Yes, fiber - and magnesium. But especially fiber. The standard American diet is very deficient in fiber. The recommended amount (which is low) is 25g, and most Americans get about 5g on average! Some good sources of fiber include oatmeal, beans (dried, prepared), Bush's Vegetarian Baked Beans (one of my favorites!) and fruits with the skins intact (such as apples and pears) or with the pulp (citrus). Fiber supplements are also available, but it is better to get fiber from whole foods whenever possible.