In general, processes that break down structural barriers to digestion reduce resistant starch content, with greater reductions resulting from processing. Whole grain wheat may contain as high as 14% resistant starch, while milled wheat flour may contain only 2%. Resistant starch content of cooked rice may decrease due to grinding or cooking. Other types of processing increase resistant starch content. If cooking includes excess water, the starch is gelatinized and becomes more digestible. However, if these starch gels are then cooled, they can form starch crystals resistant to digestive enzymes (Type RS3 or retrograded resistant starch), such as those occurring in cooked and cooled cereals or potatoes (e.g., potato salad). Cooling a boiled potato overnight increases the amount of resistant starch. Resistant starch is considered both a dietary fiber and a functional fiber, depending on whether it is naturally in foods or added. Although the U.S. Institute of Medicine has defined total fiber as equal to functional fiber plus dietary fiber, U.S. food labeling does not distinguish between them. The Institute of Medicine Panel on the Definition of Dietary Fiber proposed two definitions: functional fiber as "isolated, nondigestible carbohydrates that have beneficial physiological effects in humans", and dietary fiber as "nondigestible carbohydrates and lignin that are intrinsic and intact in plants." They also proposed that the prior classifications of soluble versus insoluble be phased out and replaced with viscous versus fermentable for each specific fiber. Starch has been consumed by people and animals for thousands of years. Thus, foods containing resistant starch are already commonly consumed. It has been estimated that average resistant starch intake in developed countries ranges from 3–6 grams/day for Northern Europeans, Australians and Americans, 8.5 grams/day for Italians and 10–15 grams/day in Indian and Chinese diets. The higher consumption of starch-containing foods like pasta and rice likely accounts for higher intake of resistant starch in Italy, India and China. Several studies have found that the traditional African diet is high in resistant starch. Rural black South Africans consume an average of 38 grams of resistant starch per day by having cooked and cooled corn porridge and beans in their diets. Isolated and extracted resistant starch and foods rich in resistant starch have been used to fortify foods to increase their dietary fiber content. Typically, food fortification utilizes RS2 resistant starch from high amylose corn, RS3 resistant starch from cassava and RS4 resistant starch from wheat and potato, as these sources can survive varying degrees of food processing without losing their resistant starch content.