Anthony L. Komaroff, M.D.
— DEAR DOCTOR K: I recently read that fiber doesn't prevent colorectal cancer. So is a high-fiber diet good for you or not?
DEAR READER: Many claims have been made about the health benefits of fiber. Yet studies have disagreed. With all the back and forth, I can understand why people are confused. And why they sometimes tune out and say, "Call me when you've discovered the truth."
Without claiming to have discovered the truth, let me tell you what I think the best evidence shows. First, a word of explanation about fiber. It's a form of indigestible carbohydrate that's found mainly in plant foods.
Fiber was once thought to play an important role in preventing colon cancer. As you've read, that turned out not to be the case. In my view, most of the evidence is against a protective effect, at least in economically developed nations like the United States.
However, diets rich in fiber are still good for your health in many other ways. For example, we know that fiber slightly reduces bad (LDL) cholesterol. It improves insulin resistance (a common precursor to diabetes). And it is linked to a lower rate of heart disease and obesity.
In addition, fiber increases the bulk of foods and creates a feeling of fullness. As a result, fiber may help you avoid overeating and becoming overweight.
You should aim to eat between 21 and 38 grams of fiber daily, based on your age and gender, as follows:
On average, Americans eat only about 15 grams of fiber a day. So chances are, you could benefit from eating more fiber.
Fruits, vegetables and whole-grain foods such as whole-wheat bread, whole-grain cereals, brown rice, bran and oats are all good sources of fiber. In particular, split peas, red kidney beans, raspberries, whole-wheat spaghetti, pears, broccoli and apples are all good choices.
We have more information on fiber in our Special Health Report, "Healthy Eating." (Learn more about this report at AskDoctorK.com, or call 877-649-9457 toll-free to order it.)
You can also consume fiber in over-the-counter supplements, which provide some of the same benefits as fiber in foods. Fiber supplements come as pills or powders. Be sure to take them with plenty of water to maximize their benefits. The recently marketed "tasteless" fiber powder, which can be added to any liquid (hot or cold) you drink, is particularly effective.
However, the scientific evidence for the health benefits of fiber comes primarily from studies of fiber in food, not fiber in supplements. And the fiber-rich foods I listed earlier also contain lots of other health-giving substances.
Bottom line: Although fiber probably does not prevent colon cancer, it really is good for your health in other ways. A diet full of fiber-rich foods will bring you loads of benefits.
Dr. Komaroff is a physician and professor at Harvard Medical School. To send questions, go to AskDoctorK.com, or write: Ask Doctor K, 10 Shattuck St., Second Floor, Boston, MA 02115.
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