What exactly does fiber do?

Editor's note: Dr. Melina Jampolis, CNN's diet and fitness expert, is a physician nutrition specialist and the author of " The Calendar Diet: A Month by Month Guide to Losing Weight While Living Your Life ."

Q: I've heard so much about fiber. What is it, what does it do and should you eat it or avoid it?

Fiber is a plant-derived carbohydrate that cannot be digested by humans, so it passes through your system relatively intact and has little to no caloric value.

There are two types of fiber: Soluble (dissolves in water) and insoluble (does not dissolve in water). Both play an important role in optimal health and occur naturally -- often together -- in foods such as whole grains, fruits, vegetables, nuts, seeds and beans.

Fiber acts as a natural laxative by increasing stool bulk, which allows stool to pass more readily through the colon. It can also help those with loose stools and may play a role in the management of irritable bowel syndrome. Many types of soluble fiber also act as prebiotics, feeding healthy bacteria in your gut, thereby further contributing to colon health.

Fiber, especially soluble fiber, also improves blood sugar control by slowing down the rate that food empties from your stomach, thereby delaying the rise in blood sugar after meals and preventing excess or exaggerated insulin release. Delayed stomach emptying may also help with weight loss by improving hunger control.

In addition, fiber adds bulk to your diet without adding calories and helps to reduce the calorie density of your diet, one of the most important strategies for long-term weight loss. Soluble fiber can help lower cholesterol, and a high-fiber diet may help reduce blood pressure, inflammation and reduce your risk of heart failure (especially fiber from whole grains).

Studies show that those with the highest intake of fiber have a significantly lower risk of dying of any cause. A recently published study by the National Cancer Institute that included almost 400,000 participants found that for every 10-gram increase in fiber intake, risk of death dropped 12% in men and 15% in women.

Women should consume about 25 grams of fiber per day and men should consume 38 grams. The majority of Americans get less than half the daily recommended amount of fiber.

Naturally occurring fiber is generally your best bet. Many products, including cookies, crackers, drinks, sugary cereals and even ice cream, are now adding fiber to appear more healthy. In most cases, these added fibers do not have the same health benefits as naturally occurring fiber.

Aim for a variety of sources of naturally fiber-rich foods including fruits, vegetables, whole grains, nuts, seeds and beans to ensure that you get all the health benefits -- along with a host of other health-promoting vitamins, minerals and phytonutrients.

Refined and highly processed grains and fruit juices are stripped of their naturally occurring fiber (and many nutrients), so it is important to choose whole foods for optimal health.

If you have trouble getting in adequate amounts of fiber daily or suffer from bowel problems or high cholesterol, talk to your doctor or a registered dietitian about adding a fiber supplement to your diet.

If you don't currently eat enough fiber, increase your fiber intake slowly to avoid unpleasant gut symptoms (such as gas and bloating) and make sure to drink plenty of water.

If you are gluten-sensitive or intolerant, there are a number of gluten-free grains, including quinoa, brown rice, oats and corn. For a full list of gluten-free grains, visit the nonprofit Whole Grains Council website .

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