David L. Katz, MD
Photo: Mackenzie Stroh
Q: I eat well and exercise, and to make sure I get plenty of fiber, I take a supplement daily. Is that safe? Is there a difference between the various brands?
— Barbara Capaldi, Havertown, Pennsylvania

A: The main drawback to supplements is that they lack the vitamins, minerals, and antioxidants you get when you eat high-fiber foods such as vegetables, fruits, whole grains, beans, and lentils. You should aim for at least 25 grams of fiber a day if you're 50 or younger, and 21 grams if you're over 51. You can get close to that with a bowl of bran cereal for breakfast and bean soup for lunch.

And frankly, those foods are far more enjoyable to consume than a powder or a pill. That's why I recommend getting fiber from your meals. Supplements should be just that—supplemental, not a substitute.

But if you're committed to pills or powders, then yes, they're generally safe—with plenty of water (at least a cup per dose). The water is key because without it, the supplements could cause a bowel obstruction.

Another concern with supplements is that fiber can reduce absorption of certain minerals and interfere with several types of drugs (antidepressants, cholesterol and diabetes medications, among others). If you take mineral pills or prescription medications, talk to your doctor about the wisdom of fiber supplements. She may suggest taking your meds at least two hours before the fiber.

Many fiber pills and powders feature what's known as soluble fiber, although some offer a mixture of soluble and insoluble. You can't digest the insoluble type; its primary job is moving waste through your digestive tract. Soluble fiber does dissolve in your intestines, and the resulting gummy substance not only acts as roughage but also slows down the absorption of fats and sugar. As a result, this type of fiber can help lower cholesterol, blood sugar, and insulin levels.

While many brands make claims about the particular benefits of their fiber blend, the supplements should behave very similarly to each other—and to the fiber you get from food—when they reach your gut.

David L. Katz, MD, is director of the Yale-Griffin Prevention Research Center and president of the nonprofit Turn the Tide Foundation.