November 1996: (11)s 1 1 5 s 1 19

Complex Carbohydrates and Resistant Starch Ian Brown, M.Sc.

Food has been known since earliest times to contribute more to an individual’s well-being than mere sustenance. Specific foods and the extracts derived from particular plants were believed to have beneficial and sometimes adverse effects on a person’s health. The increase in sci- entific knowledge of foods and food ingredients coupled with a heightened interest in traditional diets and “folk” health remedies has necessitated a detailed examination of the specific role of foods in health and nutrition, The term “functional foods” has come to refer to those foods that can provide specific nutritional, dietary, and meta- bolic benefits and potentially play a role in disease pre- vention, the mitigation of disease, and the control of dis- ease. The range of food and food ingredients that fall within this category is growing daily as our knowledge expands.

The physiological and nutritional importance of com- plex carbohydrates in our diets has gradually been identi- fied by both scientists and consumers. From the initial rec- ognition that the consumption of foods containing dietary fiber could reduce the incidence of several physiological conditions, including constipation, complex carbohydrates have been found to affect our health in many ways. Com- plex carbohydrates are a large group of compounds that include starch and nonstarch polysaccharides and have widely differing chemical structures and physiological effects. Many of the compounds have been either defined by the results of an in vitro chemical analysis or grouped according to their in vivo physiological effects.

One area of considerable interest arose from the rec- ognition that not all starch is readily digestible. The ex- amination of the cecal contents of people who had been the victims of sudden death revealed that starch and not nonstarch polysaccharides was the major complex carbo- hydrate present.’ Starches resistant to digestion have been defined by EURESTAZ (European Food-Linked Agro-In- dustrial Research-Concerted Action on Resistant Starch) as “some starches and products of starch degrada- tion that are not absorbed in the small intestine of healthy individuals.” Englyst et a1.3 proposed three categories for

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Dr. Brown is Principal Scientist-Applications Technology, Goodman Fielder Ingredients Limited, 45- 47 Green Street, Botany, NSW 2019, Australia.

the in vitro digestion of starch. Starch when gelatinized, for instance by cooking, is rapidly digested (rapidly di- gested starch), whereas the native starch granules from many cereals are slowly though completely digested (slowly digested starch). Finally, there are starches that are resistant to digestion (resistant starch).

Resistant Starch

Definition and Classification Resistant starch was originally considered to consist of three subcategories. The first category (RS 1) was based on the physical inaccessibility of the starch granules. This category includes starch granules, such as those trapped in the food matrix, that are prevented from complete swell- ing and dispersion? Inaccessible starch granules are found in whole or partially milled grains, legumes, and other starch-containing materials in which the size or composi- tion of the food particles prevents or delays the action of digestive enzymes. Englyst and Cummingss observed that after the consumption of a meal containing sweet corn, peas, and beans, “up to 20% of faecal solids may be starch contained in recognizable, undigested food.” The physi- cal structure of foods such as parboiled rice and spaghetti can also inhibit digestion.

The second category of resistant starch (RS2) refers to native starch granules. The degree of resistance to di- gestion of a starch granule appears to be related to the structure and conformation of the granule. X-ray difftac- tion crystallography has identified three possible arrange- ments of the crystalline structure of the granule: In each case, the starch a-glucan chains exist as left-handed, par- allel-stranded double helices. In the so-called A pattern the center of the hexagonal array is occupied by an addi- tional helix. This pattern is found in wheat and.com. Starches of this type are digestible when measured in vitro.’ When the center of the array is occupied by water, the B pattern is observed. Potato, banana, and high-amylose maize starches have the B pattern. Starch granules from these species are resistant to digestion. C patterns, found commonly in legume starches, are considered to be a com- bination of the A and B arrangements, and granules of this species are also resistant to enzyme degradation. These

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patterns are influenced by the chain length of the amy- lopectin fraction of the starch." The crystallinity of the granules may not be the reason for their relative resistance to digestion. Zobe19 showed that the crystallinity of maize starch decreases as the amylose content of the granule in- creases. The crystallinity of readily digestible waxy maize starch (100% amylopectin) is 40% compared with only 15% for resistant high-amylose maize starch.

Another factor influencing the resistance of native starch granules is the susceptibility of the granules to ge- latinization. In an aqueous medium, starch granules un- dergo a series of irreversible structural changes involving the disruption of the hydrogen bonds that stabilize the in- ternal crystalline structure of the granule when the gelati- nization temperature is reached. For potato and banana starches, the gelatinization temperature is <70°C.'0.'' How- ever, complete gelatinization of high-amylose maize starch does not occur until the temperature is in the range of 154- 17 1 OC.I2 This temperature range exceeds that normally encountered in food processing. The disruption or gelati- nization of the starch granules renders them accessible to enzyme hydrolysis. Once gelatinized, the starch is no longer resistant to digestion and ceases to be of impor- tance in this aspect of nutrition.

The third category of resistant starch (RS3) reflects the formation of retrograded starch material during pro- cessing. Starch is composed of two major components, amylose and amylopectin, that can be dispersed in water by heating above the gelatinization temperature. Upon cooling, the dispersed molecules of amylose and amylopec- tin spontaneously reassociate and can form crystallites that resist enzymatic hydroly~is.'~

Gelatinization of starch in aqueous solution causes disruption of the hydrogen bonds stabilizing the helical structure. The X-ray diffraction pattern formed by the ret- rograded starch can be manipulated by the conditions used to cool the gel. The formation of B-type crystalline mate- rial is favored by conditions involving solutions with low dispersed solids and low crystallization temperatures, which are found in the preparation of many types of f o ~ d . ~ ~ . ' ~ Repeated heating and cooling cycles have been applied to increase the amount of resistant starch that can be isolated from potato amylose and maize starch.'"16

Retrogradation is favored by the presence of linear polymer chains. High amylose content, the presence of amylopectin treated with a debranching pullulanase," or acid-hydrolyzed amylopectin can increase the opportunity for inter- and intrahelical hydrogen bonding in the retro- graded starch and for the formation of resistant starch. In bread for example, Siljestrom and Asp'" showed that un- der the limited moisture conditions that prevail in bread manufacture, resistant starch is formed by the retrograda- tion of amylose.

A fourth category of resistant starch has recently been added to those originally identified by Englyst and

C~mmings .~ Apart from the naturally occurring resistant starch types (i.e., RS 1, RS2, and RS3), some chemically modified starches resist enzyme hydrolysis to some de- gree (RS4). Modification of starch-for example, by cross- linking, esterification, and etherification-is commercially practiced to improve and extend the usefulness of starch in food processing, transportation, storage, and consump- tion. The ubiquitous distribution of these starches in pro- cessed foods means that they may be an important source of resistant starch in many societies.

Analysis The in vivo digestibility of the starch in the small intestine is thought to reflect the divisions suggested by in vitro analytical technique^.^ Analysts continue to refine testing procedures that seek to predict the digestion of starch in vivo.

Many common foods contain detectable amounts of resistant starch.15 However, these amounts can be variable and unpredictable depending on the degree of processing or cooking and the length of time and conditions of stor- age to which the starch is subjected. This variability has led analysts to try to separate resistant starch from the more analytically reproducible nonstarch polysa~charides.~ Ac- cepted methods for determining dietary fiber, such as the AOAC (Association of Official Analytical Chemists) en- zymatic-gravimetric method, include some resistant starch in the re~u1t.l~ This inclusion can be significant in starch- containing foods such as white bread, in which the dietary fiber content of the finished product can be up to 50% higher than in the flour used to make the bread. Siljestrom and Asp'" showed an increase of ~ 0 . 8 % (dry solids basis) in the resistant starch content of bread baked above an internal temperature of 48OC.

Crawf~rd '~ analyzed many foods for the presence of resistant starch. Prepared foods found to contain resistant starch included white and brown bread, crumpets, pastry, crisp bread, crackers, breakfast cereals (including corn flakes and shredded wheat), pasta, pizza dough, and chapatis. A survey conducted within the EURESTA pro- gram in the European Economic Community indicated that between 3 and 5 g resistant starch was being consumed daily by those surveyed. This intake of resistant starch could be in the form of kibbled or raw grains, unprocessed fruit such as bananas, starch-containing foods such as bread and breakfast cereals, and processed foods in which chemically modified starches were used to improve orga- noleptic properties.

Physiological Effects of Resistant Starch

Control of lnsulinemia In rats, the prolonged consumption of high-amylose maize starch instead of a readily digested low-amylose maize

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Table 1. Incremental Area Under the Plasma Insulin Curves After Intravenous Glucose Challenge in Australian Albino Specific-Pathogen-Free Male Wistar Rats

Area Under Plasma Insulin Curve (U/L Der 120 minutes) . a

Diet Period High-Amylose Diet Low-Amylose Diet 4 weeks 1387f72 1490f23 8 weeks 1651 f 379 2208 f 379'7 12 weeks 1955 f 207 4 194 f 309''

'Significantly different from high-amylose diet group, p < 0.05. 'Significantly different from animals at 4 weeks, p < 0.05. From Byrnes et aLzo

starch reduced both glucose and insulin responses in the small intestine. Byrnes et al.20 showed that in rats consum- ing a diet providing 55% of total energy as starch, plasma insulin responses to a glucose challenge diverged within 8 weeks (Table 1). After 12 weeks, rats consuming the diet containing low-amylose maize starch had basal plasma in- sulin concentrations and plasma insulin concentrations in response to a glucose challenge that were twice as high as those in the group consuming high-amylose maize starch (Table 1).

The experiments also showed that the degree of insu- lin resistance could vary in response to the type of maize and the age of the rat. The age-related worsening of insu- lin resistance was accentuated in rats consuming the diet containing low-amylose maize starch. Acute postprandial glycemic and insulin responses were both lower in rats consuming the high-amylose maize starch diet.20 The struc- ture of the starch in the diet appears to have a significant effect on insulin sensitivity in rats. The development of insulin resistance in humans takes many years, and further research is required to study the effect of starch structure and composition on a condition that can lead to the mani- festation of non-insulin-dependent diabetes mellitus (NIDDM).

In a recent clinical study conducted by the Common- wealth Scientific and Industrial Research Organisation Division of Human Nutrition in Australia to assess the effect of high-amylose maize starch on people with insu- lin resistance, postprandial insulin output was significantly reduced by 15% after a test meal that provided 33% of the carbohydrate as high-amylose maize starch compared with the low-amylose maize starch control.21 High-amylose maize starch may be of benefit in reducing insulin require- ments in people with insulin resistance.

Because high-amylose maize starch is not digested in the small intestine, the amount of energy available to the body will be less than if the starch were fully digested. Although experiments to date have not shown significant weight loss in subjects as the result of consumption of resistant starch as a portion of carbohydrate in the diet,

Table 2. Concentration of Starch in Cecal Digesta by Time After Feeding of Pigs Fed Either Low- or Hiah-Amvlose Maize Starch

Concentration (mug digesta) Starch l b e 5 h 7 h 9 h Low amylose Pig 1 1.1 2.2 0.8 Pig 2 15.7 5.5 8.6

Pig 3 29.6 65.9 No sample Pig 4 48.6 50.1 71.0

High amylose

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From D. L. Topping, J. Gooden, I. Brown, et al. A high amylose (amylomaize) starch raises proximal large bowel starch and increases colon length in pigs (submit- ted for publication).

this lack of observed effect is almost certainly due to the limited length of time involved. It will be interesting to observe longer-term effects of resistant starch on weight control and the relative proportion of muscle to fat in sub- jects consuming high-amylose maize starch.

Colonic Fermentation Starch granules have been observed in cecal digesta ob- tained from test animals such as pigs and from humans consuming diets containing high-amylose maize starch (D. L. Topping, J. Gooden, I. Brown, et al. A high-amylose [amylomaize] starch raises proximal large-bowel starch and increases colon length in pigs; submitted for publication).= The remnants of starch granules recovered from the cecum and the proximal end of the large bowel showed the effects of surface erosion by digestive enzymes. Ofien a major pit was evident in the granule, extending from the surface to the core. The core region of the granule appears to be more susceptible to amylolytic activity than the shell. This dif- ference appears to be due to a stability conferred by the polymer conformation of the shell and not to the crystallin- ity?

The amount of starch arriving in the large bowel has been measured in both pigs and humans. The cecal con- tents of cannulated pigs consuming a diet that provided 48.2% of available energy as starch were analyzed (Top- ping et a]., submitted for publication). Seven hours after the pigs were fed, the starch concentration of the digesta was between 50.1 and 65.9 mg/g when high-amylose starch was consumed compared with 2.2 to 5.5 mg/g with the consumption of low-amylose starch (Table 2). In studies of human subjects with ileostomies, Muir et reported that with the consumption of a low-resistant-starch meal containing 5 1.8 f 6.2 g starch, only 2.4 f 0.6 g starch was recovered in the effluent. However, with the high-resis- tant-starch meal (total starch = 52.7 f 8.8 g), ~ 1 9 . 9 f 5.2 g starch was present in the effluent.

Upon reaching the large bowel, resistant starch acts as a fermentation substrate for the native microflora and produces a range of effects that are believed to improve

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Table 3. Fecal Concentrations of Secondary Bile Acids in Humans Consuming Low- and High- Amylose Maize Starch Secondarv Bile Acids Low Amvlose High Amvlose Lithocholic acid 0.01 8 0.006

0.068 0.040 (mdmL) Deoxycholic acid (mg/mL)* From Noakes et a1.2' ' Results for deoxycholic acid were significant at p = 0.07.

bowel health. Fermentation of starch in the large bowel has been assessed indirectly through the measurement of in- creases in breath hydrogen and fecal short-chain fatty ac- ids and decreases in fecal pH. 213334 In a clinical study conducted by Phillips et al.,24 the consumption of 39.0 g resistant starch per day over a 3-week period reduced fecal pH from 6.9 to 6.3 and increased the levels of acetate (by 38% mmol/day) and butyrate (by 100% mmoVday). An in- crease of 34% in the fecal excretion of butyrate was ob- served by Noakes et a1.2L after the consumption of a diet providing 25% of carbohydrate as high-amylose maize starch. The amounts of short-chain fatty acids measured in the feces may underestimate the actual amounts pro- duced in the large bowel; experimental evidence suggests that the absorption of short-chain fatty acids may be en- hanced at low pH.25

Starch appears to be a desirable substrate for micro- bial fermentation in the bowel because it encourages those microorganisms that produce large quantities of short- chain fatty acids. In particular, starch is excellent for stimu- lating the synthesis of butyrate, which appears to be im- portant in regulating gene expression and the cell growth of colonocytes. Butyrate has also shown antineoplastic characteristics.26

Reduction in colonic pH has been shown to decrease the solubility of bile acids that are cytotoxic to colonic cells.27 The lower pH has also been shown to inhibit the bacterial transformation of primary to secondary bile ac- ids. The experiments of Noakes et aL2' and Topping et al. (submitted for publication) showed significant reductions in the concentrations of the secondary bile acids lithocholate and deoxycholate (Table 3).

Phillips et al.24 found that the consumption of high- amylose maize starch increased fecal bulk from 138 to 197 g wet weight per day. In addition, the number of bowel movements per day and the ease of defecation increased.

The effects of resistant starch in the bowel suggest why a recent correlational study comparing the dietary intake of starch with the incidence of colorectal cancer in 12 populations around the world indicated that higher in- takes of starch were associated with a lower risk of this cancer.28 No relationship was found between the intake of nonstarch polysaccharides and colon cancer. These results

Table 4. ProDerties of Hi-Maize Natural Source of dietary fiber and resistant starch White, invisible Survives most normal processing conditions Provides functional properties for foods

Good film formation Decreases oil absorption Increases product moisture retention Increases cereal bowl life

Possible nutritional benefits Opportunity for product innovation

From Brown et al.29

suggest the possibility of a protective role for starch.

Hi-Maize: A New Resistant Starch

The systematic examination of the starch extracted from Australian maize varieties revealed a direct relationship between the amylose content of the starch and the amount of resistant starch and dietary fiber present. The starch obtained from one conventionally bred very-high-amy- lose maize variety was released in 1993 under the trade name Hi-Maize (Starch Australia Ltd., Lane Cove, Aus- tralia). This starch is a natural product that has a uniquely high amount of resistant starch and dietary fiber in combi- nation with a range of properties of commercial interest (Table 4). Hi-Maize, unlike other natural sources of resis- tant starch, has a high gelatinization temperature that en- sures its ability to survive most normal food processing conditions. The small starch granule size, = lo pm, allows it to be intimately incorporated into food matrices without adversely affecting the organoleptic properties of the pro- cessed food. The availability of Hi-Maize has allowed the development of many innovative foods, including low-fat snacks; pharmaceutical preparations; high-fiber breads, noodles, pasta, and breakfast cereals; and foods for groups with special nutritional requirements, such as celiacs.

Conclusion

In Australia health authorities recommend that the public consume more dietary fiber and increase their bread in- take. However, some people, particularly children, refrain from consuming high-fiber multigrain and whole-meal breads because of their preference for white bread. Con- ventional types of dietary fiber, such as wheat bran, color the bread, change the texture by making it more fibrous and chewy, or decrease the softness and volume of the loaf. Hi-Maize allows the manufacture of a soft, high-fi- ber, white bread with excellent keeping qualities. Hi- Maize-containing bread was released in Australia in April 1994 under the trade name Wonder White (Quality Bak- ers Australia Ltd., Eastwood, Australia). Since its launch, Wonder White has been well accepted by consumers and has led not only to more people consuming white bread (8%) but also to more people eating bread in general

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(1.4%). This is the first time in many years that bread con- sumption has increased in Australia. A similar high-fiber white bread containing Hi-Maize has since been commer- cially released in New Zealand. It too has led to increased total bread consumption in that country.

The commercial success of these foods has indicated the usefblness of this new resistant starch ingredient. Con- sumers understand the importance of dietary fiber in the diet, but it remains a challenge to develop a nutritionally relevant image for resistant starch on the basis of its unique physiological benefits. Resistant starch is a component of our diets whenever we consume starchy foods. However, regional and national food preferences can influence an individual’s intake of starch. The realization that intake of resistant starch may have important implications for health has prompted intense international investigation and a search for a means by which to manipulate the resistant starch content of foods. The use of resistant starch in con- junction with innovative food technology and processing provides an opportunity to develop the potential of this functional food.

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