Resistant starch (RS) is starch, including its degradation products, that escapes (resists) digestion in the small intestine of healthy individuals. Resistant starch occurs naturally in foods, and can be used as an additive in manufactured foods. It is considered to be one of three starch types, along with rapidly digested starch and slowly digested starch. All three may affect levels of blood glucose. Some types of resistant starch (RS1, RS2, and RS3) are fermented by the large intestinal microbiota, producing short-chain fatty acids, increased bacterial mass, and promotion of butyrate-producing bacteria. Resistant starch has physiological effects similar to dietary fiber, behaving as a mild laxative and possibly causing flatulence.
Origin and history of the concept The concept of resistant starch arose in the 1970s from research supported by the European Commission.
Health effects Resistant starch does not release glucose within the small intestine but, rather, reaches the large intestine, where it is consumed or fermented by colonic bacteria (gut microbiota). On a daily basis, human intestinal microbiota encounter more carbohydrates than any other dietary component. This includes resistant starch, non-starch polysaccharide fibers, oligosaccharides, and simple sugars, which have significance in colon health. The fermentation of resistant starch produces short-chain fatty acids, including acetate, propionate, and butyrate, promoting increased bacterial cell mass. The short-chain fatty acids are produced in the large intestine where they are rapidly absorbed from the colon, then are metabolized in colonic epithelial cells, liver or other tissues. The fermentation of resistant starch produces more butyrate than other types of dietary fibers. Studies have shown that resistant starch supplementation was well tolerated. Modest amounts of gases, such as carbon dioxide, methane, and hydrogen, are also produced in intestinal fermentation. One review estimated that the acceptable daily intake of resistant starch may be as high as 45 grams in adults, an amount exceeding the total recommended intake for dietary fiber of 25–38 grams per day. When isolated resistant starch is used to substitute for flour in foods, the glycemic response of that food is reduced. There is limited evidence that RS2 and RS3 resistant starch can improve fasting glucose, fasting insulin, insulin resistance and sensitivity, especially in individuals who are diabetic, overweight or obese. In 2016, the U.S. FDA approved a qualified health claim stating that resistant starch might reduce the risk of type 2 diabetes, but with qualifying language for product labels that limited scientific evidence exists to support this claim. Because qualified health claims are issued when the science evidence is weak or not consistent, the FDA requires specific labeling language, such as the guideline concerning resistant starch: "High-amylose maize resistant starch may reduce the risk of Type 2 diabetes. FDA has concluded that there is limited scientific evidence for this claim." Natural types of resistant starch (RS2 and RS3) may reduce appetite, especially with doses of 25 grams or more, and may reduce low-density cholesterol. Natural resistant starches are under preliminary research for their possible effects on inflammatory biomarkers, including interleukin-6, tumor necrosis factor alpha, and C-reactive protein. Chemically modified resistant starches, (RS4), have been shown to reduce glycemic response in foods, but the fermentation-related effects have not yet been determined, as their fermentation-related pathways often differ compared to natural types of resistant starch.
Starch structure Plants store starch in tightly packed granules, consisting of layers of amylose and amylopectin. The size and shape of the starch granule varies by botanical source. For instance, the average size of potato starch is approximately 38 micrometers, wheat starch an average of 22 micrometers and rice starch approximately 8 micrometers.
Raw starch granules resist digestion, e.g., raw bananas, raw potatoes. This does not depend on the amylose or amylopectin content, but rather the structure of the granule protecting the starch. When starch granules are cooked, water is absorbed into the granule causing swelling and increased size. In addition, amylose chains can leak out as the granule swells. The viscosity of the solution increases as the temperature is increased. The gelatinization temperature is defined as the temperature at which maximum gelatinization or swelling of the starch granule has occurred. This is also the point of maximum viscosity. Further cooking will burst the granule apart completely, releasing all of the glucose chains. In addition, viscosity is reduced as the granules are destroyed. The glucose chains can reassociate into short crystalline structures, which typically involves rapid recrystallization of amylose molecules followed by a slow recrystallization of amylopectin molecules in a process called retrogradation. Plants produce starch with different types of structure and shape characteristics which may affect digestion. For instance, smaller starch granules are more available to enzyme digestion because the larger percentage of surface area increases the enzyme binding rate. Starch consists of amylose and amylopectin which affect the textural properties of manufactured foods. Cooked starches with high amylose content generally have increased resistant starch.
Definition and categorization Resistant starch (RS) is any starch or starch digestion products that are not digested and absorbed in the stomach or small intestine and pass on to the large intestine. RS has been categorized into five types:
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