Harvard Women's Health Watch

Ask the doctor: What are the differences between soluble and insoluble fiber?

Q. I enjoyed your article on diverticular disease and the fiber content of various foods. However, could you make some distinctions between soluble and insoluble fiber? Some fiber makes me feel very bloated.

A. Dietary fiber, sometimes referred to as roughage, consists of the indigestible parts of plant foods. As you note, there are two kinds. Soluble fiber dissolves in water; insoluble does not. Both are important for healthy digestion; both can help prevent not only diverticulitis and constipation but also heart disease, obesity, and diabetes.

Soluble fiber absorbs water in the stomach and intestine and forms a gel that slows digestion. This causes you to feel full and may help with weight loss. Soluble fiber also slows the digestion and absorption of glucose, which affects blood sugar levels and sensitivity to insulin — important factors in controlling diabetes. Finally, by interfering with the absorption of dietary cholesterol, soluble fiber decreases LDL (bad) cholesterol in the blood, which helps lower the risk of heart disease. Good sources include nuts, seeds, legumes (lentils, beans, and peas, for example), oat cereals, fruit pectin (found in citrus fruits, apples, pears, apricots, and peaches), and some vegetables, such as carrots.

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