What: Fiber is actually not one substance but a variety of compounds with different effects in the body. Plant foods contain a mix of two general types of fiber—soluble and insoluble—each with its own health benefits.
Soluble fiber forms a gel-like substance that delays absorption of glucose (sugar) in the intestines and helps prevent cholesterol absorption. Soluble types include gums (such as guar), pectins, mucilages (such as psyllium), beta-glucan and oligosaccharides (such as inulin). Insoluble fiber—parts of plant cell walls, like cellulose—increases stool bulk and speeds transit of food through the gut.
Where: The best sources of soluble fiber are oats, barley, legumes (beans, peas, lentils), psyllium, seeds and some fruits (such as apples, blueberries and citrus) and vegetables (such as okra and broccoli). Oats are particularly rich in cholesterol-lowering beta-glucan.
The best sources of insoluble fiber are cereals (especially those containing wheat bran), nuts and some fruits and vegetables (like beets, cauliflower and cabbage).
Though we don’t have the enzymes to digest fiber, some is broken down (fermented) by bacteria in our large intestines. Thus, another way of classifying fiber—a way that some experts now prefer—is by how fermentable it is (that is, how rapidly it gets broken down). Highly fermentable fibers include gums, insulin and most other soluble fibers.
How much? Aim for 14 grams of fiber for every 1,000 calories you consume—for example, 21 grams if you eat 1,500 calories a day and 35 grams if you eat 2,500 calories. People with diabetes should aim for even more fiber—15 to 25 grams per 1,000 calories.
Consume a variety of foods to get a mix of fiber compounds. Choose whole grains over refined and whole fruits over juices. Compare food labels to find products with higher amounts of fiber.
What about fiber-fortified foods? Manufacturers are increasingly adding isolated (or “functional”) fibers to foods such as yogurt, ice cream, bread, cereals and juices. These fibers include inulin, pectin, polydextrose, methylcellulose and maltodextrin. They may have some health benefits of their own—for example, some act as “prebiotics” and stimulate growth of “friendly” bacteria in the colon.
But there’s not much evidence that adding fiber to food has the same effects as eating foods that are naturally high in fiber. Moreover, different dietary fibers have different physiological effects, and many fiber-fortified foods contain only one type of fiber, not the range found in naturally high-fiber foods.
Fiber-fortified foods can help boost your overall fiber intake, but you’re better off eating fiber-rich unprocessed foods—whole grains, legumes, vegetables and fruit—which contain a range of natural fibers and other substances that have benefits beyond their fiber.