Sonidos Articulares durante los Movimientos de Lateralidad y Protrusión

In the mouth, high-fibre foods generally require more chewing. This slows down the process of eating and stimulates an increased flow of saliva. The saliva contributes to the volume of the swal- lowed food bolus. Once in the stomach, the fibre- rich food tends to absorb water and the soluble component starts to become viscous. Both of these changes delay stomach emptying. In the small intestine, the soluble fibre travels slowly because of increased viscosity; this prolongs the period of time available for the absorption of nutrients. The fibre may also bind some divalent ions in the small intestine, making them unavail- able for absorption at this point.

Classification of starch according to digestibility

TABLE 6.2

Type of starch Example of occurrence Probable digestion in small intestine Rapidly digestible Freshly cooked starchy food Rapid

Slowly digestible Mostly raw cereals Slow but complete Resistant

Physically inaccessible Partly milled grains and seeds Resistant Resistant granules Raw banana and potato Resistant Retrograded Cooled, cooked potato, bread and Resistant

cornflakes, savoury snack foods

From Englyst, H.N. and Kingman, S.M. 1990: Dietary fibre and resistant starch: a nutritional classification of plant polysac- charides. In: Kritchevsky, D., Bonfield, C., Anderson, J.W. (eds), Dietary fibre. Reproduced with kind permission of Plenum Press Publishing Corporation, New York.

DIGESTION OF CARBOHYDRATES ❚

Once in the large intestine, the soluble fibre becomes a food source for the growth and multiplication of the bacterial flora. The con- sequences of this are exactly the same as described above for resistant starch. Thus, both

resistant starch and soluble NSPs contribute to increasing bulk in the large intestine, and the production of fatty acids and gases.

Insoluble fibre, which has reached the colon largely unchanged, swells by water holding, and

adds further to the volume of the colonic con- tents. The faeces, therefore, are both bulkier and softer because of the increased water content.

Absorption of carbohydrates After digestion, the resulting monosaccharides are absorbed from the gut lumen across the mucosa into the blood by one of three mechanisms:

■ simple diffusion;

■ facilitated diffusion; or

■ active transport.

The latter two processes allow faster absorp- tion of the simple sugars than could be achieved by simple diffusion alone. This becomes particu- larly important in the later stages of absorption, as concentrations in the gut lumen fall. Active transport involves the breakdown of ATP and the presence of Na.

Absorption of sugars causes a variable rise in blood sugar. When given individually, glucose and maltose produce the greatest increase, fol- lowed by sucrose, lactose, galactose and fruc- tose. The effects are not necessarily the same when these sugars are consumed as part of a meal. The level of glucose in the blood rises to a maximum in about 30 minutes and falls to fast- ing levels after 90–180 minutes. The rate of rise to the maximum and the rate of fall vary with the nature of the meal, and are related to the digestion rates occurring in the small intestine and the speed of release of glucose.

It is possible to measure the relative effects of different carbohydrate foods on the blood sugar level. The rise in blood glucose following ingestion of a portion of a test food containing 50 g of available carbohydrate is compared with the effect on blood glucose of a 50 g available carbohydrate portion of a standard, such as glu- cose or white bread. Comparison of the areas under the two glucose curves obtained produces a ‘glycaemic index’ (Figure 6.6).

The glycaemic index of a large number of foods has been determined. Glycaemic responses vary between individuals, but the ranking of response to different foods can be predicted from the standard results. Foods grouped according to their glycaemic index are shown in Chapter 16.

Diets with a low glycaemic index have been shown to have various health benefits, including reduction of blood lipids and improved blood glucose control in diabetic subjects. They also enhance satiety and increase athletic endurance. More research is needed to increase the evidence base on relationships between glycaemic index and health, in specific conditions and for the general population. There are moves to introduce labelling of products with their glycaemic index, although there needs to be public education about the significance of this measure before any such initiative.

By definition, we would not expect the non- starch polysaccharides to be absorbed. However, these compounds do not travel through the digestive tract completely unchanged. Physical breakdown and bacterial fermentation are the main changes that alter both the soluble and insoluble fibres as they pass through the digest- ive tract. Some of the fatty acids released as a result of fermentation are absorbed and provide usable energy.

Figure 6.6Rise in blood glucose after eating, and the calculation of glycaemic index.

■ Why might diets with a low glycaemic index be beneficial in diabetic subjects?

■ What foods should be recommended? ■ Why might diets with a low glycaemic index

help to promote satiety?

■ In what ways might diets with low glycaemic index be beneficial in endurance athletes?

METABOLISM OF CARBOHYDRATES ❚

Metabolism of carbohydrates In discussing carbohydrate metabolism in the body, it is simplest to consider glucose, fructose and galactose. These travel in the bloodstream from the small intestine to the liver, where they are stored as long chains of glucose units in the form of glycogen. The liver stores one-third of the body’s total glycogen (about 150 g). The remainder of the glucose may pass on to the muscles, where it is also stored as glycogen. Storage of glycogen is encouraged by insulin, the hormone produced by the  cells of the pancreas. Liver glycogen is readily transformed back into glucose whenever the blood sugar level falls below about 4 mmol/l. Thus, glucose can continue to supply energy to the brain,

central nervous system and other organs whether the person has eaten or not. The glucose from the blood passes into these tissues where it is oxidized and energy is released by means of one of several pathways (glycolysis, tricarboxylic acid cycle, hexose monophosphate pathway), depending on circumstances. A number of vita- mins are needed to achieve this oxidation, most notably thiamin, riboflavin and nicotinic acid. Insulin is also needed to facilitate the entry of the glucose into tissues (see Figure 6.7). Muscle glycogen is not used to maintain blood sugar levels, rather its role is to provide energy directly for muscle contraction.

Glycogen is stored in association with water and is a bulky way of storing energy. Thus, the

Figure 6.7Metabolism of carbohydrates.

body only contains enough glycogen to provide energy for relatively short periods of time, although new research suggests that glycogen stores are well controlled. Longer term energy stores are maintained in the form of fat and, as a last resort, as protein. If we take in more carbo- hydrate than we need, the body will use the glu- cose to fill its glycogen stores and then could convert the remaining glucose into the more per- manent storage form – fat. Unlike the limited storage capacity for glycogen, the body can store unlimited amounts of fat. In practice, this does not occur in humans to any significant extent and carbohydrate metabolism is stimulated to utilize the excess carbohydrate. As a consequence, energy from fat is not used and fat may be stored. Fat synthesis from carbohydrate only occurs when extremely large amounts of carbohydrate are consumed over a period of days.

Why do we need

carbohydrates in the diet? From the above discussion, it is clear that carbo- hydrates are an important source of energy for the body, providing glucose for immediate use and glycogen reserves (see Figure 6.8). All the cells of the body require glucose and some, such as the brain, nervous system and developing red blood cells, are ‘obligatory’ users of glucose. We are able to make some glucose from proteins and fats in the process of gluconeogenesis. This enables the body to survive when the glycogen stores are depleted and no carbohydrate has been eaten. Almost all the body’s amino acids (those

known as glucogenic) and the glycerol part of triglycerides (about 5 per cent of the weight of fat) but not the fatty acids can be converted to glucose. However, using protein to make glucose is potentially harmful, since tissue protein may be broken down. This happens in starvation both in the early stages before the body adapts to using more fats for essential energy, and in the final stages when body fat stores have been depleted, and the body’s structural protein is being used for energy.

A further problem arises when there is insufficient carbohydrate available to complete fat metabolism. In the absence of carbohydrate, acetyl coenzyme A accumulates and condenses to form ketone bodies. This state, known as ketosis, is associated with mild disturbances of cellular function and is an early indication of insufficient carbohydrate availability in the body. So, even though glucose can be produced from non-carbohydrate sources, the processes are inefficient and potentially harmful, and indicate a specific need for carbohydrate in the diet to supply energy.

Carbohydrates are also used in the synthesis of various metabolically active complexes. Glycoproteins are important components of cellular membranes, in particular on the extra- cellular surface. They are also found as circulat- ing proteins in blood or plasma. Glycolipids, such as sphingolipids and gangliosides, have roles at receptor sites on cells and in synaptic transmission.

Mucopolysaccharides have important water- holding or binding properties in many sites of

Figure 6.8Why do we need carbohydrates in the diet?

CARBOHYDRATES AND HEALTH ❚

the body; they occur in basement membranes and in intercellular cement and form an integral part of cartilage, tendon, skin and synovial fluid. Disorders of mucopolysaccharide metabolism have been associated with a number of disease states. Little is currently known about the influ- ence of dietary sugars on these compounds or on specific quantitative requirements.

How much carbohydrate should we have?

The intake of dietary carbohydrate must not only be sufficient to provide the necessary energy for the survival of the body, but must also contain sufficient specific sugars to allow the synthesis of essential complex molecules. However, this is difficult to quantify; it is much easier to calculate the amount of protein needed by the body to maintain nitrogen balance, and the amounts of fat to supply the essential fatty acids.

The only true requirement for carbohydrates that current knowledge has identified is for the prevention of ketosis. Estimates of the minimum amount of carbohydrate needed by an adult are in the range of 150–180 g carbohydrate per day. This does not necessarily need to be supplied entirely from the diet: 130 g could be synthe- sized by gluconeogenesis, with the remaining 50 g provided exogenously from food. The total figure represents 29 per cent of the total energy expenditure.

Studies on pregnant rats indicate that a min- imum amount of carbohydrate, up to 12 per cent of glucose, is needed to sustain pregnancy and lactation and avoid high mortality rates in the offspring. This points to other specific require- ments for synthesis of carbohydrate-containing complexes.

In the UK, the recommendations made about carbohydrates use an ‘optimum’ intake approach, which includes an amount sufficient to: prevent ketosis; avoid starvation but not induce obesity; avoid adverse effects on the large intestine, and on lipid and insulin metabolism; and to avoid caries. In addition, the intake should contribute to an enjoyable diet. The sources of carbohydrate should be as unprocessed as possible, as any increase in the degree of processing is linked with

possible adverse effects. The dietary reference values report suggests that:

■ dietary carbohydrate should supply 50 per cent of energy;

■ sugars not contained within cellular struc- tures (the non-milk extrinsic sugars) should constitute no more than 10 per cent of the energy; and

■ the balance should be made up from com- plex carbohydrates and other sugars, such as those in fruits and milk.

Dietary guidelines published in other coun- tries on the whole adopt a similar approach to sugar intake with a level of approximately 10 per cent being recommended. Some scientists believe that such a low level is not achievable alongside the goal to lower fat intakes. Studies of the British diet have shown that a reciprocal relationship exists between intakes of fat and refined sugars, and that lowering the sugar intake is likely to cause an increase rather than the desired reduction in the intake of fats. This highlights one of the dilemmas associated with looking at individual nutrient components of the diet to compile ‘whole diet’ guidelines.

The National Food Survey 2000 (DEFRA, 2001) showed the reciprocal trend in carbohy- drate and fat intakes in the UK over the past 60 years, with a decreasing percentage of the energy from carbohydrate being accompanied by an increased percentage from fat. Recently, this trend has reversed and, currently, carbohy- drate percentage is rising, as fat percentage falls. The most recent survey showed that car- bohydrates provided 46.6 per cent of total energy. The total intake of carbohydrates was 218 g from household food, which contained 131 g of starch and 87 g of total sugars (includ- ing 47 g of non-milk extrinsic sugars). A further 21 g of carbohydrate was consumed from foods eaten outside the household. These figures show that the proportions of carbohydrates consumed in the UK do not match the dietary guidelines.

Carbohydrates and health There is a great deal of confusion surrounding the links between carbohydrates and health.

Opinions about carbohydrates include the following:

■ ‘they are fattening’;

■ ‘they provide instant energy’;

■ ‘they are bad for the teeth’;

■ ‘they are essential to life’’.

In order to evaluate these views, it is necessary to distinguish between the various types of carbohydrates in the diet, since each behaves differently in the body.