The insoluble fiber in apples provides bulk in the intestinal tract, holding water to cleanse and move food quickly through the digestive system, rich in many other essential vitamins, minerals and antioxidants which prevent illness and disease. Apples are a source of both soluble and insoluble fiber. Soluble fiber such as pectin actually helps to prevent cholesterol buildup in the lining of blood vessel walls, thus reducing the risk for heart disease.