Resistant starch

Resistant starch (RS) is starch and starch degradation products that escape from digestion in the small intestine of healthy individuals.^[1] 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 undigestible 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 and high amylose corn.

• RS3 Resistant starch that is formed when starch-containing foods are cooked and cooled, such as legumes, bread, cornflakes, potatoes, sushi rice or pasta salad.^[2] Occurs due to retrogradation, which refers to the collective processes of dissolved starch becoming less soluble after being heated and dissolved in water and then cooled.

• 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.

There is some discussion about resistant dextrins being described as "resistant starch." Resistant dextrins are not starches, and they can be soluble or insoluble. They might be described as "starch degradation products," a phrase which is included in the EURESTA definition, but their characteristics and performance are very different from those of insoluble resistant starches.

As functional fiber

Resistant starch is considered both a dietary fiber and a functional fiber, depending on whether it is naturally in foods or added.^[3][4][5] The U.S. Institute of Medicine has defined total fiber as equal to functional fiber plus dietary fiber,^[6] and U.S. food labeling doesn't distinguish between them.^[7]

In 1971, Painter and Burkitt suggested^[10] that a significant gap exists between the amount of dietary fiber urbanized people consume and the optimal amount of fiber for health and wellness, but some skepticism remains.^[11][12] In 1982, Englyst et al.^[13] gelatinized starch then post-processed it with both alpha-amylase and pullulanase in order to analyze it, found that some starch remained, and called it resistant starch.^[14][15] In 1986, Berry formed^[16] functional RS3^[17] dietary fibers by a process of heating and cooling^[18] a variety of starch sources, one of which was amylopectin pre-processed with the enzyme pullulanase. That source had the second highest alpha-amylase resistant starch level, while amylomaize or high-amylose starch had the highest.^[19][20][21] In 2007, the Federal Register published a 2001 U.S. Institute of Medicine (IOM) Panel on the Definition of Dietary Fiber's response to a request from the U.S. Food and Drug Administration. The IOM Panel 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 with respect to each specific fiber.^[22]

The National Academy of Sciences of the Institute of Medicine within the United States has recommended a daily fiber intake of 38 grams for adult men and 25 grams for adult women. Many countries around the world recommend 25-30 grams for their populations.

Potential health benefits

Public health authorities and food organizations such as the Food and Agricultural Organization, the World Health Organization,^[23] the British Nutrition Foundation^[15] and the U.S. National Academy of Sciences^[5] recognize resistant starch as a beneficial carbohydrate. The WIC program (Women, Infants, Children) includes many foods high in resistant starch. 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."^[23]

Research of RS2 resistant starches from high amylose corn indicates benefits in intestinal/colonic health as well as metabolic benefits in glycemic management,^{[24][25][26][27][28][29]} satiety and hunger ^[30][31][32] and eye health.^[33][34] Different types and sources of resistant starch are digested and/or fermented differently and thus must be considered individually.^{[35][36][37][38][39][40]}

Weight management

Consumption of foods containing natural resistant starch seems to positively affect weight management in six 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.^[41] 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 altering the secretion of hormones related to food digestion, are considered likely mechanisms.^[42]

• Calorie reduction: Resistant starch can be used to replace higher calorie food ingredients, such as 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).^[43][44] Consequently, resistant starch is a valuable tool for formulators of reduced-calorie foods.

• Satiety: Multiple recent studies have shown that naturally occurring resistant starch (from high amylose corn and from barley kernels) increases satiety and reduces food intake in the short term (within a few hours) and longer-term (for 20–24 hours).^{[32][45][46][47]} However, chemically modified resistant wheat starch, chemically modified resistant potato starch and retrograded, acid-treated high amylose resistant corn starch was shown to have no impact on subjective satiety and no impact on subsequent food intake.^[48][49][50]

• Lipid oxidation: Resistant starch may help burn fat and may lead to lower fat accumulation. One clinical trial with high amylose corn resistant starch showed that it increased fat oxidation after a meal. These findings suggest a possible metabolic effect of resistant starch that may affect body weight.^[51]

• Fat storage: Resistant starch from high amylose corn has been shown to improve fatty acid metabolism within adipose tissue. A human clinical trial^[29] found increased levels of lipases (hormone-sensitive lipase (HSL), lipoprotein lipase (LPL), and adipose triglyceride lipase (ATGL)) which may indicate increased adipocyte differentiation within the adipose tissue.^[52]

Glycemic (blood sugar) management

Consumption of natural resistant starch by humans has been shown to result in

• Decreased glycemic response in healthy individuals,^[53] decreased glycemic response in diabetics.^[54]

• Increase insulin sensitivity in healthy individuals,^[24][25] individuals with Type II diabetes^[27] as well as insulin resistant individuals.^[28][29] One study found a 50% increase in insulin sensitivity in overweight men consuming 15 grams of resistant starch/day for 4 weeks.^[55] One additional study found that 12 grams of ingredients containing resistant starch did not improve insulin sensitivity in African American adults at risk for diabetes.^[56]

• Increase glycemic health of next generation when fed to pregnant mothers An animal study demonstrated that when RS2 resistant starch from high amylose corn was fed to pregnant, diabetic rats, it resulted in increased insulin sensitivity and preservation of beta-cell mass within the pancreas of the dams as well as lower fasting blood glucose levels in the pups.^[57]

• Improves first phase insulin secretion A human clinical trial demonstrated improved first phase insulin secretion when RS2 resistant starch from high amylose corn was fed to overweight, insulin resistant adults.^[58]

Animal studies have shown that RS2 resistant starch from high amylose corn prevented the development of insulin resistance (compared to high glycemic starch), but does not reverse insulin resistance that has already developed.^[59][60][61]

Digestive system

Natural resistant starch appears to help maintain a healthy colon and a healthy digestive system, possibly via several mechanisms.

• It encourages the growth of healthy bacteria in the bowel and, therefore, is called "prebiotic fiber." The fermentation of natural resistant starch reduces intestinal pH and the production of potentially harmful secondary bile acids, ammonia, and phenols.^[62]

• It is predicted to help maintain "regularity" with a mild laxative effect due to increased microbial activity in the large intestine. Its bulking effects are generated by increasing the bacterial mass, and not through water holding, which accounts for its milder regularity effects compared to non-fermenting, bulking fibers like cellulose.^[63][64][65]

• It may help 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-carcinogenic as well as anti-inflammatory properties^[66] that are important for keeping colon cells healthy.^[67][68] 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,^[69] and induces apoptosis or programmed cell death of human colorectal cancer cells.^[70][71]

• Resistant starch may contribute to oral rehydration solutions for the treatment of diarrhea.^[72][73]

• Laboratory studies indicate that RS protects against experimental colo-rectal cancer^[74] and diet-induced colonocyte genetic damage through short-chain fatty acids ^[75] However, resistant starch did not prevent colon cancer in two international clinical trials in individuals with Lynch Syndrome,^[76][77] or in young people with familial adenomatous polyposis.^[78][79] It has been speculated that the colon cancer prevention trials did not feed their subjects sufficient quantities of resistant starch.^[80] Human studies suggest that >20 g of RS/day is needed to raise SCFA as reported by faecal levels.^[81] The clinical trial in individuals with Lynch Syndrome only fed their subjects 9 grams of resistant starch/day.

References