You need both types of fiber in your diet. Soluble, which dissolves in water, can help lower cholesterol and glucose levels. Good sources of soluble fiber include oats, beans, apples, citrus fruits, carrots and barley. Insoluble fiber, which doesn't dissolve in water, promotes the movement of material through your digestive system and can help with constipation or irregular stools. Good sources of insoluble fiber include whole-wheat flour, wheat bran, nuts and many vegetables.