Q. I know that I should get about 30 grams of fiber a day. What I don’t know is how much of this fiber should be soluble and how much should be insoluble. What is the exact difference between the two? Is there any difference in how these different fibers are used by the body?

A. There are a dozen or so different types of compounds that fall under the general heading of dietary fiber, including gums, pectins, lignans, beta-glucans, cellulose, etc. Nutritionists tend to categorize different types of fiber as either soluble or insoluble fiber, depending on whether or not the fibers will dissolve in water (soluble) or not (insoluble). The two have very different functions in the body.

Soluble fiber helps slow down digestion (which keeps you from getting hungry as quickly after a meal) and helps regulate blood sugar and cholesterol levels. A diet high in soluble fiber can help reduce your risk of heart disease, diabetes, and may help you lose weight. Good sources of soluble fiber include oranges, apples, carrots, oats and oat bran, psyllium husk, nuts, legumes, and flaxseed.

Insoluble fiber, on the other hand, is the type that helps prevent constipation, colon cancer and other digestive diseases. Good sources of insoluble fiber include wheat bran, corn bran, nuts and seeds, dark leafy vegetables, fruit and vegetable skins (for example, apple and potato skins).

The USDA doesn’t make a specific recommendations for how much of your fiber intake should be soluble or insoluble. I think your best strategy is to eat a wide variety of fiber-rich foods to satisfy your total daily requirements, thereby getting the best of both worlds.

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