Digestion and absorption of carbohydrates

Carbohydrates are compounds of carbon, hydrogen and oxygen in the ratio Cn:H „'O„. The basic unit of the carbohydrates is the sugar molecule or monosaccharide. Note that sugar is used here in a chemical sense, and includes a variety of simple carbohydrates that are collectively known as sugars. Ordinary table sugar (cane sugar or beet sugar) is correctly known as sucrose; as discussed in section 4.2.1.3, it is a disaccharide. It is just one of a number of different sugars found in the diet.

4.2.1 The classification of carbohydrates

Dietary carbohydrates can be considered in two main groups: sugars and polysaccharides. As shown in Figure 4.3, the polysaccharides can be further subdivided into starches and non-starch polysaccharides.

The simplest type of sugar is a monosaccharide — a single sugar unit (section 4.2.1.1). Monosaccharides normally consist of between three and seven carbon atoms (and the corresponding number of hydrogen and oxygen atoms). A few larger monosaccharides also occur, although they are not important in nutrition and metabolism.

Disaccharides (section 4.2.1.3) are formed by condensation between two monosaccharides to form a glycoside bond. The reverse reaction, cleavage of the glycoside bond to release the individual monosaccharides, is a hydrolysis.

Oligosaccharides consist of three or four monosaccharide units (trisaccharides and tetrasaccharides), and occasionally more, linked by glycoside bonds. Nutritionally, they are not particularly important, and indeed they are generally not digested, although they may be fermented by intestinal bacteria and make a significant contribution to the production of intestinal gas.

Nutritionally, it is useful to consider sugars (both monosaccharides and disaccharides) in two groups:

  • Sugars contained within plant cell walls in foods. These are known as intrinsic sugars.
  • Sugars that are in free solution in foods, and therefore provide a substrate for oral bacteria, leading to the formation of dental plaque and caries. These are known as extrinsic sugars. As discussed in section 7.3.3.1, it is considered desirable to reduce the consumption of extrinsic sugars because excessive amounts are associated with dental decay as well as obesity (section 6.3) and possibly also an increased risk of developing diabetes mellitus (section 10.7).
Amylose And Diabetes Mellitus
Figure 4.3 Nutritional classification of carbohydrates.

A complication in the classification of sugars as intrinsic (which are considered desirable in the diet) and extrinsic (which are considered undesirable in the diet) is that lactose (section 4.2.1.3) occurs in free solution in milk, and hence is an extrinsic sugar. However, lactose is not a cause of dental decay, and milk is an important source of calcium (section 11.15.1), protein (see Chapter 9) and vitamin B2 (section 11.7). It is not considered desirable to reduce intakes of milk, which is the only significant source of lactose, and extrinsic sugars are further subdivided into milk sugar and nonmilk extrinsic sugars.

Polysaccharides are polymers of many hundreds of monosaccharide units, again linked by glycoside bonds. The most important are starch and glycogen (section 4.2.1.5), both of which are polymers of the monosaccharide glucose. There are also a number of other polysaccharides, composed of other monosaccharides or of glucose units linked differently from the linkages in starch and glycogen. Collectively these are known as non-starch polysaccharides. They are generally not digested but have important roles in nutrition (section 4.2.1.6).

4.2.1.1 Monosaccharides

The classes of monosaccharides are named by the number of carbon atoms in the ring, using the Greek names for the numbers, with the ending '-ose' to show that they are sugars. The names of all sugars end in '-ose'.

  • Four-carbon monosaccharides are tetroses.
  • Five-carbon monosaccharides are pentoses.
  • Six-carbon monosaccharides are hexoses.
  • Seven-carbon monosaccharides are heptoses.

In general, trioses, tetroses and heptoses are important as intermediate compounds in the metabolism of pentoses and hexoses. Hexoses are the nutritionally important sugars.

The pentoses and hexoses can either exist as straight-chain compounds or can form heterocyclic rings, as shown in Figure 4.4. By convention, the ring of sugars is drawn with the bonds of one side thicker than on the other. This is to show that the rings are planar and can be considered to lie at right angles to the plane of the paper. The boldly drawn part of the molecule is then coming out of the paper, while the lightly

Molecule Arrangement Galactose
Figure 4.4 The nutritionally important monosaccharides.

drawn part is going behind the paper. The hydroxyl groups lie above or below the plane of the ring, in the plane of the paper. Each carbon has a hydrogen atom attached as well as a hydroxyl group. For convenience in drawing the structures of sugars, this hydrogen is generally omitted when the structures are drawn as rings.

The nutritionally important hexoses are glucose, galactose and fructose. Glucose and galactose differ from each other only in the arrangement of one hydroxyl group above or below the plane of the ring. Fructose differs from glucose and galactose in that it has a C = O (keto) group at carbon 2, whereas the other two have an H—C=O (aldehyde) group at carbon 1.

There are two important pentose sugars, ribose and deoxyribose. Deoxyribose is unusual, in that it has lost one of its hydroxyl groups. The main role of ribose and deoxyribose is in the nucleotides (see Figure 3.1) and the nucleic acids: RNA, in which the sugar is ribose (section 9.2.2), and DNA, in which the sugar is deoxyribose (section 9.2.1). Although pentoses do occur in the diet, they are also readily synthesized from glucose (section 5.4.2)

4.2.1.2 Sugar alcohols

Sugar alcohols are formed by the reduction of the aldehyde group of a monosaccharide to a hydroxyl (—OH) group. The most important of these is sorbitol, formed by the reduction of glucose. It is absorbed from the intestinal tract and metabolized only slowly, so that it has very much less effect on the concentration of glucose in the bloodstream than other carbohydrates. Because of this, it is widely used in preparation of foods suitable for use by diabetics, as it tastes sweet and can replace sucrose and other sugars in food manufacture. However, sorbitol is metabolized as a metabolic fuel, with an energy yield approximately half that of glucose, because it is poorly absorbed, so that it is not suitable for the replacement of carbohydrates in weight-reducing diets.

Xylitol is the sugar alcohol formed by reduction of the five-carbon sugar xylose, an isomer of ribose. It is of interest because, so far from promoting dental caries, as does sucrose (section 7.3.3.1), xylitol has an anti-cariogenic action. The reasons for this are not well understood, but sucking sweets made from xylitol results in a significant reduction in the incidence of caries — such sweets are sometimes called 'tooth-friendly' because of this.

4.2.1.3 Disaccharides

The major dietary disaccharides, shown in Figure 4.5, are:

  • sucrose, cane or beet sugar, which is glucosyl-fructose;
  • lactose, the sugar of milk, which is galactosyl-glucose;
  • maltose, the sugar originally isolated from malt, which is is glucosyl-glucose;
  • isomaltose, which is glucosyl-glucose linked 1^6;
sucrose (glucosyl-fructose) trehalose (glucosyl-glucoside)
lactose (galactosyl-glucose) maltose (glucosyl-glucose)

CH,OH

CH,OH

OH

Figure 4.5 The nutritionally important disaccharides.

• trehalose, the sugar found especially in mushrooms, but also as the blood sugar of some insects, which is glucosyl-glucoside.

Both maltose and isomaltose arise from the digestion of starch. 4.2.1.4 Reducing and non-reducing sugars

Chemically, the aldehyde group of glucose is a reducing agent. As shown in Figure 4.6, this provides a simple test for glucose in urine. Glucose reacts with copper (Cu2+) ions in alkaline solution, reducing them to Cu+ oxide, and itself being oxidized. The original solution of Cu2+ ions has a blue colour; the copper oxide forms a yellow— brown precipitate.

This reaction is not specific for glucose. Other sugars with a free aldehyde group at carbon-1, including vitamin C (section 11.14) and a number of pentose sugars that occur in foods, can undergo the same reaction, giving a false-positive result.

Glucose Positive
Figure 4.6 Measurement of glucose using alkaline copper reagents and glucose oxidase.

While alkaline copper reagents are sometimes used to measure urine glucose in monitoring diabetic control (section 10.7), a test using the enzyme glucose oxidase measures only glucose. As shown in Figure 4.6, glucose oxidase reduces oxygen to hydrogen peroxide; in turn, this reacts with a colourless compound to yield a coloured dyestuff that can readily be measured. High concentrations of vitamin C (section 11.14), as may occur in the urine of people taking supplements of the vitamin, can react with hydrogen peroxide before it oxidizes the colourless precursor, or can reduce the dyestuff back to its colourless form. This means that tests using glucose oxidase can yield a false-negative result in the presence of high concentrations of vitamin C (see Problem 4.1 at the end of this chapter).

It is important to realize that the term 'reducing sugars' reflects a chemical reaction of the sugars — the ability to reduce a suitable acceptor such as copper ions. It has nothing to do with weight reduction and slimming, although some people erroneously believe that reducing sugars somehow help one to reduce excessive weight. This is not correct — the energy yield from reducing sugars and non-reducing sugars is exactly the same, and excess of either will contribute to obesity.

4.2.1.5 Polysaccharides: starches and glycogen

Starch is a polymer of glucose, containing a large, but variable, number of glucose units. It is thus impossible to quote a relative molecular mass for starch, or to discuss amounts of starch in terms of moles. It can, however, be hydrolysed to glucose, and the result expressed as moles of glucose.

The simplest type of starch is amylose, a straight chain of glucose molecules, with glycoside links between carbon-1 of one glucose unit and carbon-4 of the next. Some types of starch have a branched structure, in which every 30th glucose molecule has glycoside links to three others instead of just two. The branch is formed by linkage between carbon-1 of one glucose unit and carbon-6 of the next, as shown in Figure 4.7. This is amylopectin.

Starches are the storage carbohydrates of plants, and the relative amounts of amylose and amylopectin differ in starches from different sources, as indeed does the size of the overall starch molecule. On average, about 20—25% of starch in foods is amylose, and the remaining 75—80% is amylopectin.

CH2OH

ch2oh a1^6 links: branch points in amylopectin and glycogen

CH2OH

ch2oh a1^6 links: branch points in amylopectin and glycogen

CH2OH

What Ch2oh Called

Figure 4.7 The branched structure of starch and glycogen.

CH2OH

Figure 4.7 The branched structure of starch and glycogen.

Glycogen is the storage carbohydrate of mammalian muscle and liver. It is synthesized from glucose in the fed state (section 5.6.3), and its constituent glucose units are used as a metabolic fuel in the fasting state. Glycogen is a branched polymer, with essentially the same structure as amylopectin, except that it is more highly branched, with a 1—6 bond about every 10 th glucose.

4.2.1.6 Non-starch polysaccharides (dietary fibre)

There are a number of other polysaccharides in foods. Collectively they are known as non-starch polysaccharides, the major components of dietary fibre (section 7.3.3.2). Non-starch polysaccharides are not digested by human enzymes, although all can be fermented to some extent by intestinal bacteria, and the products of bacterial fermentation may be absorbed and metabolized as metabolic fuels. The major non-starch polysaccharides (shown in Figure 4.8) are:

  • cellulose, a polymer of glucose in which the configuration of the glycoside bond between the glucose units is in the opposite configuration (^1—>4) from that in starch (a1—4) and cannot be hydrolysed by human enzymes;
  • hemicelluloses, branched polymers of pentose (five-carbon) and hexose (six-carbon) sugars;
  • inulin, a polymer of fructose that is the storage carbohydrate of Jerusalem artichoke and some other root vegetables;
  • pectin, a complex polymer of a variety of monosaccharides, including some methylated sugars;
  • plant gums such as gum Arabic, gum tragacanth, acacia, carob and guar gums — complex polymers of mixed monosaccharides;
  • mucilages such as alginates, agar and carrageen, complex polymers of mixed monosaccharides found in seaweeds and other algae.

Cellulose, hemicelluloses and inulin are insoluble non-starch polysaccharides, whereas pectin and the plant gums and mucilages are soluble. The other major constituent of dietary fibre, lignin, is not a carbohydrate at all but a complex polymer of a variety of aromatic alcohols.

4.2.2 Carbohydrate digestion and absorption

The digestion of carbohydrates is by hydrolysis to liberate small oligosaccharides, then free mono- and disaccharides. The extent and speed with which a carbohydrate is hydrolysed and the resultant monosaccharides absorbed is measured as the glycaemic index — the increase in blood glucose after a test dose of the carbohydrate compared with that after an equivalent amount of glucose.

Glucose and galactose have a glycaemic index of 1, as do lactose, maltose, isomaltose and trehalose, which give rise to these monosaccharides on hydrolysis. However, because cellulose - glucose polymer linked (31->4

cellulose - glucose polymer linked (31->4

chitin - /V-acetylglucosamine polymer linked pi->4

pectin - galacturonic acid polymer linked cc1-»4, partially methylated; some glactose and/or arabinose branches

Figure 4.8 The major types of dietary non-starch polysaccharide.

ch2oh

oh ch,oh

oh oh sO

cooh

oh oh

inulin - fructose polymer linked p2-

oh hoch2 o 9h2

ft oh ho9h2 o ch2

oh i oh i oh plant cell walls are largely cellulose, which is not digested, intrinsic sugars in fruits and vegetables have a lower glycaemic index. Other monosaccharides (e.g. fructose) and the sugar alcohols are absorbed less rapidly (section 4.4.2.3) and have a lower glycaemic index, as does sucrose, which yields glucose and fructose on hydrolysis. As discussed in section 4.4.2.1, the glycaemic index of starch is variable, and that of non-starch polysaccharides is zero.

Carbohydrates with a high glycaemic index lead to a greater secretion of insulin after a meal than do those with a lower glycaemic index; this results in increased synthesis of fatty acids and triacylglycerol (section 5.6.1), and is therefore a factor in the development of obesity (see Chapter 6). There is also some evidence that habitual consumption of carbohydrates with a high glycaemic index may be a factor in the development of non-insulin-dependent diabetes (section 10.7).

4.2.2.1 Starch digestion

The enzymes that catalyse the hydrolysis of starch are amylases, which are secreted in both the saliva and the pancreatic juice. (Salivary amylase is sometimes known by its old name of ptyalin.) Both salivary and pancreatic amylases catalyse random hydrolysis of glycoside bonds, yielding initially dextrins and other oligosaccharides, then a mixture of glucose, maltose and isomaltose (from the branch points in amylopectin).

The digestion of starch begins when food is chewed, and continues for a time in the stomach. As discussed in section 1.3.3.1, hydrolysis of starch to sweet sugars in the mouth may be a factor in determining food and nutrient intake.

The gastric juice is very acid (about pH 1.5—2), and amylase is inactive at this pH; as the food bolus is mixed with gastric juice, so starch digestion ceases. When the food leaves the stomach and enters the small intestine, it is neutralized by the alkaline pancreatic juice (pH 8.8) and bile (pH 8). Amylase secreted by the pancreas continues the digestion of starch begun by salivary amylase. Starches can be classified as:

  • rapidly hydrolysed, with a glycaemic index near 1 — these are more or less completely hydrolysed in the small intestine;
  • slowly hydrolysed, with a glycaemic index significantly less than 1, so that a significant proportion remains in the gut lumen and is a substrate for bacterial fermentation in the colon;
  • resistant to hydrolysis, with a glycaemic index near to zero, so that most remains in the gut lumen and is a substrate for bacterial fermentation in the colon.

A proportion of the starch in foods is still enclosed by plant cell walls, which are mainly composed of cellulose. Cellulose is not digested by human enzymes, and therefore this starch is protected against digestion. Similarly, intrinsic sugars (section 4.4.2.1) have a lower glycaemic index than would be expected, because they are within intact cells.

Uncooked starch is resistant to amylase action, because it is present as small insoluble granules. The process of cooking swells the starch granules, resulting in a gel on which amylase can act. However, as cooked starch cools, a proportion undergoes crystallization to a form that is again resistant to amylase action — this is part of the process of staling of starchy foods.

Much of the resistant and slowly hydrolysed starch is fermented by bacteria in the colon, and a proportion of the products of bacterial metabolism, including short-chain fatty acids, may be absorbed and metabolized. As discussed in section 7.3.3.2, butyrate produced by bacterial fermentation of resistant starch and non-starch polysaccharides has an antiproliferative action against tumour cells in culture, and may provide protection against the development of colorectal cancer.

4.2.2.2 Digestion of disaccharides

The enzymes that catalyse the hydrolysis of disaccharides (the disaccharidases) are located on the brush border of the intestinal mucosal cells; the resultant monosaccharides return to the lumen of the small intestine, and are absorbed together with dietary monosaccharides and glucose arising from the digestion of starch (section

4.2.2.1). There are four disaccharidases:

  • Maltase catalyses the hydrolysis of maltose to two molecules of glucose.
  • Sucrase—isomaltase is a bifunctional enzyme that catalyses the hydrolysis of sucrose to glucose and fructose, and of isomaltose to two molecules of glucose.
  • Lactase catalyses the hydrolysis of lactose to glucose and galactose.
  • Trehalase catalyses the hydrolysis of trehalose to two molecules of glucose.

Deficiency of the enzyme lactase is common. Indeed, it is only in people of north European origin that lactase persists after childhood. In most other people, and in a number of Europeans, lactase is gradually lost through adolescence — alactasia (see Problem 4.2). In the absence of lactase, lactose cannot be absorbed. It remains in the intestinal lumen, where it is a substrate for bacterial fermentation to lactate (section

5.4.1.2). This results in a considerable increase in the osmolality of the gut contents, as 1 mol of lactose yields 4 mol of lactate and 4 mol of protons. In addition, bacterial fermentation produces carbon dioxide, methane and hydrogen, and the result of consuming a moderate amount of lactose is an explosive watery diarrhoea and severe abdominal pain. Even the relatively small amounts of lactose in milk may upset people with a complete deficiency of lactase. Such people can normally tolerate yoghurt and other fermented milk products, as much of the lactose has been converted to lactic acid. Fortunately for people who suffer from alactasia, milk is the only significant source of lactose in the diet, so it is relatively easy to avoid consuming lactose.

Rarely, people may lack sucrase—isomaltase, maltase and/or trehalase. This may be either a genetic lack of the enzyme or an acquired loss as a result of intestinal infection, when all four disaccharidases are lost. These people are intolerant of the sugar(s) that cannot be hydrolysed and suffer in the same way as alactasic subjects given lactose. It is relatively easy to avoid maltose and trehalose, as there are few sources in the diet. People who lack sucrase have a more serious problem because, as well as the obvious sugar in cakes and biscuits and jams, many manufactured foods contain added sucrose.

Genetic lack of sucrase—isomaltase is very common among the Inuit of North America. On their traditional diet this caused no problems, as they had no significant sources of sucrose or isomaltose. With the adoption of a more Western diet, sucrose-induced diarrhoea has become a significant cause of undernutrition among infants and children.

4.2.2.3 The absorption of monosaccharides

As shown in Figure 4.9, there are two separate mechanisms for the absorption of monosaccharides in the small intestine.

Glucose and galactose are absorbed by a sodium-dependent active process (section 3.2.2.3). The sodium pump and the sodium/potassium ATPase create a sodium gradient across the membrane; the sodium ions then re-enter the cell together with glucose or galactose. These two monosaccharides are carried by the same transport protein, and compete with each other for intestinal absorption.

Other monosaccharides are absorbed by carrier-mediated diffusion; there are at least three distinct carrier proteins: one for fructose, one for other monosaccharides and one for sugar alcohols. Because they are not actively transported, fructose and sugar alcohols are absorbed only to a limited extent, and after a moderately high intake a significant amount will avoid absorption and remain in the intestinal lumen, acting as a substrate for colon bacteria and, like unabsorbed disaccharides in people with disaccharidase deficiency, causing abdominal pain and diarrhoea.

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