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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 RS1) 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)

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