Resistant starch
Resistant starch (RS) is starch that escapes digestion in the small intestine of healthy individuals.  Resistant starch is considered the third type of dietary fiber, as it can deliver some of the benefits of insoluble fiber and some of the benefits of soluble fiber.
Some carbohydrates, such as sugars and most starch, are rapidly digested and absorbed as glucose into the body through the small intestine and subsequently used for short-term energy needs or stored. Resistant starch, on the other hand, resists digestion and passes through to the large intestine where it acts like dietary fiber.
Resistant starch has been categorized into four types:
- RS1 Physically inaccessible or digestible resistant starch, such as that found in seeds or legumes and unprocessed whole grains
- RS2 Resistant starch that occurs in its natural granular form, such as uncooked potato, green banana flour and high amylose corn
- RS3 Resistant starch that is formed when starch-containing foods are cooked and cooled such as in bread, cornflakes and cooked-and-chilled potatoes or retrograded high amylose corn
- RS4 Starches that have been chemically modified to resist digestion.  This type of resistant starches can have a wide variety of structures and are not found in nature.

# As fiber
Resistant starch is one of three types of dietary fiber.
Soluble Fiber (e.g., pectins, gums, mucillages, and some hemicellulose)  Soluble fiber is found in fresh and dried fruit, vegetables, oats, legumes and seeds.  Some soluble fibers increase viscosity of the intestinal contents and assist in reducing cholesterol absorption. Other soluble fibers are fermented by bacteria within the large intestine and can assist in maintaining colon health and increasing mineral absorption.
Insoluble Fiber (e.g., cellulose, lignin and hemicellulose)  Insoluble fiber is found in the plant cell walls of whole grain bread, whole grain cereals, fruits, vegetables, unprocessed bran and wheat germ.  Insoluble fiber provides bulking and helps keep people regular.  Many insoluble fibers, including cellulose and psyllium, are not fermented.
Resistant Starch  Starch that resists digestion is found in foods such as legumes, bananas (especially under-ripe, slightly green bananas), and unprocessed whole grains.  Natural resistant starch is insoluble, is fermented in the large intestine and is a prebiotic fiber.  Other types of resistant starch may be soluble or insoluble, and may or may not have prebiotic properties.
Examples of Naturally-Occurring Resistant Starch
Public health and scientific experts widely recognize that a significant gap exists between the amount of fiber most of people consume and the optimal amount of fiber for health and wellness. The National Academy of Sciences of the Institute of Medicine within the United States has recommended a daily intake of 38 grams/day for adult men and 25 grams/day for adult women. Many countries around the world recommend 25-30 grams of fiber/day for their populations.  Resistant starch assists in increasing dietary fiber consumption because it can be incorporated into foods without impacting the taste or texture of the food.

# Health benefits
Many public health authorities and food organizations such as the Food and Agricultural Organization, the World Health Organization, the British Nutrition Foundation and the U.S. National Academy of Sciences recognize resistant starch as a beneficial carbohydrate. The Joint Food and Agricultural Organization of the United Nations/World Health Organization Expert Consultation on Human Nutrition stated, "One of the major developments in our understanding of the importance of carbohydrates for health in the past twenty years has been the discovery of resistant starch.
Substantial research of natural resistant starches from high amylose corn indicates benefits in intestinal/colonic health as well as metabolically important benefits in glycemic management and energy. Studies have shown that different classes of resistant starch are digested and/or fermented differently and thus must be considered individually.

# Weight management
Consumption of foods containing natural resistant starch positively affects weight management in three ways.
Fiber fortification: When added to foods such as bread, biscuits, sweet goods, pasta, nutritional bars and cereal, resistant starch can increase fiber content without affecting taste or texture. In 2003, the World Health Organization concluded that dietary fiber was the only dietary component that had convincing evidence showing a protective effect against weight gain and obesity.  While the exact mechanisms of fiber protecting against weight gain are still under investigation, its ability to increase satiety and decrease subsequent hunger, along with the altering the secretion of hormones related to food digestion, are considered likely mechanisms.
Calorie reduction:  Resistant starch lowers the caloric content of foods when it is used to replace flour or other rapidly digested carbohydrates. Natural resistant starch delivers between 2-3 kilocalories/gram (8-12 kilojoules/gram) versus 4 kilocalories/gram (16 kilojoules/gram).
Lipid oxidation:  Resistant starch helps burn fat and may lead to lower fat accumulation. A recent clinical trial with high amylose corn resistant starch showed that it increased fat oxidation after a meal. It also changed the sequence in which the body burns food  with fat burning being placed at the top of the list relative to carbohydrates and protein. These findings suggest a possible metabolic effect of resistant starch that may impact body weight.

# Energy management
Eating foods with natural resistant starch helps balance energy in the hours following a meal, mitigating a drop in blood sugar.
Resistant starch releases part of its energy in the small intestine as glucose, the exclusive energy source for the brain, and part of its energy in the large intestine as fermentation by-products used as an energy source in muscle and fat tissue.
Resistant starch delivers slowly released energy, providing moderate rises in blood sugar levels. Resistant starch also helps moderate the rapid rise in blood glucose resulting from consumption of processed carbohydrates.  Rapid change in glycemic impact is commonly experienced as fluctuation in energy, particularly as people age.

# Blood sugar response/glycemic management
Natural resistant starch helps maintain healthy blood sugar levels by increasing insulin sensitivity in healthy people. Incorporation of resistant starch into processed foods (i.e. as a flour substitute) reduces the glycemic impact of that food and increases insulin sensitivity, which research suggests may help to reduce the risk of type 2 diabetes.
Studies suggest continual exposure to elevated levels of insulin as a result of a high glycemic diet may contribute to reduced sensitivity by cells to the insulin (insulin resistance) and a higher risk of diabetes. As insulin resistance increases, the body produces more insulin to maintain adequate blood sugar control. With rising resistance, even more insulin is required, and the body may not be able to keep up or the pancreatic cells producing insulin may stop functioning.
Consumption of natural resistant starch by humans has been shown to result in decreased glycemic response in healthy individuals, decreased glycemic response in diabetics, and increased insulin sensitivity in healthy individuals.

# Digestive/colonic health
Natural resistant starch helps maintain a healthy colon and a healthy digestive system via several mechanisms.
It helps to keep you "regular with a mild laxative effect.
It encourages the growth of healthy bacteria in the bowel and discourages the growth of potentially harmful bacteria, called prebiotic fiber. The fermentation of natural resistant starch reduces intestinal pH and the production of potentially harmful secondary bile acids, ammonia and phenols.
It helps to keep colon tissue healthy by producing protective compounds called short-chain fatty acids.  One of these, called butyrate, is particularly important for colon health because it is the primary energy source for colonic cells and has anti-inflammatory properties that are important for keeping colon cells healthy.  In addition, butyrate has anti-carcinogenic properties.  Published research has shown that butyrate inhibits the growth and proliferation of tumor cell lines in vitro, induces differentiation of tumor cells, producing a phenotype similar to that of the normal mature cell,  and induces apoptosis or programmed cell death of human colorectal cancer cells.
It contributes to oral rehydration solutions for the treatment of diarrhea.

# Gluten free
Resistant starch can act as a replacement for wheat products in foods that are required to be gluten-free.  Recent scientific studies suggest that resistant starchs fermentation within the colon may be important because it produces more butyrate than other fibers tested.  Butyrate, a short-chain fatty acid, has been shown to have anti-carcinogenic properties and anti-inflammatory properties, which may be useful for preventing and/or treating Celiac disease and inflammatory bowel disease.