There are two types of fiber in food: soluble, which dissolves in water, and insoluble, which doesn't dissolve in water. Soluble fiber can help lower blood cholesterol and blood glucose levels. Good sources of soluble fiber include oats, peas, beans, apples, citrus fruits, carrots, barley and psyllium. Insoluble fiber promotes the movement of material through your digestive system and increases stool bulk, so it can be of benefit if you struggle with constipation or irregularity. Good sources of insoluble fiber include whole-wheat flour, wheat bran, nuts and many vegetables.