General principles of nutrition
Every individual has his own personalized genetic constitution and, correspondingly, his own unique pattern of enzymes that render him/her more or less susceptible to particular external influences. These include the food he/she eats, the diseases to which he/she is exposed, and the drugs he/she may take. Although it is not possible to predict the requirements and reactions of any particular person, certain nutritional principles have been established that are valid for populations taken as a whole. This chapter describes the various functions of food: to provide energy, without which no living structure can be built or maintained; to provide structural materials that are required for the growth and maintenance of the tissue; to provide regulatory substances; and to provide water. To sustain life and health, the body requires carbohydrate, fat, protein, minerals, vitamins, and water, but it is very difficult to determine how much of these nutrients is required. Many factors need to be considered, including the types of carbohydrate, fat, and protein; the age and sex of the individual; the nature and extent of his activities; and the climatic conditions. A diet should be considered as a whole and should be balanced with respect to its various constituents. The recommended daily amount of a nutrient is defined as the average amount that should be provided per head in a group of people if the needs of practically all members of the group are to be met.
In the field of human nutrition there are few hard facts, and clearcut conclusions are almost impossible. Apart from the difficulty of performing experiments on human subjects there is the problem of their biological variability. Every individual has his own ‘personalized’ genetic constitution and, correspondingly, his own unique pattern of enzymes which render him/her more or less susceptible to particular external influences. These include the food he/she eats, the diseases to which he/she is exposed and the drugs he/she may take. However, although it is not possible to predict the requirements and reactions of any particular person certain nutritional principles have been established which are valid for populations taken as a whole. This was successfully demonstrated in World War II when, as a result of prudent food imports, rationing and such measures as the fortification of margarine with vitamins and bread with calcium, the general standard of nutrition was better than it has ever been before or since. This was the result of it being virtually impossible to eat any food in excess, so that the national diet, though restricted, was well-balanced.
2. Structural materials which are required for the growth and maintenance of the tissues. The main materials falling in this category are proteins, although minerals, notably calcium and phosphorus, also fulfil a structural role.
4. Water. This is of over-riding importance in both structural and regulatory roles but its nutritional functions are often taken for granted. Death from lack of water occurs within 4–5 days, whereas starvation results in death only after 30 days or more.
It is easy to state that in order to sustain life and health the body requires carbohydrate, fat, protein, minerals, vitamins and water but very difficult to determine how much of these nutrients is required. Many factors need to be considered including the types of carbohydrate, fat and protein, the age and sex of the individual, the nature and extent of his activities and the climatic conditions. Furthermore a diet should be considered as a whole and should be balanced with respect to its various constituents.
The recommended daily amount (RDA) of a nutrient has been defined as ‘the average amount which should be provided per head in a group of people if the needs of practically all members (sic) of the group are to be met’. From this it can be seen that individual variations are recognized and, since in any group there will be some people who require substantially more of the nutrient than others, the recommended amount should include a considerable safety margin to allow for this. The RDA for energy and for various specific nutrients for males and females of different ages living in the United Kingdom is given in Table 10.1. The figures are taken from the Report of the Committee on Medical Aspects of Food Policy published by the Department of Health and Social Security in 1979.
|Age range (years)||Energy*||Protein (g)||Thiamin (mg)||Riboflavin (mg)||Nicotinic acid equivalents(mg)†||Ascorbic acid(mg)||Vitamin A retinol equivalents(μg)‡||Vitamin D cholecalciferol(μg)§||Calcium(mg)||Iron(mg)|
|Assuming a sedentary life||10·0||2400||60||1·0||1·6||18||30||750||—||500||10|
|Assuming a sedentary life||8·0||1900||47||0·8||1·3||15||30||750||—||500||10|
§No dietary sources may be necessary for children and adults who are sufficiently exposed to sunlight, but during the winter children and adolescents should receive 10 μg (400 i.u.) daily by supplementation. Adults with inadequate exposure to sunlight, for example those who are housebound, may also need a supplement of 10 μg daily.
The main energy sources are carbohydrates from plants and fats of animal or vegetable origin. The use of protein as a source of energy is normally incidental to the continuous turnover of body protein which occurs throughout life (page 185). However, if insufficient carbohydrate and fat are available to cover essential energy requirements, protein will be called upon for this purpose instead of being used for the synthesis of nitrogenous compounds.
The energy used by all living creatures is ultimately derived from the sun. Photosynthetic autotrophs (self-feeders) use carbon dioxide as their sole source of carbon, and solar energy to convert it into glucose according to the overall equation:
Energy derived from the breakdown of glucose is used for all their other energy-requiring reactions. The heterotrophs (feeding on others), which include animals and most microorganisms, are only able to use chemical energy, stored in the form of organic compounds, which also provide the carbon required for synthetic purposes. Consequently the heterotrophs are totally dependent on the autotrophs. In fact, photosynthetic and heterotrophic organisms are interdependent, since while the former use carbon dioxide and produce organic compounds and oxygen, the latter utilize organic compounds and oxygen and produce carbon dioxide. This results in a cycle of food materials and a flow of energy that is derived externally.
The quantity of carbon used annually in the photosynthetic production of glucose is enormous, being estimated at some 150 × 109 tons, requiring about 5 × 1019 calories of solar energy. This conversion is achieved largely in forests, which are consequently responsible for controlling the level of oxygen and carbon dioxide in the atmosphere.
|1 g carbohydrate*||16 kJ (3·75 kcal)|
|1 g protein||17 kJ (4 kcal)|
|1 g fat||37 kJ (9 kcal)|
|1 gethanol||29 kJ (7 kcal)|
Although good health may be maintained on diets of widely varying composition, in the adequately nourished western world protein usually supplies between 10 and 13% of the total energy. This value appears to be independent of income because, where there is a greater consumption of expensive protein-rich animal foods, there is also an increase in the consumption of fats and refined sugars.
Dietary carbohydrate is derived almost entirely from plant sources mainly as starch and sucrose. Carbohydrate-rich foods are mostly relatively cheap and are readily digested and absorbed. Cereals and root crops give the highest yields of consumable energy per unit of land and contain appreciable amounts of protein (wheat 8–13%, rice 6% and maize 9%) as well as minerals and vitamins.
In the developing countries, large amounts of cereals and cereal products are consumed and carbohydrate supplies up to 75% of the total energy. However, in high income countries, where only 50–60% of the energy is obtained from carbohydrate, starchy foods tend to be replaced by sucrose, widely used for sweetening. This trend, now also beginning in the developing countries, is detrimental because while refined sugar provides energy it contains none of the ‘protective’ nutrients, i.e. protein, minerals and vitamins, hence the expression ‘empty calories’. According to the British nutritionist Yudkin, sugar and sugar-containing foods are nutritionally disastrous, contributing not only to obesity, dental caries and diabetes but also to coronary thrombosis and ischaemic heart disease. The truth of these latter claims has not been fully established.
Fat is important because of its high energy content. Diets containing little fat are bulky and unpalatable; since fat reduces the motility of the stomach and delays the secretion of gastric juice, fat-free diets tend to lack ‘staying power’. Natural fats also supply essential fatty acids, fat-soluble vitamins and choline in the form of lecithin.
Fat may contribute from 10% to 40% of the total energy and well-fed families in Britain consume 80–100 g per day (3·0–3·7 MJ or 720–900 kcal). Although diets in which fat supplies less than 20% of the total energy tend to be unappetizing, low-fat diets have no ill-effects provided that the requirements for essential fatty acids and fat-soluble vitamins are met. An excessive intake of fat, e.g. more than 150 g per day, may result in ketosis (page 262). However, in cold climates the natural tendency to increase fat consumption is accompanied by adaptation to the higher intake. The importance of dietary fat in relation to coronary heart disease is discussed on page 267 and the problem of energy balance and obesity on page 268.
It has been found that, when rats are fed on a diet completely devoid of fat, they fail to thrive and develop characteristic skin lesions; when fat is restored to the diet the lesions heal and normal growth is resumed. The factors present in fat that are essential for rat growth are certain polyunsaturated fatty acids. Linoleic (18:2) and arachidonic (20:4) acids have the highest biological activity and are able both to restore growth and to cure the skin lesions; linolenic acid (18:3), only does the former. The polyunsaturated fatty acids (PUFA) whose structure is shown in Figure 10.1 must be supplied in the diet because there is no system in the body for introducing double bonds between the central double bond of oleic acid and the methyl end of the fatty acid chain. Linoleic acid is present in large amounts in many vegetable oils (Table 10·2), while arachidonic acid is found only in animal fats and in small amounts. Human milk fat contains about 8% of linolenic acid but cow’s milk is a very poor source.
|Animal fats||Linoleic acid||Other polyunsaturated fatty acids|
|Butter, milk and cream||2||1|
Until recently the essential fatty acids were considered to be of little practical importance in human nutrition and their function was largely unknown. The discovery of the prostaglandins (page 364) suggested that their role as precursors of this group of compounds might explain why they must be supplied in the diet. However, estimation of the amounts required for prostaglandin synthesis showed that they were some orders of magnitude lower than the estimated overall dietary requirement of 2–10 g linoleic acid per day for humans. Moreover, since inhibitors of prostaglandin synthetase do not induce symptoms of essential fatty acid deficiency other functions are implied.
Deficiency states seem to induce membrane disorders such as abnormalities of the erythrocytes, increased skin permeability and mitochondrial damage resulting in an increased metabolic rate. The essential fatty acids also play a part in the transport and metabolism of cholesterol (page 267). Recently, evidence has been obtained that the various esential fatty acids may not have identical functions and it is suggested that linolenic acid and the ω-3 fatty acids may be required in their own right. The polyunsaturated fatty acids are protected against peroxidation by vitamin E (page 158).
In normal circumstances an outright dietary deficiency of essential fatty acids is unlikely to occur but currently there is considerable speculation as to the role of subclinical deficiency in the aetiology of a number of diseases including ischaemic heart disease (page 267), cystic fibrosis and multiple sclerosis.
Protein requirements depend on the nature of the proteins and their composition. Strictly speaking the need for protein is a need for the essential amino acids (page 277) which the body is unable to synthesize for itself. However, the so-called ‘dispensable’ amino acids also fulfil an important nutritional role, since not only are they directly incorporated into proteins but they also provide a non-specific source of nitrogen for the synthesis of a great variety of compounds. If they were not available, the essential amino acids would be used for this purpose and the requirements for these would be greatly increased. However, since every protein contains large amounts of the non-essential amino acids, consideration need only be given to the supply of essential amino acids which have a more restricted distribution. Animal proteins are relatively rich in the essential amino acids and are normally considered to be nutritionally more desirable than plant proteins which tend to be deficient in tryptophan, lysine and the sulphur-containing amino acids and may contain other amino acids in excess. Major imbalances with respect to leucine and glutamic acid can have harmful effects. Notable exceptions to the general rule that animal proteins are nutritionally superior to plant proteins are collagen and its derivative, gelatin, which are almost totally deficient in tryptophan, tyrosine and the sulphur-containing amino acids. The amino acid composition of some representative proteins is given in Table 10·3.