What is dietary fiber?

Dietary fiber is a nutrient known as a carbohydrate. Fiber includes the parts of plant foods that the body can't digest or absorb. This makes it different from nutrients such as fats, proteins, and other carbohydrates including starches and sugars. The body breaks down these nutrients and absorbs them. Instead, fiber passes somewhat intact through the stomach, small intestine and colon and out of the body.

There are two main types of fiber:

• Soluble fiber. This type of fiber dissolves in water. It forms a gel-like material in the stomach that slows down digestion. It can help lower cholesterol and blood sugar. Soluble fiber is found in oats, peas, beans, apples, bananas, avocados, citrus fruits, carrots, barley and psyllium.
• Insoluble fiber. This type of fiber doesn't dissolve in water. It supports the movement of material through the digestive system and adds bulk to stool. So it can be helpful for people who have constipation or don't regularly pass stool. Some good sources of insoluble fiber are whole-wheat flour, wheat bran, nuts, beans, and vegetables such as cauliflower, green beans and potatoes.

Most high-fiber plant foods contain both soluble and insoluble fiber. The amount of each type of fiber varies with the type of plant, such as whether it's a fruit, vegetable or whole grain. But you can get both types of fiber by eating a variety of fiber-rich foods.