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—he 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. And for more shocking truths about the food you eat, read this list of the 30 "healthy" foods that aren't.

All Fiber is Created Equal

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.

More: High-Fiber Breakfast Recipes

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. Learn how thousands of average guys lost millions of pounds—and how you can, too.

More: Foods That Burn Belly Fat

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."

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