Resistant Starch Definition

In 1992, a concerted action of European researchers defined resistant starch as ''the sum of starch and the products of starch degradation not absorbed in the small intestine of healthy individuals.'' This concept completely changed our understanding of the action of carbohydrates in the diet because up until the early 1980s, it was thought that starches were completely digested and absorbed in the human small intestine. Three important considerations are attached to this physiological definition. First, resistant starch is made up not only of high-molecular weight polymers but also can include dextrins, small oligosaccharides, and even glucose, all derived from digested starch that escapes absorption. Second, resistant starches reach the human large intestine where they are metabolized by the complex colonic microflora. Finally, the actual amount of resistant starch in a food (i.e., the amount reaching the colon) depends on the physiology of the individual and it may be affected by age.

Classification and Dietary Sources

Food starches can be classified according to the way they are metabolized by the human small intestine into those that are rapidly digested, those that are slowly digested, and those that are resistant to digestion. Similarly, resistant starch has been classified into three types: physically inaccessible starch, resistant starch granules, and retrograded starch (Table 1).

Physically inaccessible starch (RS1) Type I resistant starch is physically inaccessible and is protected from the action of a-amylase, the enzyme that hydrolyzes the breakdown of starch in the human small intestine. This inaccessibility is due to the presence of plant cell walls that entrap the starch, for example, in legume seeds and partially milled and whole grains. RS1 can also be found in highly compact processed food like pasta. The RS1 content is affected by disruption of the food structure during processing (e.g., milling) and, to some extent, by chewing.

Resistant granules (RS2) Starch granules are plant organelles where starch is produced and stored. Each plant has characteristic starch granules that differ in size, shape, amylose to amylopectin ratio, crystalline to amorphous material ratio, starch supramolecular architecture, and amylose-lipid complexes, amongst other features. It is believed that combinations of these factors make some granules more resistant to the attack of digestive enzymes than other granules. Type II resistant starch is found in unripe bananas, uncooked potatoes, and high amylose starches. RS2 disappears during cooking, especially in water, because a combination of water and heat make the starch gelatinize, giving more access to amylases.

Retrograded starch (RS3) Type III resistant starch is the most abundant of the resistant starches present in food. It is formed during usual food processing by cooking and then cooling. When starch is cooked in an excess of water, it gelatinizes, i.e., the granular structure is disrupted, the granule swells, and amy-lose leaks out of the amylopectin matrix. Then, when the food is cooled down, amylose (and more slowly amylopectin) recrystallizes to a new ordered and more compact structure (process known as retrogradation), which decreases access for digestive enzymes. RS3 production can be affected by the amylose to amylopectin ratio, amount of water, and temperature during cooking, and the number of repeated cooking and cooling cycles. Retrograded starch can be found in bread, some brands of corn flakes, cooked-cooled potatoes, and legumes.

Others sources of resistant starch In recent years, amylose-lipid complex and modified starches have also been recognized as other sources of resistant starches (Table 1). Amylose-lipid complexes occur when fatty acids (12-18 carbons) are held within the helical structure of amylose. They are formed naturally during starch biosynthesis, but may also be produced during cooking. Lipids may interfere with amylose retrogradation, impairing the production of retrograded starch during processing. However, these complexes themselves have lower digestibility than cooked starch.

As well as naturally resistant starch complexes, there are different types of modified starches that are manufactured by the food industry for a variety of reasons. They can be defined as native starches that have been submitted to one or more physical, chemical, or enzymatic treatments promoting granular disorganization, polymer degradation, molecular rearrangements, oxidation, or chemical group addition. Modified starches can be classified into four main categories accordingly to their main physi-cochemical characteristic: pregelatinized, derivatized, cross-linked, and dextrinized starches (Table 2). However, they usually are known as physically, chemically, or enzymatically modified starch because of the way they are produced (Table 3). The digestibility of these modified starches is variable and depends on the type and extent of the treatment. Some authors have proposed a new category, type IV resistant starch, to include chemically modified starches. Indeed, it has been shown that cross-linked starches have a 15-19% decrease in in vitro digestibility when compared with their native starches, and hydroxypropylated starch is only 50% digestible. However, pregelatinized starches produced by drum drying and extrusion have a 3-6% and 5-11% decrease in digestibility, respectively. Part but not all of this reduction in digestibility is due to the formation of retrograded starch; therefore, physically modified starches should also be considered as a category of resistant starch.

In addition to the starch properties already described, several starchy foods (for instance, cereals and legumes) have antinutritional factors, such as lectins, tannins, phytates, and enzyme inhibitors (both protease and amylase inhibitors). Amylase inhibitors present in raw pulses may reduce the activity of amylase in the human small intestine. However, most of these factors, especially enzyme inhibitors, are inactivated during food processing and cooking.

Analysis

The definition of resistant starch is based on its physiological behavior in the human small intestine, i.e., resistant starch is a heterogeneous group of molecules from small monosaccharides to large polymers with different molecular weight, degree of polymerization, and supramolecular architecture.

Table 2 Classification of modified starches

Starch

Modifying agent

Physicochemical characteristic

Use in food

Pregelatinized

Extrusion Drum drying

Soluble in cold water

Cake and instant products

Hydroxypropyl Phosphate

Stable at freeze-thawing cycles

Canned and frozen food

Cross-linked

Epiclorhydrine

Stable at higher temperatures,

Meat sauce thickeners

Trimetaphosphate

extreme pH, and higher shear forces

Instant soup Weaning infant food Dressings

Dextrinized

Acid hydrolysis

Soluble in cold water

Chewing gums

Oxidizing agents

Lower or nil viscosity

Jelly

Irradiation

Syrups

Heat (pyrodextrins)

Amylolytic enzymes

Table 3 Methods of modified starch production

Treatment

Modification

Description

Physically Pregelatinization Starch paste is precooked modified and dried by extrusion or drum drying Dextrinization Starch polymers are hydrolyzed to smaller molecules by irradiation Chemically Derivatizationa Lateral groups are added modified to starch lateral chains

Cross-linkinga Multifunctional groups are used to link two different starch molecules together Dextrinization Starch polymers are hydrolyzed by oxidizing agents, acid hydrolysis, pyrodextrinization

Enzymatically Dextrinization Starch polymers are modified hydrolyzed to smaller molecules by incubation with amylases aDouble-derived starches are produced by combination of these two processes.

This complexity makes it difficult to quantify accurately. All in vitro methods therefore need to be corroborated against in vivo models; however, in vivo models are also very difficult to validate.

In general, in vitro methods try to imitate human small intestine digestion using different sample preparation (i.e., milling, chewing, etc.), sample pretreatment (i.e., simulation of oral or stomach digestion), sample treatment (i.e., different enzymes mixtures), sample post-treatment (i.e., different resistant starch solubilizing agents and enzyme mixtures), and incubation conditions (i.e., shaking/stirring, pH, temperature, time) (Table 4). The choice of each of these multiple factors represents a huge analytical problem because not only a compromise between physiological conditions and analytical handling has to be achieved, but also because the resistant starch content values must be in agreement with in vivo data.

On the other hand, in human in vivo methods, samples of digested food that reach the end of the small intestine are taken for analysis, either from ileostomy patients (i.e., where the large intestine has been removed) or from healthy volunteers using special cannulas in the ileum. Animals can also be employed for in vivo experiments, such as gnotobiotic (i.e., germ-free) and pseudognotobiotic (i.e., antibiotic treated) rats. In these cases, colonic bacterial fermentation is absent or suppressed by antibiotics and it is assumed that what reaches the end of the small intestine appears in feces. The main difficulty with these methods is that in vivo starch digestion may occur during the whole transit through the small intestine, which varies between individuals and the type of meal consumed. Moreover, these studies are difficult to perform in healthy volunteers and the physiological significance of using ileostomy patients is debatable, for example, it may not relate to infants and children who have decreased digestive capacity.

The initial in vitro assays were adapted from the enzymatic-gravimetric method used for dietary fiber assessment, but could only measure RS3. Soon new approaches to assess other types of resistant starch were developed. The Berry method, for instance, measures both RS3 and RS2 using an exhaustive incubation (16 h) of milled sample with a-amylase and pullulanase, following by centrifugation to separate the insoluble residue, which contains the resistant starch. This residue is treated with KOH to disperse retrograded and native starches, which are then hydrolyzed to glucose with amyloglucosidase. Finally, released glucose is quantified by a colori-metric assay. The Berry method has been subsequently modified by Faisant et al. and Goni et al.: the pullulanase was eliminated from the enzyme mixture and a pretreatment with pepsin added to decrease starch-protein interactions (Table 4).

Other methods have been developed to assess all types of resistant starch. Indeed, the Englyst method was developed to assess all nutritionally important starch fractions, such as rapidly digestible and slowly digestible starches, along with the three types of resistant starches described above. In this method, resistant starch fractions are estimated altogether by difference between total and digestible starches. Sample preparation is kept to a minimum in an attempt to mimic the way food is consumed. After pretreatment with pepsin, the sample is incubated with a mixture of amylogluco-sidase, invertase, and pancreatic enzymes for 2 h. Glucose released is then used to estimate the digestible starch. Next, total starch is measured as glucose released after solubilization of the nondigestible fractions with KOH, followed by amyloglucosidase hydrolysis. The Englyst method also allows evaluation of RS1, RS2, and RS3. The main problem with this method is its low reproducibility, especially between laboratories, because of the technical difficulties involved. Two other methods include chewing by volunteer subjects as sample preparation. In the Muir method, for instance, the chewed sample is sequentially treated with pepsin and an amyloglucosidase-pancreatic amy-lase mixture to obtain the nondigestible fraction, which is then boiled with Termamyl (a thermostable a-amy-lase) and solubilized with dimethyl sulfoxide followed by another amyloglucosidase-pancreatic amylase mixture step to yield finally glucose. The Akerberg method is similar to the Muir method, but it includes other

Table 4 Comparison between different methods to measure resistant starch in vitro

Method

Sample

Treatment incubation

Types of RS measureda

Preparation

Pretreatment

Treatment

Post-treatment

Berry (1986)

Milling

None

Pancreatic «-amylase and

KOH"

Shaking for 16 h at

Sum of RS2 and

pullulanase

Amyloglucosidase

37°C, pH 5.2

rs3

Faisant et al. (1995)

Same as

Same as above

Same as above, but without

Same as above

Same as above, but

Same as above

above

pullulanase

pH 6.9

Goni etal. (1996)

Same as

Pepsin

Same as above

Same as above

Same as above

Same as above

above

Englyst et al. (1992)

Minced or

Pepsin

Pancreatic «-amylase,

Same as above

Shaking for 2 h at

RS-|, RS2, rs3,

as eaten

amyloglucosidase, and

37°C, pH 5.2

and total RS

invertase

Muir & O'Dea (1992)

Chewing

Salivary «-amylase

Pancreatic «-amylase and

Thermostable «-amylase

Stirring for 15h at

Total RS

then pepsin

amyloglucosidase

Dimethyl sulfoxide''

37°C, pH 5.0

Amyloglucosidase and

pancreatic «-amylase

Akerberg et al. (1998)

Same as

Same as above

Same as above

KOH"

Stirring for 16 h at

Same as above

above

Thermostable «-amylase

40°C, pH 5.0

Amyloglucosidase

McCleary & Monaghan (2002)

Milling

None

Same as above

KOH"

Shaking for 16 h at

Sum of RS2 and

Amyloglucosidase

37°C, pH 6.0

rs3

aRS, resistant starch; RS-i, physically inaccessible starch; RS2, resistant granules; RS3, retrograded starch. ^KOH and dimethyl sulfoxide are used as resistant starch solubilizing agents.

steps that permit the estimation of available starch and dietary fiber along with resistant starch (Table 4).

Recently, the most commonly used in vitro methods were extensively evaluated and a simplified version was proposed (McCleary method). Here, samples are treated with an amyloglucosidase-pan-creatic amylase mixture only and the insoluble residue, after washing with ethanol, is dispersed with KOH, followed by the amyloglucosidase step to yield glucose. This protocol has been accepted by AOAC International (AOAC method 2002.02) and the American Association of Cereal Chemists (AACC method 32-40) (Table 4).

Regarding the quantification of the resistant fractions in modified starches, care must be taken because some nondigestible fractions are soluble in water and they can be lost during washing steps. This is particularly important with pregelatinized starches and pyrodextrins. One suitable way to look at the impact of the modification on the starch availability is measure total starch before and after the modification.

gases (H2, CO2, and CH4). Acetate is the main SCFA produced (50-70%) and is the only one to reach peripheral circulation in significant amounts, providing energy for muscle and other tissues. Propionate is the second most abundant SCFA and is mainly metabolized by the liver, where its carbons are used to produce glucose (via gluconeogenesis). Propionate has also been associated with reduced cholesterol and lipid synthesis. Finally, butyrate is mainly used as fuel by the colonic enterocytes, but has been shown in vitro to have many potential anticancer actions, such as stimulating apoptosis (i.e., programed cell death) and cancer cell differentiation (i.e., increasing expression of normal cell function), and inhibiting histone deacetylation (this protects the DNA). Resistant starch fermentation has been shown to increase the molar proportion of butyrate in the colon.

The main physiological effects of digestion and fermentation of resistant starch are summarized in Table 5. However, most of these effects have

Dietary Intake

It is very difficult to assess resistant starch intake at present, because there are not enough data on the resistant starch content of foods. In addition, as the resistance of the starch to digestion depends on the method of cooking and the temperature of the food as eaten, the values gained from looking at old dietary intake data may be misleading. Despite this, an average value for resistant starch intake across Europe has been estimated as 4.1 gday-1. Figures comparable with this estimation have been made in other countries, for instance, Venezuela (4.3gday-1). It is very difficult to separate the benefits of slowly, but completely, digestible starches from those that are resistant. In some groups like small children, whose small intestinal digestive capacity is reduced, the very same food may provide more starch that is resistant to digestion than it would in normal adults.

Quantification of modified starch intake is even more difficult. First, food labels do not usually provide information about the nature of the modification used. Second, the commonly used method to estimate resistant starch can underestimate any nondigestible fractions that became soluble in water because of the modification. At present, there is no data available on how much modified starch is eaten.

Fermentation in the Colon

The main nutritional properties of resistant starch arise from its potential fermentation in the colon. The diverse and numerous colonic microflora ferments unabsorbed carbohydrates to short-chain fatty acids (SCFA), mainly acetate, propionate and butyrate, and

Table 5 Physiological effects of resistant starch intake

Energy

Glycemic and insulinemic response

Lipid metabolism

Fermentability

SCFA production

CO2 and H2 production Colonic pH

Bile salts

Colon cell proliferation

Fecal excretion

Transit time Nitrogen metabolism

Minerals

Disease prevention

8-13 kJg-1; cf. 17kJg-1 for digestible starches

Depends on food, e.g., legumes (high in RS-i) and amylose-rich starchy foods (which tend to produce RS3 on cooking) increase glucose tolerance, but cornflakes and cooked potatoes, both with high and similar glycemic indexes, have different resistant starch content Decreases plasma cholesterol and triacylglyceride levels in rat, but not in humans Complete, although some RS3 are more resistant Increased production, especially butyrate Occurs

Decreased, especially by lactate production Deoxycholate, a secondary bile salt with cytotoxic activity, precipitated due to the low pH Stimulated in proximal colon, but repressed in distal colon; may be mediated by butyrate At high dose, fecal bulk increases due to an increase in bacteria mass and water retention Increased intestinal transit at high dose Increased bacterial nitrogen and biomass

May increase calcium and magnesium absorption in large intestine Epidemiological studies suggest prevention against colorectal cancer and constipation been observed with a resistant starch intake of around 20-30 g day-1, which represents from 5 to 7 times the estimated intake for the European population.

all enter the colon intact (nondigestible oligosac-charides). Table 6 shows several examples of oli-gosaccharides (and disaccharides, for comparison purposes), their chemical structure, and source.

Oligosaccharides Definition and Classification

Oligosaccharides are carbohydrate chains containing 3-10 sugar units. However, some authors also include carbohydrates with up to 20 residues or even disaccharides. Oligosaccharides can be made of any sugar monomers, but most research has been carried out on fructooligosaccharides (e.g., oligofructose) and galactooligosaccharides (e.g., raffinose, human milk oligosaccharides). Few oligosaccharides are hydrolyzed and absorbed in the small intestine (e.g., maltotriose), but nearly

Dietary Sources and Intake

The first source of oligosaccharides in the human diet is mother's milk, which contains approximately 12 gl-1. In human breast milk, there are over 100 different oligosaccharides with both simple and complex structures. They are composed of galactose, fucose, sialic acid, glucose, and N-acetylglucosamine. Most are of low molecular weight, but a small proportion are of high molecular weight. Ninety per cent of breast milk oligosaccharides are neutral; the remainder are acidic. Interestingly, the nature of these oligosac-charide structures is determined by the mother's blood group. These oligosaccharides may have

Table 6 Chemical structure and source of sugars and oligosaccharides

Common name Simplified structurea Source NDOb

Table 6 Chemical structure and source of sugars and oligosaccharides

Common name Simplified structurea Source NDOb

Sugars (disaccharides)

Lactose

Gal/31 !

4Glc

Milk, milk products

No

Maltose

Glca1 !

4Glc

Glucose syrups, hydrolysis

No

of starch

Sucrose

Fru^2 !

1Glc

Table sugar

No

Cellobiose

Glc^l !

4Glc

Hydrolysis of cellulose

Yes

Trehalose

Glcal !

1Glc

Mushrooms, yeast

No

Melibiose

Gala1 !

6Glc

Hydrolysis of raffinose

Yes

Gentiobiose

Glc^l !

6Glc,3

Plant pigments, like

Yes

saffron

Trisaccharides

Maltotriose

Glcal !

4Glca1 ! 4Glc

Glucose syrups, hydrolysis

No

of starch

Umbelliferose

Galal !

2Glca1 !

-> 2Fru,3

Plant tissues

Yes

Raffinose

Gala1 !

6Glca1 !

-> 2Fru,3

Legume seeds

Yes

Planteose

Gala1 !

>6Fru^2 !

-> 1Glc

Plant tissues

Yes

Sialyla(2-3)lactose

NeuAca2 ! 3Gal^1 ! 4Glc

Human milk

Yes

Tetrasaccharides

Stachyose

Gala1 !

6Gala1 !

-> 6Glca1 ! 2Fru,3

Legume seeds

Yes

Lychnose

Gala1 !

6Glca1 !

-> 2Fru^1 ! 1Gal

Plant tissues

Yes

Isolychnose

Gala1 !

6Glca1 !

-> 2Fru^3 ! 1Gal

Plant tissues

Yes

Sesamose

Gala1 !

6Gala1 !

-> 6Fru^2 ! 1Glc

Plant tissues

Yes

Pentasaccharides

Verbacose

Gala1 !

6Gala1 !

-> 6Gala1 ! 6Glca1 ! 2Fru,3

Plant tissues

Yes

Lacto-N-fucopentaose I

Fuca1 !

>2Gal^1 -

! 3GlcNAc^1 ! 3Gal^1 ! 4Glc

Human milk

Yes

Lacto-N-fucopentaose II

Gal/31 !

3[Fuca1

! 4]GlcNAc^1 ! 3Gal^1 ! 4Glc

Human milk

Yes

Fructans

Oligofructose

[Fru^2 -

^ 1]Fru^2

! 1Glc with 1-9 [Fru^2 ! 1]

Hydrolysis of inulin or

Yes

residues

synthesis from sucrose

Inulin (polysaccharide)

[Fru^2 !

^ 1]Fru^2

! 1Glc with 10-64 [Fru^2 ! 1]

Artichokes

Yes

residues

aFru, D-fructose; Fuc, L-fucose; Gal, D-galactose; Glc, D-glucose; GlcNAc, N-acetylglucosamine; NeuAc, N-acetylneuraminic acid (or sialic acid).

bNDO, nondigestible oligosaccharides.

important function in the small intestine, where they can bind to the mucosa or to bacteria, interfering with pathogenic bacterial attachment and thus acting as anti-infective agents. As they are nondigestible, they enter the colon and may act as a major energy for the colonic microflora and promote the growth of typical lactic acid bacteria that are characteristic of the normal breast-fed infant. More recently, oligosaccharides have been added to some infant formulas to mimic the actions of those in human milk. Recently, several studies have shown that these promote the growth of bifidobacteria in feces and make the stools more like those of breast-fed infants in terms of consistency, frequency, and pH.

In adults, the main dietary sources of oligosacchar-ides are chicory, artichokes, onions, garlic, leeks, bananas, and wheat. However, much research has been carried out on purified or synthetic oligosaccharide mixtures, mostly fructooligosaccharides derived from inulin. The normal dietary intake of oligosaccharides is difficult to estimate, as they are not a major dietary component. Around 3gday-1 has been suggested in the European diet. However, with the increasing information on the health benefits of isolated oligosacchar-ide sources (see below) they are being incorporated into functional foods.

Analysis

In general, oligosaccharides are a less heterogeneous group of compounds than resistant starches. Almost all nondigestible oligosaccharides (some fructooligo-saccharides are an exception) are soluble in 80% (v/v) ethanol solution, which makes them relatively easy to isolate from insoluble components. Liquid chromatography, more specifically high-performance anion exchange chromatography (HPAEC), has been extensively employed not only to separate mixtures of different oligosaccharides, but also to separate, identify, and quantify individual carbohydrate moieties after appropriate hydrolysis of the oligosaccharide to its individual monomers. A more comprehensive study of the oligosaccharide structure can be achieved using more sophisticated techniques, like nuclear magnetic resonance and mass spectrometry. However, from a nutritional viewpoint, where simpler methods are needed for quality control and labeling purposes, HPAEC is usually applied to quantify the monomers (and dimers) present before and after hydrolysis of the studied oligosaccharide with appropriate enzymes and then the oligosaccharide level is worked out by difference.

Fermentation in the Colon and Health Benefits

Most oligosaccharides escape digestion in the small intestine and are fermented by the colonic bacteria. They are rapidly fermented resulting in a low pH and have been shown to increase the survival of so-called probiotic organisms, i.e., lac-tobacilli and bifidobacteria. Probiotic bacteria have been show to have strain-specific effects, including reduction in duration of rotavirus and other infective diarrhea and reduction in symptoms of atopic eczema. They may also have some anticarcinogenic effects, but these have not been demonstrated in human in vivo studies. This action of oligosaccharides to promote the growth of bifidobacteria and lactobacilli defines them as prebiotics. Some studies are now investigating the synergistic effects of probiotics mixed with prebio-tics. These mixtures are termed synbiotics. In addition to these actions, some oligosaccharides have similar health benefits to fermentable dietary fiber and resistant starch by increasing colonic fermentation, production of SCFA (especially butyrate), and reduction in colonic pH.

The Most Important Guide On Dieting And Nutrition For 21st Century

The Most Important Guide On Dieting And Nutrition For 21st Century

A Hard Hitting, Powerhouse E-book That Is Guaranteed To Change The Way You Look At Your Health And Wellness... Forever. Everything You Know About Health And Wellness Is Going To Change, Discover How You Can Enjoy Great Health Without Going Through Extreme Workouts Or Horrendous Diets.

Get My Free Ebook


Post a comment