Ask the Doctors - Just why is fiber good for you?


Dear Doctors: Our family went out to dinner the other night and when I urged our kids to order a salad instead of French fries because of the dietary fiber in salads, they challenged me to explain why. I was embarrassed that all I could come up with about the benefits of fiber was “because it’s good for you.” Can you help?

Don’t feel bad! With your brief answer you got right to the heart of the matter – dietary fiber is integral to a well-balanced diet and to a healthy life. It lowers your risk of heart disease, diabetes, and certain cancers, prevents constipation, stabilizes blood sugar, and can help maintain a healthy weight.

But kids are often natural skeptics when it comes to any kind of rules, so we’re happy to help you out with the science of why dietary fiber matters.

Let’s start with what we’re referring to when we talk about fiber. Dietary fiber is the part of the fruit, vegetable or grain that your body cannot digest and absorb. It falls into two basic categories. Fiber that can dissolve in water is known as soluble fiber. Fiber that cannot dissolve in water, and which passes through the body pretty much intact, is known as insoluble fiber.

Each type of fiber offers a specific health benefit. Soluble fiber dissolves in water to form a kind of gel matrix, which puts the brakes on the entire digestive process. The presence of soluble fiber slows the absorption of sugars, which stabilizes blood glucose levels. It binds cholesterol so that instead of being absorbed by the body, cholesterol is excreted. And because the stomach takes longer to empty when soluble fiber is present, you wind up feeling fuller for longer.

Insoluble fiber, meanwhile, has an equally important job. Not only do the texture and "chew" that insoluble fiber adds to food make a meal more interesting, it travels relatively unchanged through the digestive tract and adds much-needed bulk to stool. This results in easier and more regular elimination. Insoluble fiber also helps to control the pH of the colon, and helps to prevent microbes from producing cancerous substances.

How much fiber do you need? Current recommendations put the number at 25 grams per day for women under 50, and 38 grams per day for men under 50. For adults over 50, the number is 21 grams per day for women, and 30 grams per day for men.

Although all kinds of fiber supplements are available, they lack vitamins, minerals and micronutrients, so it’s better to eat whole foods. Most fruit, vegetables, beans, legumes and grains contain both soluble and insoluble fiber, but in differing amounts. For the best results, eat from a wide range of foods.

If fiber has been missing from your diet, it’s wise to add it slowly. Make the change to a high-fiber diet too quickly and you run the risk of unpleasant side effects, such as gas. And be sure to drink plenty of liquid to keep the fiber – and your digestive tract – moving smoothly.

Eve Glazier, MD., MBA, and Elizabeth Ko, MD., are internists at UCLA Health. Dr. Glazier is an associate professor of medicine; Dr. Ko is an assistant professor of medicine.

Ask the Doctors is a syndicated column first published by UExpress syndicate.