Resistant Starch 101

About 25 years ago, scientists studying cereal grains discovered a starch that didn't exactly act like a starch;it acted more liker a fiber. Specifically, it was resistant to digestion and didn't break down until it reached the lower intestinal tract. Starch … resistant to digestion. Ergo: resistant starch.

Resistant starch occurs naturally in a number of agricultural products and in three forms. RS1 is prevalent in seeds, legumes and unprocessed or minimally processed whole grains. In RS1, the starch molecules are packed into dense granules and have more of the cell walls intact, creating a physical barrier that leaves few ends for the enzyme amylase to access.

RS2 contains much of the carbohydrate amylose. It, too, is packed into dense granules like RS1 but is not gelatinized – therefore, the starch does not break down and absorb water. RS2 therefore is digested very slowly.
It's typically found in potatoes, corn, underripe bananas and flour.

The third form of resistant starch, RS3, only becomes resistant when portions of the starch chain expand and then contract during food preparation or processing. It's another high-amylose starch, but the amylose forms during cooking, when the starch gelatinizes -- and at that point, it's digestible. However, cool the starch and the amylose condenses and becomes undigestible.
Potatoes, breads and some cereals (such as corn flakes) are common food sources of RS3.

There is a fourth form of resistant starch, RS4, but it does not occur naturally. It's formed by chemical or heat treatment and has both soluble and insoluble properties and does not always behave like a fiber.