Before discussing the digestive system it is necessary to have an understanding of the nutritional needs of the body, i.e. the dietary constituents and their functions.
A nutrient is any substance that is digested, absorbed and used to promote body function. These substances are:
• carbohydrates
• proteins
• fats
• vitamins
• mineral salts, trace elements and water.
Many foods contain a number of nutrients, e.g. potatoes and bread are mainly carbohydrate, but both contain protein and some vitamins. Foods are described as carbohydrate or protein because they contain a higher proportion of one or the other. Fibre, more correctly known as non-starch polysaccharide (NSP), consists of indigestible material. It is not a nutrient, as it is not digested or absorbed, but it has many beneficial effects on the digestive tract.
The diet is the selection of foods eaten by an individual. A balanced diet is essential for health. It provides the appropriate amounts of all nutrients in the correct proportions to meet body requirements. An essential nutrient is a substance that cannot be made by the body and must therefore be included in the diet.
Many health problems arise as the result of poor diet. In developed countries obesity is increasingly common, while in other countries malnutrition is widespread. The final section of the chapter considers some consequences of poor nutrition.
The balanced diet
Learning outcomes
After studying this section, you should be able to:
list the constituent food groups of a balanced diet
calculate body mass index from an individual’s weight and height.
A balanced diet contains all nutrients required for health in appropriate proportions, and is normally achieved by eating a variety of foods. This is because no single food contains the correct proportions of all essential nutrients. If any nutrient is eaten in excess, or is deficient, health may be adversely affected. For example, a high-energy diet can lead to obesity, and an iron-deficient one to anaemia.
A balanced diet is important in maintaining a healthy body weight, which can be assessed by calculating body mass index (BMI) (Box 11.1).
Box 11.1 Body mass index: WHO classification
Calculation of BMI
Interpretation of BMI
|
<16 |
Severely underweight |
|
16–18.4 |
Underweight |
|
18.5–24.9 |
Normal range |
|
25–29.9 |
Overweight |
|
30–39.9 |
Obese |
|
>40 |
Severely obese |
Healthy eating, i.e. eating a balanced diet, requires a certain amount of knowledge and planning. An important dietary consideration is the amount of energy required. This should meet individual requirements. Daily energy requirements depend on several factors including basal metabolic rate (p. 306), age, gender and activity levels. Dietary carbohydrates, fats and proteins are the principal energy sources and fat is the most concentrated form. Dietary energy is correctly expressed in joules or kilojoules (kJ) although the older terms calories and kilocalories (kcal or Cal) are also still used in the UK.
This section is based on the recommendations of the British Nutrition Foundation (2009). Recommendations for daily food intake sort foods of similar origins and nutritional values into food groups, and advise that a certain proportion from each group be eaten daily (Fig. 11.1). If this plan is followed, the resulting dietary intake is likely to be well balanced. The five food groups are:
• bread, rice, potatoes, pasta
• milk and dairy foods
• foods and drinks high in fat and/or sugar
• meat, fish, eggs, beans
• fruit and vegetables.
Figure 11.1 The eatwell plate. The main food groups and their recommended proportions within a balanced diet.
Reproduced with permission from the Food Standards Agency, UK.
Bread, rice, potatoes, pasta
The British Nutrition Foundation recommends that this group should make up one-third of the diet and that each meal should be based around one food from this group. Potatoes, yams, plantains and sweet potato are classified as ‘starchy carbohydrates’ and are, therefore, considered within this group rather than as fruit and vegetables. These foods are sources of carbohydrate and fibre that provide sustained energy release. Some also contain iron and B-group vitamins including folic acid (p. 271).
Milk and dairy foods
Foods in this group provide protein and minerals including calcium and zinc: some are also a source of vitamins A, B2 and B12. They include milk, cheese, fromage frais and yoghurt, and often contain considerable amounts of fat. Intake should be limited to three servings per day.
Foods and drinks high in fat and/or sugar
These foods are illustrated in Figure 11.1 and also include oils, butter, margarine (including low-fat spreads), mayonnaise, fried food including chips, crisps, sweets, chocolate, cream, ice cream, puddings, jam, sugar and soft drinks, but not diet drinks. Fats are classified as saturated or unsaturated and the differences between these are explained on page 269. Foods in this group should only be used sparingly, if at all, as they are high in energy and have little other nutritional value.
Meat, fish, eggs, beans
In addition to the food shown in Figure 11.1, this group includes meat products such as bacon, sausages, beefburgers, salami and paté. Moderate amounts are recommended because many have a high fat content. It is suggested that fish, including one portion of oily fish, e.g. salmon, mackerel, trout, sardines or fresh tuna, is eaten twice weekly. This food group provides protein, iron, vitamins B and D and sometimes minerals. Vegetarian alternatives include tofu, nuts, beans and pulses, e.g. lentils. Beans and pulses are also a good source of fibre.
Fruit and vegetables
Foods in this group include fresh, frozen and canned products, 100% fruit or vegetable juices and pure fruit smoothies. These foods provide carbohydrate, fibre, vitamin C, folic acid (p. 9) and fibre. A minimum of five portions per day is recommended.
Additional recommendations
The British Nutrition Foundation makes other specific recommendations about salt (p. 272) and fluid intake (1.5 to 2 l per day). This includes water, tea, coffee, squash and fruit juice. Alcohol intake should not exceed 3–4 units per day for men and 2–3 units per day for women.
Groups of people with specific dietary requirements
Certain groups of people may require a diet different from the principles outlined above. For example, pregnant and lactating women have higher energy requirements to support the growing baby and milk production. Menstruating women need more iron in their diet than non-menstruating women to compensate for blood loss during menstruation. Babies and growing children have higher energy requirements than adults because they have relatively higher growth and metabolic rates. In some gastrointestinal disorders there is intolerance of certain foods, which restricts that person’s dietary choices, e.g. coeliac disease (p. 323).
Digestion, absorption and use of nutrients are explained in Chapter 12. Structures and chemistry of carbohydrates, proteins and fats are described in Chapter 2.
Carbohydrates
Learning outcomes
After studying this section, you should be able to:
describe the main mono-, di- and polysaccharides
list the nutritional function of digestible carbohydrates.
Carbohydrates are mainly sugars and starches, which are found in a wide variety of foods, e.g. sugar, jam, cereals, bread, biscuits, pasta, convenience foods, fruit and vegetables. They consist of carbon, hydrogen and oxygen, the hydrogen and oxygen being in the same proportion as in water. Carbohydrates are classified according to the complexity of the chemical substances from which they are formed.
Monosaccharides
Carbohydrates are digested in the alimentary canal and absorbed as monosaccharides. Examples include glucose (see Fig. 2.7, p. 23), fructose and galactose. These are, chemically, the simplest form in which a carbohydrate can exist.
Disaccharides
These consist of two monosaccharide molecules chemically combined, e.g. sucrose (see Fig. 2.7, p. 23), maltose and lactose.
Polysaccharides
These consist of complex molecules made up of large numbers of monosaccharides in chemical combination, e.g. starches, glycogen, cellulose and dextrins.
Not all polysaccharides can be digested by humans; e.g. cellulose and other substances present in vegetables, fruit and some cereals pass through the alimentary canal almost unchanged (see NSP, p. 273).
Functions of digestible carbohydrates
These include:
• provision of energy and heat; the breakdown of monosaccharides, preferably in the presence of oxygen, releases energy for metabolic work – glucose is the main fuel molecule used by body cells
• ‘protein sparing’; i.e. when there is an adequate supply of carbohydrate in the diet, protein does not need to be used to provide energy and heat
• provision of a store of energy when carbohydrate is eaten in excess of the body’s needs as it is converted to:
– glycogen – as a short-term energy store in the liver and skeletal muscles (see p. 307)
– fat and stored in the fat depots, e.g. under the skin.
Proteins (nitrogenous foods)
Learning outcomes
After studying this section, you should be able to:
list the sources of animal and vegetarian protein
list the nutritional functions of dietary proteins.
Proteins are broken down into their constituent amino acids by digestion and it is in this form that they are absorbed into the bloodstream. A constant supply of amino acids is needed to build new proteins, e.g. structural proteins, enzymes and some hormones.
Amino acids (see Fig. 2.8)
These are composed of the elements carbon, hydrogen, oxygen and nitrogen. Some contain minerals such as iron, copper, zinc, iodine, sulphur and phosphate. They are divided into two categories: essentialand non-essential.
Essential amino acids cannot be synthesised in the body, therefore they must be included in the diet. Non-essential amino acids are those that can be synthesised in the body. The essential and non-essential amino acids are shown in Box 11.2.
Box 11.2 Essential and non-essential amino acids
|
Essential amino acids |
Non-essential amino acids |
|
Histidine |
Alanine |
|
Isoleucine |
Arginine |
|
Leucine |
Asparagine |
|
Lysine |
Aspartic acid |
|
Methionine |
Cysteine |
|
Phenylalanine |
Cystine |
|
Threonine |
Glutamic acid |
|
Tryptophan |
Glutamine |
|
Valine |
Glycine |
|
Hydroxyproline |
|
|
Proline |
|
|
Serine |
|
|
Tyrosine |
Nitrogen balance
Excess amino acids are broken down. The amino group (~NH2) is converted to the nitrogenous waste product urea and excreted by the kidneys. The remainder of the molecule is converted to either glucose or a ketone body (see ketosis, Ch. 12), depending on the amino acid. Negative nitrogen balance occurs when amino acid supply does not meet body needs. This situation may arise either when intake is inadequate, e.g. dietary amino acids are absent or deficient, or protein requirement is increased, e.g. during growth spurts and following injury or surgery.
Biological value of protein
The nutritional value of a protein, its biological value, is measured by how well it meets the nutritional needs of the body. Protein of high biological value is usually of animal origin, easily digested and contains all essential amino acids in the proportions required by the body. A balanced diet, containing all the amino acids required, may also be achieved by eating a range of foods containing proteins of lower biological values, provided that deficiencies in amino acid content of any one of the constituent proteins of the diet is supplied by another. A balanced vegetarian diet, which consists primarily of proteins with lower biological values, e.g. vegetables, cereals and pulses, is based on this principle.
Functions of proteins
Amino acids are used for:
• growth and repair of body cells and tissues
• synthesis of enzymes, plasma proteins, antibodies (immunoglobulins) and some hormones
• provision of energy. Normally a secondary function, this becomes important only when there is not enough carbohydrate in the diet and fat stores are depleted.
When protein is eaten in excess of the body’s needs, the nitrogenous part is detached, i.e. it is deaminated, and excreted by the kidneys. The remainder is converted to fat for storage in the fat depots, e.g. in the fat cells of adipose tissue (p. 37).
Fats
Learning outcomes
After studying this section, you should be able to:
outline the main sources of dietary fat
list the functions of fats in the body.
Fats consist of carbon, hydrogen and oxygen, but they differ from carbohydrates in that the hydrogen and oxygen are not in the same proportions as in water. There are several groups of fats and lipids important in nutrition.
Fats (triglycerides)
Commonly known as ‘fats’, triglycerides consist of one glycerol and three fatty acid molecules (see Fig. 2.9, p. 24). Depending on the type and relative amounts of fatty acids they contain, fats are classified as saturated or unsaturated. In general, saturated fats are solid at room temperature and originate from animal sources, while unsaturated fats are oils, usually derived from vegetables or plants. A high intake of saturated fat can predispose to coronary heart disease (Ch. 5)
Linoleic, linolenic and arachadonic acids are essential fatty acids, which cannot be synthesised by the body in significant amounts, but are needed for synthesis of prostaglandins, phospholipids and leukotrienes. These fatty acids are found in oily fish.
Cholesterol
Unlike other lipids whose molecules are composed of chains of atoms, this molecule contains four rings, which give it the characteristic steroid structure. It can be synthesised by the body and is a constituent of full-fat dairy products, fatty meat and egg yolk. Cholesterol is needed for synthesis of steroid hormones, e.g. glucocorticoids and mineralocorticoids (Ch. 9), and is an important constituent of the cell membrane.
Functions of fats
These include:
• provision of the most concentrated source of chemical energy and heat
• support of some organs, e.g. the kidneys, the eyes
• transport and storage of the fat-soluble vitamins: A, D, E, K
• constituent of myelin sheaths (p. 39) and of sebum (p. 356)
• formation of steroid hormones from cholesterol
• storage of energy as fat in adipose tissue under the skin and in the mesentery, especially when eaten in excess of requirements
• insulation – as a subcutaneous layer it reduces heat loss through the skin
• satiety value – the emptying time of the stomach is prolonged after eating food that is high in fat, postponing the return of hunger.
Vitamins
Learning outcomes
After studying this section, you should be able to:
outline the sources and functions of the fat-soluble vitamins: A, D, E and K
describe the sources and functions of the water-soluble vitamins: the vitamin B complex and C.
Vitamins are chemical compounds, required in very small quantities, which are essential for normal metabolism and health. They are in found a wide range of foods and are divided into two groups:
• fat-soluble vitamins: A, D, E and K
• water-soluble vitamins: B complex and C.
Guidance from DoH (1991) is used in this section.
Fat-soluble vitamins
Bile is needed for absorption of these vitamins from the small intestine. The presence of mineral oils in the intestine and malabsorption impair this process.
Vitamin A (retinol)
This vitamin is found in such foods as cream, egg yolk, liver, fish oil, milk, cheese and butter. It is absent from vegetable fats and oils but is added to margarine during manufacture. Vitamin A can be formed in the body from certain carotenes, the main dietary sources of which are green vegetables, fruit and carrots. Although some is synthesised in the body the daily dietary requirement is 600 to 700 μg. The main roles of vitamin A in the body are:
• generation of the light-sensitive pigment rhodopsin (visual purple) in the retina of the eye
• cell growth and differentiation; this is especially important in fast-growing cells, such as the epithelial cells covering both internal and external body surfaces
• promotion of immunity and defence against infection
• promotion of growth, e.g. in bones.
The first sign of vitamin A deficiency is night blindness due to formation of abnormal retinal pigment. Other consequences include xerophthalmia, which is drying and thickening of the conjunctiva and, ultimately, ulceration and destruction of the conjunctiva. This is a common cause of blindness in developing countries. Atrophy and keratinisation of other epithelial tissues leads to increased incidence of infections of the ear, and the respiratory, genitourinary and alimentary tracts. Immunity is compromised and bone development may be abnormal and delayed.
Vitamin D
Vitamin D is found mainly in animal fats such as eggs, butter, cheese, fish liver oils. Humans can synthesise vitamin D by the action of the ultraviolet rays in sunlight on a form of cholesterol (7-dehydrocholesterol) in the skin (see p. 358).
Vitamin D regulates calcium and phosphate metabolism by increasing their absorption from the gut and stimulating their retention by the kidneys. It therefore promotes the calcification of bones and teeth.
Deficiency causes rickets in children and osteomalacia in adults (p. 421), due to impaired absorption and use of calcium and phosphate. The daily requirement is 10 μg and stores in fat and muscle are such that deficiency may not be apparent for several years.
Vitamin E
This is a group of eight substances called tocopherols. They are found in nuts, egg yolk, wheat germ, whole cereal, milk and butter.
Vitamin E is an antioxidant, which means that it protects body constituents such as membrane lipids from being destroyed in oxidative reactions. Recently, vitamin E has been shown to protect against coronary heart disease. Recommended daily intake is 10 mg for men and 8 mg for women, but this should be increased in high-fat diets.
Deficiency is rare, because this vitamin is present in many foods, and is usually seen only in premature babies and in conditions associated with impaired fat absorption, e.g. cystic fibrosis. Haemolytic anaemia (p. 67) occurs, as abnormal red blood cell membranes rupture. White blood cells can likewise be affected, and vitamin E supplements boost immune function. Neurological abnormalities such as ataxia and visual disturbances may occur if the deficiency is severe.
Vitamin K
The sources of vitamin K are liver, some vegetable oils and leafy green vegetables. It is synthesised in the large intestine by microbes and significant amounts are absorbed. The normal daily requirement is 1 μg/kg body weight and only a small amount is stored in the liver.
Vitamin K is required by the liver for the production of prothrombin and factors VII, IX and X, all essential for the clotting of blood (p. 64). Deficiency therefore prevents normal blood coagulation. It may occur in adults when there is obstruction to the flow of bile, severe liver damage and in malabsorption, e.g. in coeliac disease. Newborn infants may be given vitamin K because their intestines are sterile and require several weeks to become colonised with vitamin K-producing bacteria.
Water-soluble vitamins
Vitamin B complex
This is a group of water-soluble vitamins that promote activity of enzymes involved in the chemical breakdown (catabolism) of nutrients to release energy.
Vitamin B1 (thiamin)
This vitamin is present in nuts, yeast, egg yolk, liver, legumes, meat and the germ of cereals. It is rapidly destroyed by heat. The daily requirement is 0.8 to 1 mg and the body stores only about 30 mg. Thiamin is essential for the complete aerobic release of energy from carbohydrate. When it is absent there is accumulation of lactic and pyruvic acids, which may lead to accumulation of tissue fluid (oedema) and heart failure. Thiamin is also important for nervous system function because of the dependency of these tissues on glucose for fuel.
Deficiency causes beriberi, which occurs mainly in countries where polished rice is the chief constituent of the diet. In beriberi there is:
• severe muscle wasting
• delayed growth in children
• polyneuritis, causing degeneration of motor, sensory and some autonomic nerves
• susceptibility to infections.
If untreated, death occurs owing to cardiac failure or severe microbial infection.
The main cause of thiamin deficiency in developed countries is alcoholism, where the diet is usually poor. Neurological symptoms, which are usually irreversible, include memory loss, ataxia and visual disturbances.
Vitamin B2 (riboflavin)
Riboflavin is found in yeast, green vegetables, milk, liver, eggs, cheese and fish roe. The daily requirement is 1.1 to 1.3 mg; only small amounts are stored in the body and it is destroyed by light and alkalis. It is concerned with carbohydrate and protein metabolism, especially in the eyes and skin. Deficiency leads to cracking of the skin, commonly around the mouth (angular stomatitis), and inflammation of the tongue (glossitis).
Vitamin B3 (niacin)
This is found in liver, cheese, yeast, whole cereals, eggs, fish and nuts; in addition, the body can synthesise it from the amino acid tryptophan. It is associated with energy-releasing reactions in cells. In fat metabolism it inhibits the production of cholesterol and assists in fat breakdown. Deficiency occurs mainly in areas where maize is the chief constituent of the diet because niacin in maize is in an unusable form. The daily requirement is 12 to 17 mg and it is fairly stable.
Pellagra develops within 6 to 8 weeks of severe deficiency. It is characterised by:
• dermatitis – redness of the skin in parts exposed to light, especially the neck
• anorexia, nausea, dysphagia and inflammation of the oral mucosa
• delirium, mental disturbance and dementia.
Vitamin B6 (pyridoxine)
This stable vitamin is found in egg yolk, peas, beans, soya beans, yeast, meat and liver. The daily requirement is about 1.2 to 1.4 mg and dietary deficiency is rare, although alcohol and antituberculous drugs antagonise the vitamin and can induce deficiency states. Vitamin B6 is associated with amino acid metabolism, including the synthesis of non-essential amino acids and molecules such as haem and nucleic acids.
Vitamin B12 (cobalamin)
Vitamin B12 consists of a number of cobalamin compounds (containing cobalt). It is found in liver, meat, eggs, milk and fermented liquors, and is destroyed by heat. The normal daily requirement is 1.5 μg.
Like folic acid, vitamin B12 is essential for DNA synthesis, and deficiency also leads to a megaloblastic anaemia, which is correctable with supplements. However, vitamin B12 is also required for formation and maintenance of myelin, the fatty substance that surrounds and protects some nerves. Deficiency accordingly causes irreversible damage such as peripheral neuropathy and/or subacute spinal cord degeneration. The presence of intrinsic factor in the stomach is essential for vitamin B12 absorption, and deficiency is usually associated with insufficient intrinsic factor.
Folic acid (folate)
This is found in liver, kidney, fresh leafy green vegetables and yeast. It is synthesised by bacteria in the large intestine, and significant amounts derived from this source are believed to be absorbed. It is destroyed by heat and moisture. The daily requirement is 200 μg, and, as only a small amount is stored in the body, deficiency is evident within a short time. It is essential for DNA synthesis, and when lacking mitosis (cell division) is impaired. This manifests particularly in rapidly dividing tissues such as blood, and folate deficiency therefore leads to a type of megaloblastic anaemia (p. 67), which is reversible with folate supplements. Deficiency at conception and during early pregnancy is linked to an increased incidence of spina bifida (p. 182).
Pantothenic acid
This is found in many foods and is associated with amino acid metabolism. The daily safe intake is 3 to 7 mg and no deficiency diseases have been identified. It is destroyed by excessive heat and freezing.
Biotin
This is found in a wide range of foods including yeast, egg yolk, liver, kidney and tomatoes and is synthesised by microbes in the intestine. It is associated with the metabolism of carbohydrates, lipids and some amino acids. The daily safe intake is 10 to 20 μg, deficiency is rare and it is a stable compound.
Vitamin C (ascorbic acid)
This is found in fresh fruit, especially blackcurrants, oranges, grapefruit and lemons, and also in rosehips and green vegetables. The vitamin is very water soluble and is easily destroyed by heat, ageing, chopping, salting and drying. These processes may predispose to the development of scurvy (deficiency). The daily requirement is 40 mg and after 2 to 3 months deficiency becomes apparent.
Vitamin C is associated with protein metabolism, especially the laying down of collagen fibres in connective tissue. Vitamin C, like vitamin E, acts as an antioxidant, protecting body molecules from damaging oxidative reactions. When scurvy occurs, collagen production is affected, leading to fragility of blood vessels, delayed wound healing and poor bone repair. Gums become swollen and spongy and the teeth loosen in their sockets.
Minerals, trace elements and water
Learning outcomes
After studying this section, you should be able to:
list the commonest mineral salts required by the body
describe their functions
explain the distribution of water within the body
describe the functions of water within the body.
Minerals and trace elements
Minerals are inorganic substances needed in small amounts by all cells for normal functioning. Those required in only tiny quantities are known as trace elements or trace minerals, e.g. iron, iodine, zinc, copper, cobalt, selenium and fluoride. The main minerals and trace elements are outlined below.
Calcium
This is found in milk, cheese, eggs, green vegetables and some fish. An adequate supply should be obtained from a normal, well-balanced diet, although requirements are higher in pregnant women and growing children. 99% of body calcium is found in the bones and teeth, where it is an essential structural component. Calcium is also involved in coagulation of blood, and nerve and muscle function. Deficiency of calcium causes rickets in children and osteomalacia in adults (p. 421).
Phosphate
Sources of phosphate include cheese, oatmeal, liver and kidney. If there is sufficient calcium in the diet it is unlikely that there will be phosphate deficiency.
It is associated with calcium and vitamin D in the hardening of bones and teeth; 85% of body phosphate is found in these sites. Phosphates are an essential part of nucleic acids (DNA and RNA, see Ch. 17) and energy storage molecules inside cells as adenosine triphosphate (ATP, Fig. 2.10, p. 24).
Sodium
Sodium is found in most foods, especially fish, meat, eggs, milk, most processed foods and also added during cooking or as table salt. Intake of sodium chloride usually exceeds recommendations and excess is normally excreted in the urine. The recommended daily salt (sodium chloride) intake for adults should not exceed 6 g. In practice, food is usually labelled with sodium content, and to convert this to salt, the sodium content is multiplied by 2.5.
It is the most commonly occurring extracellular cation and is involved in muscle contraction, transmission of nerve impulses along axons and maintenance of water and electrolyte balance.
Potassium
This substance is found widely distributed in all foods, especially fruit and vegetables, and intake usually exceeds potassium requirements.
It is the most commonly occurring intracellular cation and is involved in many chemical activities inside cells including muscle contraction, transmission of nerve impulses and maintenance of water and electrolyte balance.
Iron
Iron, as a soluble compound, is found in liver, kidney, beef, egg yolk, wholemeal bread and green vegetables. In normal adults about 1 mg of iron is lost from the body daily. The normal daily diet contains more, i.e. 9 to 15 mg, but only 5–15% of intake is absorbed. Iron is essential for the formation of haemoglobin in red blood cells. It is also necessary for oxidation of carbohydrates and the synthesis of some hormones and neurotransmitters.
Iron deficiency is a relatively common condition, and causes anaemia (p. 67) if iron stores become sufficiently depleted. Menstruating and pregnant women have increased iron requirements, as do young people experiencing growth spurts. Iron deficiency anaemia may also occur in chronic bleeding, e.g. peptic ulcer disease.
Iodine
Iodine is found in salt-water fish and in vegetables grown in soil containing iodine. In some parts of the world where iodine is deficient in soil, very small quantities are added to table salt to prevent goitre (p. 223). Daily iodine requirement depends upon metabolic rate. Some people have a higher normal metabolic rate than others and their iodine requirements are greater.
It is essential for the formation of thyroxine and tri-iodothyronine, two hormones secreted by the thyroid gland (p. 213).
Water
Water is the most abundant constituent of the human body, it makes up about 60% of the body weight in men and about 55% in women. A man weighing 70 kg contains about 40 litres of water, 28 of which are intracellular and 12 extracellular. Extracellular water consists of 2 to 3 litres in plasma and the remainder, interstitial fluid (see Fig. 2.14, p. 26).
A large amount of water is lost each day in urine, sweat and faeces. This is normally balanced by intake in food and fluids, to satisfy thirst. Water requirements are increased following exercise and in high environmental temperatures. Dehydration, with serious consequences, may occur if intake does not balance loss.
Functions of water
These include:
• provision of the moist internal environment required by all living cells
• providing the aqueous (water-based) conditions in which all metabolic reactions take place
• moistening of food for swallowing (see saliva, p. 286)
• regulation of body temperature – as a constituent of sweat, which is secreted onto the skin, it evaporates, cooling the body surface (Ch. 14)
• being the major constituent of blood and tissue fluid, it transports substances round the body and to and from body cells
• dilution of waste products and poisonous substances in the body
• providing the medium for the excretion of waste products, e.g. urine and faeces.
Non-starch polysaccharide (NSP)
Learning outcome
After studying this section, you should be able to:
describe the sources and functions of non-starch polysaccharide.
Non-starch polysaccharide (NSP) is the correct term for dietary fibre although the latter term continues to be more commonly used in the UK. It is the indigestible part of the diet and consists of bran, cellulose and other polysaccharides found in fruit and vegetables. It is widely distributed in wholemeal flour, the husks of cereals and in fruit and vegetables. Dietary fibre is partly digested by microbes in the large intestine with gas (flatus) formation. The daily requirement is at least 20 g.
Functions of NSP (dietary fibre)
Dietary fibre:
• provides bulk to the diet and helps to satisfy the appetite
• stimulates peristalsis (see p. 281), which prevents constipation
• attracts water, increasing bulk and softness of faeces
• increases frequency of defaecation, preventing constipation
• prevents some gastrointestinal disorders, e.g. colorectal cancer and diverticular disease (p. 320).
Disorders of nutrition
Learning outcome
After studying this section, you should be able to:
describe the main consequences of malnutrition, malabsorption and obesity.
The importance of nutrition is increasingly recognised as essential for health, and illness often alters nutritional requirements.
Malnutrition
This may be due to:
• protein-energy malnutrition (PEM)
• vitamin deficiencies
• both PEM and vitamin deficiencies.
The degree of malnutrition can be assessed from measurement of body mass index (see Box 11.1).
Protein-energy malnutrition (Fig. 11.2)
This is the result of inadequate intake of protein, carbohydrate and fat. It occurs during periods of starvation and when dietary intake is inadequate to meet increased requirements, e.g. trauma, fever and illness. Infants and young children are especially susceptible as they need sufficient nutrients to grow and develop normally. If dietary intake is inadequate, it is not uncommon for vitamin deficiency to develop at the same time. Poor nutrition (malnutrition) reduces the ability to combat other illness and infection.
Figure 11.2 Features of protein-energy malnutrition.
Kwashiorkor
This is mainly caused by protein deficiency, and occurs in infants and children in some developing countries, especially when there has been serious drought and crop failure. Reduced plasma proteins lead to ascites and oedema (p. 117) in the lower limbs that masks emaciation. There is severe liver damage. Growth stops and there is loss of weight and loss of pigmentation of skin and hair accompanied by listlessness, apathy and irritability. Affected individuals are susceptible to infection and recovery from injury and infection takes longer.
Marasmus
This is caused by deficiency of both protein and carbohydrate. It is often caused by incorrect bottle feeding (overdilution of milk) or gastroenteritis, although people of any age can be affected. Marasmus is characterised by severe emaciation due to breakdown (catabolism) of muscle and fat. Growth is retarded, the skin becomes wrinkled due to absence of subcutaneous fat and hair is lost.
Malabsorption
The causes of malabsorption vary widely, from short-term problems such as gastrointestinal infections (p. 317) to chronic conditions such as cystic fibrosis (p. 258). Malabsorption may be specific for one nutrient, e.g. vitamin B12 in pernicious anaemia (p. 67), or it may apply across a spectrum of nutrients, e.g. in tropical sprue (p. 323).
Obesity
In developed countries, this is a very common nutritional disorder in which there is accumulation of excess body fat. Clinically, obesity is present when body mass index exceeds 29.9 (see Box 11.1). It occurs when energy intake exceeds energy expenditure, e.g. in inactive individuals whose food intake exceeds daily energy requirements.
Obesity predisposes to:
• gallstones (p. 326)
• cardiovascular diseases, e.g. ischaemic heart disease (p. 120), hypertension (p. 124)
• hernias (p. 321)
• varicose veins (p. 116)
• osteoarthritis (p.424)
• type 2 (non-insulin dependent) diabetes mellitus (p. 227)
• increased incidence of postoperative complications.
Conditions with dietary implications
In addition to nutritional disorder there are many conditions where dietary modifications are needed. Some of these are listed in Box 11.3.
Box 11.3 Conditions that require dietary modification
Obesity
Malnutrition
Diabetes mellitus (p. 227)
Diverticular disease (p. 320)
Coeliac disease (p. 323)
Phenylketonuria (p. 435)
Acute renal failure (p. 345)
Chronic renal failure (p. 346)
Liver failure (p. 326)
Lactose intolerance
For a range of self-assessment exercises on the topics in this chapter, visitwww.rossandwilson.com
Further reading
British Nutrition Foundation. The Eatwell Plate. Available online at http://www.nutrition.org.uk/home.asp?siteId=43§ionId=299&which=1. Accessed 21 January 2009
Department of Health. Dietary reference values of food energy and nutrients for the UK: COMA report. London: HMSO, 1991.
World Health Organization. Global database on body mass index. Available online at http://www.who.int/bmi/index.jsp?introPage=intro_3.html. Accessed 17 January 2009