Jai Ho

Why fiber reduces the risk of early death is unclear. Perhaps it's because fiber lowers levels of "bad" LDL cholesterol, improves blood glucose levels, reduces inflammation, and binds to potential cancer-causing agents, helping to flush them out of the body, says lead author Yikyung Park, a staff scientist at the National Cancer Institute.

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.