Average intakes of sugars (and especially sucrose) are generally considered to be higher than is desirable. The adverse effects of an excessive intake of sugars include:
Because of this, it is considered desirable that sucrose should provide no more than average desirable protein 15%
Figure 7.14 Average and desirable percentage of energy intake from different types of carbohydrate.
10% of energy (compared with the current average of 14%). Intakes of other sugars (mainly glucose and fructose in fruits, and lactose from milk) are considered to be appropriate. Therefore, as shown in Figure 7.14, the increased proportion of energy from carbohydrates should be from increased consumption of starches.
18.104.22.168 Undigested carbohydrates (dietary fibre)
The residue of plant cell walls is not digested by human enzyme, but provides bulk in the diet (and hence in the intestines). It is measured by weighing the fraction of foods that remains after treatment with a variety of digestive enzymes. This is what is known as dietary fibre. It is a misleading term, as not all the components of dietary fibre are fibrous; some are soluble and form viscous gels.
A more precise analytical method permits measurement of the specific polysaccharides other than starch (section 22.214.171.124) that are the main constituents of dietary fibre; the results of such analysis are quoted as non-starch polysaccharides (nsp).
The two methods of analysis give different results. Measurement of non-starch polysaccharides in the diet gives average intakes in Britain of between 11 and 13 g/ day, compared with an intake of dietary fibre of about 20 g/day as measured by the less specific method. Non-starch polysaccharides are found only foods of vegetable origin, and vegetarians have a higher intake than omnivores.
Non-starch polysaccharides have little nutritional value in their own right, as they are compounds that are not digested or absorbed to any significant extent. Nevertheless, they are a valuable component of the diet, and some of the products of fermentation by colonic bacteria can be absorbed and utilized as metabolic fuel. Together with non-starch polysaccharides, we have to consider that proportion of starch that is (relatively) resistant to digestion in the small intestine (section 126.96.36.199), because it too is a substrate for bacterial fermentation.
The main products of bacterial fermentation of non-starch polysaccharides and resistant starch are short-chain fatty acids such as propionate and butyrate. In addition to being absorbed, and hence used as metabolic fuels, they have an antiproliferative effect on tumour cells in culture, and there is some evidence that they provide protection against the development of colorectal cancer.
Diets low in non-starch polysaccharides are associated with the excretion of a small bulk of faeces, and frequently with constipation and straining while defecating. This has been linked with the development of haemorrhoids, varicose veins and diverticular disease of the colon. These diseases are commoner in Western countries, where people generally have a relatively low intake of non-starch polysaccharide, than in parts of the world where the intake is higher.
Some types of non-starch polysaccharides bind a number of potentially undesirable compounds in the intestinal lumen, and so reduce their absorption. Again this may be protective against colorectal cancer. A number of compounds that are believed to be involved in causing or promoting cancer of the colon occur in the contents of the intestinal tract, both because they are present in foods and as a result of bacterial metabolism in the colon. They are adsorbed by non-starch polysaccharides, and so cannot interact with the cells of the gut wall but are eliminated in the faeces.
Although epidemiological studies show that diets high in non-starch polysaccharides are associated with a low risk of colon cancer, such diets also provide relatively large amounts of fruit and vegetables, and are therefore rich in vitamins C and E and carotene, which may have protective effects against the development of cancers (section 7.4.3). Furthermore, because they provide more fruit and vegetables, and less meat, such diets are also relatively low in saturated fats, and there is evidence that a high intake of saturated fats is a separate risk factor for colon cancer.
As discussed in section 188.8.131.52, the bile salts required for the absorption of dietary fat are synthesized in the liver from cholesterol. Normally about 90—95% of the 30 g of bile salts secreted daily is reabsorbed and reutilized. Non-starch polysaccharides adsorb bile salts, so that they are excreted in the faeces. This means that there has to be increased synthesis de novo from cholesterol to replace the lost bile salts, so reducing the total body content of cholesterol.
A total intake of about 18 g of non-starch polysaccharides per day is recommended (equivalent to about 30 g/day of dietary fibre). In general, this should come from fibre-rich foods — whole-grain cereals and wholemeal cereal products, fruits and vegetables — rather than supplements. This is because as well as the nsp, these fibre-rich foods are valuable sources of a variety of nutrients. There is no evidence that intakes of fibre over about 30 g/day confer any benefit, other than in the treatment of established bowel disease. Above this level of intake it is likely that people would reach satiety (or at least feel full, or even bloated) without eating enough food to satisfy energy needs. This may be a problem for children fed on a diet that is very high in fibre — they may be physically full but still physiologically hungry.
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