The Truth About Fiber

You hear the advice constantly: You need fiber. It’s crucial to your health. Fine, but how much fiber, and how crucial is it? Maybe you’re wondering, What is fiber, exactly?

Let’s start with the basics. Fiber is a type of carbohydrate that makes up the structural material in the leaves, stems, and roots of plants. But unlike sugar and starch—the other two kinds of carbs—fiber stays intact until it nears the end of your digestive system.  This, it seems, is what makes fiber beneficial, and why you’ve probably heard you can’t eat enough of it. Now read on to separate the facts from the fiction.

FALSE: There are two basic types of fiber, with different functions. Insoluble fiber is found in wheat bran, nuts, and many vegetables. Its structure is thick and rough, and it won’t dissolve in water, so it zips through your digestive tract and increases stool bulk. Soluble fiber is found in oats, beans, barley, and some fruits. It dissolves in water- to form a gel-like material in your digestive tract. This allows it to slow the absorption of sugar into your bloodstream. What’s more, soluble fiber, when eaten regularly, has been shown to slightly lower LDL (bad) cholesterol levels.

Fiber Has No Calories

FALSE: Fiber is essentially composed of a bundle of sugar molecules. These molecules are held together by chemical bonds that your body has trouble breaking. In fact, your small intestine—can’t break down soluble or insoluble fiber; both types just go right through you. That’s why some experts say fiber doesn’t provide any calories. However, this claim isn’t entirely accurate. In your large intestine, soluble fiber’s molecules are converted to short-chain fatty acids, which do provide a few calories. A gram of regular carbohydrates has about 4 calories, as does a gram of soluble fiber, according to the FDA. (Insoluble fiber has essentially zero calories.)

Fiber Can Help You Lose Weight

TRUE: Fiber’s few calories are more than offset by its weight-control benefits. The conclusion of a review published in the journal Nutrition is clear: People who add fiber to their diets lose more weight than those who don’t. Fiber requires extra chewing and slows the absorption of nutrients in your gut, so your body is tricked into thinking you’ve eaten enough, says review author Joanne Slavin, Ph.D., R.D. And some fibers may also stimulate CCK, an appetite-suppressing hormone in the gut.

Fiber is All-Natural Goodness

SORT OF: Fiber is showing up in everything these days—yogurt, grape juice, artificial sweetener. If this seems impossible, remember that these are molecules; you don’t have to see or feel fiber for it to be present. Scientists now have a new class of fiber they refer to as “functional” fiber, meaning it’s created and added to processed foods. “You can make fiber from bacteria or from yeast,” says Slavin. “And as long as you prove that it can lower cholesterol or feed the good bacteria in your gut or increase stool weight, it’s fiber.”

Supplemental Fiber is Healthy

TRUE: Foods with added fiber don’t necessarily provide the benefits you might expect. Inulin, for example, a soluble fiber extracted from chicory root, can be found in products like Fiber One bars. In addition to boosting fiber content, it’s also commonly used to replace fat. Inulin is known as a prebiotic, which means it promotes the growth of healthy bacteria in your gut. That’s good, of course. “But,” says Slavin, “inulin doesn’t have the same cholesterol-lowering effect as the fiber found in oat bran.”

Food Companies are Jumping on the Fiber Bandwagon

DUH: In 2007, the FDA declared that polydextrose can be called fiber. Polywhat? Polydextrose is made from glucose, sorbitol (a sugar alcohol), and citric acid. It’s what puts the fiber in Fruity Pebbles (not actual pebbles). Polydextrose received FDA approval because it mimics some attributes of dietary fiber: It isn’t absorbed in the small intestine, and it increases stool weight. Polydextrose mainly bulks up foods so they’re not as high in calories. However, there’s no research to prove that polydextrose is as beneficial as the fiber found in whole foods.

Fiber Helps Prevent Colon Cancer

MAYBE: This idea arose in the 1960s when it was noted that fiber-scarfing Ugandans rarely developed colon cancer. But nearly five decades later, it still hasn’t been proven.

In 1999, Harvard researchers found no link between dietary fiber intake and colon cancer. But a European study that tracked more than a half million people correlated a high-fiber diet with up to a 40 percent reduced risk of colorectal cancer. Then a 2005 review in the Journal of the American Medical Association found that people who ate the same amount of fiber as those in the European study didn’t experience any benefit. The American Institute for Cancer Research calls protection “probable.” This controversy aside, high-fiber diets are associated with preventing many chronic diseases, so it’s smart to boost your intake, says Arthur Schatzkin, M.D., Dr.P.H., of the National Cancer Institute.

You Need 38 Grams of Fiber a Day

FALSE: That’s the recommendation from the Institute of Medicine. Scientists there crunched data from three studies and squeezed out the number 38 in 2005. It equals 9 apples, or 12 bowls of instant oatmeal. (Most people eat about 15 grams of fiber daily.) The studies found a correlation between high fiber intake and lower incidence of heart disease. But none of the high-fiber-eating groups in those studies averaged as high as 38 grams, and, in fact, people saw maximum benefits with a daily gram intake averaging from the high 20s to the low 30s. Also, it’s worth noting that these studies don’t show cause and effect, and that unless you’re taking a supplement, it’s hard for even those who eat the healthiest of diets to consume 38 grams of fiber. It’s fine to shoot for that amount, but you’re certainly not failing if you don’t meet it.

This is Complicated

FALSE: A simple strategy: Eat sensibly. Favor whole, unprocessed foods. Make sure the carbs you eat are fiber-rich—this means produce, legumes, and whole grains—to help slow the absorption of sugar into your bloodstream. “The more carbohydrates you eat, the more fiber becomes important to help minimize the wide fluctuations in blood-sugar levels,” says Jeff Volek, Ph.D., R.D., a nutrition researcher at the University of Connecticut.