripe bananas. And they're also being added to foods to increase fiber content without affecting taste, as well as to reduce caloric density; a product called Hi-maize, for example, is added to pastas and energy bars. In addition, resistant starch is a "prebiotic" that, when fermented in the large intestine, increases beneficial bacteria, says Hope Warshaw, a nutritionist and author of the Real-Life Guide to Diabetes. (She's also a consultant to National Starch, the maker of Hi-maize.) It doesn't, however, seem to have the cardiovascular effects of other soluble fibers, says Anderson.
Yogurts with added fiber actually contain inulin, a group of simple sugars that are not digested. Inulin occurs naturally in chicory root and other plants and grains and is a form of soluble fiber but, like resistant starches, doesn't have the same anticholesterol effects, says Slavin. It, too, has prebiotic effects.
So what's the bottom line? The federal government's just-released Dietary Guidelines for Americans call for about 25 grams of daily fiber for women and 38 for men, and research shows we are getting only about 15 grams. But because the health benefits of different types of fiber vary--and in many cases are not clear or consistent--the best advice is to eat an array of plant-based foods, including fruits, vegetables, legumes, and whole grains. That's despite the new findings that only fiber from grains is linked to a lower risk of early death. Worry less about targeting specific types of fiber and "get [it] from as many different foods as you can," advises Slavin. And if you are selecting packaged foods on the basis of their fiber content, be sure that they are healthful in and of themselves. Cracklin' Oat Bran, for example, has 6 grams of fiber per serving, but 30 percent of its calories come from sugar and it has 3 grams of saturated fat.
--This article was originally published on 5/5/2009. It has been updated.