Sugars, starches and the glycaemic index

What would you expect to see if the subjects had consumed an equivalent amount of glucose?

The carbohydrate in meal 2 is obviously more rapidly and completely digested than that in meal 1.

Glucose does not require digestion, and is rapidly absorbed, so the response of blood glucose to a test dose of glucose would be similar to that after test meal 1 - or perhaps a higher peak achieved slightly more rapidly.

The glycaemic index of a food is the extent to which it raises the blood concentration of glucose (i.e. the area under the curve in the graph shown above), compared with that for a test dose of the equivalent amount of glucose or a standard carbohydrate food that is rapidly digested, such as freshly boiled mashed potato or white bread.

Insulin is secreted in response to increasing blood glucose concentrations, so you would indeed expect to see a lower concentration of serum insulin at each time point after meal 1, which causes less increase in blood glucose, than after meal 2.

We can classify carbohydrates into three main groups:

• the sugars (mono- and disaccharides),
• oligosaccharides (polymers of 3 - 10 monosaccharide units)
• polysaccharides (polymers of several hundred monosaccharide units).

It is conventional to consider the sugar alcohols separately from the simple sugars.

The nutritionally and metabolically important monosaccharides are glucose, galactose and fructose:

Galactose differs from glucose in the orientation of the hydroxyl group at carbon-4.

Fructose has a keto group at carbon-2 (and hence is a ketose), rather than an aldehyde group at carbon-1, as in glucose and galactose (which are aldoses). This means that while glucose and galactose are chemically reducing compounds, fructose is not. It also means that when ring closure occurs, glucose and galactose form 6-member pyranose rings, while fructose forms a 5-member furanose ring.

Glucose, galactose and fructose are all 6-carbon sugars (hexoses). Two 5-carbon sugars (pentoses) are biochemically important: ribose and dexxyribose.

The sugar alcohols are formed by reduction of the aldehyde or keto group of a sugar to a hydroxyl group, and therefore are often called polyols. For example, the sugar alcohol derived from glucose is sorbitol:

The main dietary disaccharides are sucrose, lactose, trehalose, maltose and isomaltose:

The classical technique is the "everted gut sac". The small intestine of a rat or other experimental animal is dissected out, washed with an appropriate buffer solution, then everted - turned inside out by rolling it over a metal rod. Small sections of the everted gut (4 - 6 cm long) are then tied off at one end, filled with buffer solution and tied off at the other end. They are then suspended in a buffer solution containing the compound of interest, as well as a small amount of glucose to permit them to metabolise. The contents of the sac represent what has been transported from the mucosal surface of the gut (which now faces outwards into the incubation medium). This means that sampling the contents of the sac after an appropriate time of incubation permits you to measure what has been transported.

If there is a higher concentration of a sugar inside the sac than outside then this must be active transport - directly or indirectly linked to utilisation of ATP. Both glucose and galactose are actively transported across the intestinal mucosa, linked to the transport of sodium ions down their concentration gradient. In turn, the sodium gradient has been set up linked to the hydrolysis of ATP to ADP and phosphate.

In a series of experiments it was found that when both glucose and galactose were present in the incubation medium, they competed with each other - the more galactose was present, the less glucose was absorbed. Similarly, the more glucose was present, the less galactose was absorbed.

If glucose and galactose compete with each other then they must be transported by the same carrier. The total amount of sugar (glucose + galactose) transported will be more or less constant as the proportions of each change. If they were transported by separate carriers then there would not be competition, but rather when both sugars were present in the incubation there would be increased total sugar transport.

Fructose, sugar alcohols and various other monosaccharides are absorbed by carrier-mediated diffusion, and so cannot be transported against their concentration gradient.

A relatively large amount of fructose or sugar alcohol exceeds the capacity to absorb it. This leaves a relatively high concentration of sugar (or sugar alcohol) in the intestinal lumen. This has an osmotic effect, drawing water into the intestinal lumen. It also provides a substrate for fermentation by intestinal bacteria to 2- and 3-carbon acids, so increasing yet further the osmotic effect. The net result of this is an osmotic diarrhoea - diarrhoea caused by the relatively large amount of water drawn into the intestinal lumen by the unabsorbed sugar.

Again you could use the everted gut sac. Now you would place the disaccharide of interest in the incubation medium and measure what appears inside the sac - i.e. what has been transported from the mucosal side (which faces outwards) to the serosal side, which is the inside of the sac.

If you added sucrose to the incubation medium, you would find glucose and fructose, but virtually no sucrose, inside the sac.

Obviously, sucrose is hydrolysed to its constituent monosaccharides at some stage in transport from the mucosal to serosal side of the gut. There are three possibilities:

1. Sucrose is transported into the mucosal cell, and then undergoes hydrolysis to glucose and fructose, which are transported to the serosal side, and (in vivo) into the bloodstream;
2. Sucrose is hydrolysed by an enzyme secreted into the intestinal lumen by mucosal cells and the resultant glucose and fructose are taken up by the same transporters as take up monosaccharides;
3. Sucrose is hydrolysed by an enzyme at the mucosal surface of the cell (the brush border) and the resultant glucose and fructose are taken up by the same transporters as take up monosaccharides.

1. If sucrose is transported intact into the mucosal cell then there will only be sucrose, and no glucose or fructose in the incubation medium
2. If sucrose is hydrolysed by an enzyme secreted into the intestinal lumen by mucosal cells then there will be some free glucose and fructose detectable in the incubation medium. Furthermore, a sample of the incubation medium, after removing the everted gut sac should still be capable of hydrolysing sucrose because it will contain the secreted enzyme. (Note that because you only have a section of small intestine in the incubation then you cannot be considering an enzyme secreted by the pancreas - it must be an enzyme secreted by the mucosal cells.)
3. If sucrose is hydrolysed by an enzyme at the brush border of the mucosal cell, the resultant glucose and fructose are taken up by the same transporters as take up monosaccharides then again you would expect to find some free glucose and fructose in the incubation medium. However, in this case a sample of the incubation medium after removing the everted gut sac will not hydrolyse sucrose, because you have removed the enzyme when you removed the gut sac.

When you add sucrose to the incubation medium you find some glucose and fructose in the incubation medium, but after removing the everted gut sac the incubation medium does not catalyse any further hydrolysis of sucrose to glucose and fructose.

Obviously, sucrose is hydrolysed to glucose and fructose at the mucosal surface, but by an enzyme that is attached to the cell (in the brush border) and is not secreted into the intestinal lumen.

This enzyme is sucrase, which is actually a bifunctional enzyme, with two catalytic sites. It also catalyses the hydrolysis of isomaltose to 2 mol of glucose, and is correctly called sucrase-isomaltase.

There are three other disaccharidases that are also attached to the brush border of the mucosal cells:

1. lactase, which catalyses the hydrolysis of lactose to glucose and galactose;
2. maltase, which catalyses the hydrolysis of maltose to 2 mol of glucose;
3. trehalase, which catalyses the hydrolysis of trehalose to 2 mol of glucose.

If you repeated the experiments with the everted gut sac, but this time adding a small amount of solubilised starch to the incubation medium, you would not see any appearance of glucose in the incubation medium or inside the sac.

Obviously, starch is neither hydrolysed by a cell surface enzyme nor taken up into the mucosal cell to be hydrolysed. Therefore the enzyme catalysing the hydrolysis of starch must be secreted into the intestinal lumen.

This enzyme is amylase, and it is secreted in saliva (salivary amylase, sometimes called by its old name ptyalin) and also in the pancreatic juice.

It is unlikely that salivary amylase accounts for a significant amount of hydrolysis of dietary starch, since the food is in the mouth for only a relatively short time. The hydrolysis of starch catalysed by salivary amylase will continue in the middle of the food bolus in the stomach, but will cease when the food is mixed with the very acidic gastric juice.

The main importance of salivary amylase is that the small amount of starch that is hydrolysed in the mouth yields glucose, which is sweet, and can be detected by the sweetness taste buds. This provides a mechanism for sensing that the food being eaten is a source of carbohydrate, and hence a valuable source of energy.

If you mixed solubilised starch with either salivary or pancreatic amylase, you would find a mixture of glucose, maltose and isomaltose, as well as smaller fragments (dextrins) after incubating for several minutes.

Starch must contain glucose units linked 1-4 (so yielding maltose as a result of random hydrolysis by amylase) and also glucose units linked 1-6 (so yielding isomaltose as a result of random hydrolysis by amylase).

There are two forms of starch:

1. Amylose, which is a linear polymer of glucose linked 1-4
2. Amylopectin, which is a branched polymer, with straight chains of glucose linked 1-4 and branch points with 1-6 linkage.

Most starchy foods contain about 20 - 25% amylase and 75 - 80% amylopectin.

If you stir 5 g of starch into 100 mL of water, it will rapidly precipitate out. However, if you heat the suspension to about 100C, it will set to a viscous gel.

Heating the suspension of starch granules allows water to penetrate between the chains of the highly branched molecule of amylopectin, swelling the molecule. This is the process of gelatinisation or solubilisation of starch. It is now maintained in suspension because of hydrogen bonding between the many hydroxyl groups of the glucose monomers and the water.

Ungelatinised starch is crystalline and does not present much surface for amylase to act on. By contrast, the expanded structure of gelatinised starch presents a large surface area and amylase can readily penetrate between the branches, hydrolysing bonds and releasing small dextrins, glucose, maltose and isomaltose. The gelatinisation of starch is reversible, and gradually after heating the amylopectin will begin to recrystallise - this is what occurs, for example, when bread "goes stale".

The graph on the right shows the glycaemic response to two test meals - both contained the same amount of starch.

Three reasons may explain why starch in some foods in less accessible to amylase, and hence more slowly hydrolysed, than in other foods:

1. From the discussion above, one reason why starch may be resistant to digestion is that it is less gelatinised - for example, the starch in raw potato is almost completely undigested, while that in boiled potato is almost completely digested.
2. Equally, the recrystallisation that occurs in gelatinised starch will reduce its accessibility to amylase and so reduce its glycaemic index.
3. In many foods the starch is within cell walls, which are largely cellulose, a carbohydrate that is not hydrolysed by mammalian enzymes. This means that the starch is not accessible to amylase.

Some may remain intact throughout its passage through the gastrointestinal tract, and be excreted in faeces. However, a considerable amount of resistant starch provides a substrate for metabolism by intestinal bacteria (mainly in the colon). The bacteria metabolise starch (and other carbohydrates that have escaped digestion) to yield a variety of compounds, including short-chain fatty acids (e.g. the 4-carbon compound butyrate, the 3-carbon compounds pyruvate and lactate, and the 2-carbon compound acetate), as well as carbon dioxide and small amounts of hydrogen and methane.

There is a considerable body of evidence that production butyrate by intestinal bacteria is beneficial. It provides a significant metabolic fuel to intestinal mucosal cells and there is some evidence that it provides protection against the development of colorectal cancer.

Foods contain a variety of polysaccharides other than starch - collectively these are known as non-starch polysaccharides, and they are the main constituents of dietary fibre.

None of these non-starch polysaccharides is a substrate for human enzymes, so, like resistant starch they will arrive in the colon intact. Here, again like resistant starch, they provide substrates for bacterial fermentation yielding short-chain fatty acids, carbon dioxide and small amounts of hydrogen and methane. They encourage the growth and proliferation of beneficial bacteria at the expense of pathogens.

Some non-starch polysaccharides, such as pectin and the various plant gums and mucilages, are soluble. They increase the viscosity of the intestinal contents, and so slow the absorption of the products of digestion. A number of studies have shown that consuming a modest amount of soluble non-starch polysaccharide before or with a meal lowers the post-prandial rise in blood glucose, and improves glycaemic control in people with diabetes mellitus.

Insoluble non-starch polysaccharides increase the bulk of the intestinal contents, so aiding peristalsis - this is valuable in the prevention or treatment of diverticular disease of the colon. Excessive pressure developing as a result of small hard intestinal contents can lead to the development of diverticula - as shown in the lower diagram on the right. This is the condition of diverticulosis.

Particles of food and faeces can then become trapped in the diverticula, which then become inflamed, leading to diarrhoea and intense pain. This is the condition of diverticulitis.

An additional benefit of insoluble non-starch polysaccharides is that they can bind bile salts and potential carcinogens, so preventing their absorption. You will see in a later exercise how binding of bile salts to non-starch polysaccharide so that they are not reabsorbed for resecretion leads to the need for increased synthesis of bile salts from cholesterol, and thus has a useful action in lower the total body burden of cholesterol.

Key points from this exercise:

• Different carbohydrate foods have a different effect on blood glucose; the glycaemic index of a carbohydrate food is the extent to which it raises blood glucose compared with the equivalent amount of glucose or a reference carbohydrate that is rapidly digested and absorbed.
• The insulin response to carbohydrate ingestion follows the glucose response.
• Carbohydrates can be classified as sugars (mono- and disaccharides and sugar alcohols), oligosaccharides and polysaccharides.
• The major monosaccharides are the hexoses glucose, galactose and fructose; the pentoses ribose and deoxyribose have important biochemical roles.
• Glucose and galactose are absorbed by sodium-linked active transport, sharing the same carrier. Fructose, sugar alcohols and other monosaccharides are absorbed by passive carrier-mediated diffusion.
• The major disaccharides are sucrose, lactose, maltose, isomaltose and trehalose. They are hydrolysed by enzymes in the brush border of intestinal mucosal cells, and the resulting monosaccharides are absorbed.
• Starch is a large polymer of glucose. There are two main forms of starch: amylose is a linear polymer of glucose linked 1-4; amylopectin is a branched polymer, with branch points formed by 1-6 glycoside bonds.
• Starch digestion is catalysed by amylase in saliva and pancreatic juice. Amylase attacks randomly, yielding a mixture of glucose, maltose, isomaltose and small oligosaccharides (dextrins).
• Starch may be rapidly or slowly digested, or more or less completely resistant to digestion. This depends largely on whether or not the starch is gelatinised or crystalline. Starch enclosed in plant cell walls is inaccessible to amylase. Gelatinised starch is more accessible than crystalline.
• Both resistant starch and non-starch polysaccharides provide a substrate for intestinal bacterial fermentation, and the resultant short-chain fatty acids provide a major fuel for colonic enterocytes. Butyrate may also have antiproliferative actions and provide protection against colorectal cancer.
• Soluble non-starch polysaccharides increase the viscosity of the intestinal contents and slow the absorption of the products of digestion.
• Insoluble non-starch polysaccharides provide bulk to the intestinal contents, so aiding peristalsis and preventing or treating diverticular disease of the colon. They also bind bile salts and potential carcinogens, preventing their absorption.