NUTRITION AND METABOLISM

'Introduction to Nutrition and Metabolism'

 An adult eats about a tonne of food a year. This book attempts to answer the question ‘Why?’—by exploring the need for food and the uses to which food is put in the body. Some discussion of chemistry and biochemistry is obviously essential in order to investigate the fate of food in the body, and why there is a continual need for food throughout life. Therefore, in the following chapters various aspects of biochemistry and metabolism will be discussed. This should provide not only the basis of our present understanding, knowledge and concepts in nutrition, but also, more importantly, a basis from which to interpret future research findings and evaluate new ideas and hypotheses as they are formulated. We eat because we are hungry. Why have we evolved complex physiological and psychological mechanisms to control not only hunger, but also our appetite for different types of food? Why do meals form such an important part of our life?

1.1

The need for energy

There is an obvious need for energy to perform physical work. Work has to be done to lift a load against the force of gravity, and there must be a source of energy to perform that work. The energy used in various activities can readily be measured, as can the metabolic energy yield of the foods that are the fuel for that work (see Table 1.1). This means that it is possible to calculate a balance between the intake of energy, as metabolic fuels, and the body’s energy expenditure. Obviously, energy intake has to be appropriate for the level of energy expenditure; neither excess intake nor a deficiency is desirable.
Quite apart from this visible work output, the body has a considerable requirement for energy, even at rest. Only about one-third of the average person’s energy expenditure is for obvious work. Two thirds is required for maintenance of the body’s functions, homeostasis of the internal environment, and metabolic integrity. This energy requirement, the basal metabolic rate, can be measured by the output of heat when the subject is completely at rest.
Part of this basal energy requirement is obvious: the heart beats to circulate the blood; respiration continues; there is considerable electrical activity in nerves and muscles, whether they are ‘working’ or not; and the kidneys expend a considerable amount of energy during the filtration of waste products from the bloodstream. All of these processes require a metabolic energy source. Less obviously, there is also a requirement for energy for the wide variety of biochemical reactions occurring all the time in the body: laying down reserves of fat and carbohydrate; turnover of tissue proteins; transport of substrates into, and products out of, cells; and the production and secretion of hormones and neurotransmitters.

1.1.1

Units of energy

Energy expenditure is measured by the output of heat from the body. The unit of heat used in the early studies was the calorie—the amount of heat required to raise the temperature of 1 g of water by 1°C. The calorie is still used to some extent in nutrition; in biological systems the kilocalorie, kcal (sometimes written as ‘Calorie’, with a capital C) is used. One kilocalorie is 1000 calories (103 cal), and hence the amount of heat required to raise 1 kg of water through 1°C. Correctly, the joule (J) is used as the unit of energy. The joule is an SI unit, named after James Prescott Joule, who first showed the equivalence of heat, mechanical work and other forms of energy. In biological systems the kilojoule (kJ, =103 J=1000 J) and megajoule (MJ, =106 J=1000000 J) are used.
To convert between calories and joules:
1 kcal=4.186 kJ (normally rounded off to 4.2 kJ)
1 kJ=0.239 kcal (normally rounded off to 0.24 kcal)
Average energy expenditure of adults is between 7.5 and 10 MJ day−1 for women and 8 to 12 MJ day−1 for men.

1.2

Metabolic fuels

The dietary sources of metabolic energy (the metabolic fuels) are carbohydrates, fats, protein and alcohol. The metabolism of these fuels results in the production of carbon dioxide and water (and also urea in the case of protein. They can be converted to the same end-products chemically, by burning in air. Although the process of metabolism in the body is more complex, it is a fundamental law of chemistry that, if the starting material and end-products are the same, the energy yield is the same, regardless of the route taken. Therefore, the energy yield of foodstuffs can be determined by measuring the heat produced when they are burnt in air, making allowance for the extent to which they are digested and absorbed from foods. The energy yields of the metabolic fuels in the body, allowing for metabolic efficiency, are shown in Table 1.1.

1.2.1

The need for carbohydrate and fat

Although there is a requirement for energy sources in the diet, it does not matter unduly how that requirement is met. There is no requirement for a dietary source of carbohydrate, the body can make as much carbohydrate as it requires from proteins. Similarly, there is no requirement for a dietary source of fat, apart from the essential fatty acids, and there is certainly no requirement for a dietary source of alcohol.
Although there is no requirement for fat in the diet, fats do have nutritional importance:
• It is difficult to eat enough of a very low-fat diet to meet energy requirements. As shown in Table 1.1, the energy yield of 1 g of fat is more than twice that of 1 g of carbohydrate or protein. It would be necessary to eat a considerably larger amount of a very low-fat diet to meet energy needs from carbohydrate and protein alone; indeed, it is unlikely that it would be possible to eat a sufficient bulk of food to meet energy requirements from a diet that was devoid of fat. The problem in Western countries is an undesirably high intake of fat, contributing to the development of obesity and the diseases of affluence.
• Four of the vitamins, A, D, E and K, are fat soluble, and are found in fatty and oily foods. More importantly, because they are absorbed dissolved in fat, their absorption requires an adequate intake of fat. On a very low-fat diet the absorption of these vitamins will be inadequate to meet requirements.
• There is a requirement for small amounts of the essential fatty acids. These are constituents of fats which are required for specific functions; they cannot be formed in the body, so they must be provided in the diet.
• In many foods much of the flavour (and hence the pleasure of eating) is carried in the fat.
• Fats lubricate food and make it easier to chew and swallow.

1.2.2

The need for protein

Unlike fats and carbohydrates, there is a requirement for protein in the diet. In a growing child this need is obvious. As the child grows, and the size of its body increases, so there is an increase in the total amount of protein in the body. Adults also require protein in the diet. There is a continual small loss of protein from the body, for example in hair, shed skin cells, enzymes and other proteins secreted into the gut and not completely digested, and so on. More importantly, there is turnover of tissue proteins, which are continually being broken down and replaced. Although there is no change in the total amount of protein in the body, an adult with an inadequate intake of protein will be unable to replace this loss, and will lose tissue protein.

1.2.3

The need for minerals and vitamins

In addition to metabolic fuels and protein, the body has a requirement for a variety of mineral salts, in very much smaller amounts. Obviously, if a metal or ion has a function in the body, it must be provided by the diet, since the different elements cannot be interconverted. Again, the need is obvious for a growing child; as the body grows in size, so the total amounts of minerals in the body will increase. In adults there is a turnover of minerals in the body, and losses must be replaced from the diet. There is a requirement for a different group of nutrients, also in very small amounts—the vitamins. These are relatively complex organic compounds that have essential functions in metabolic processes. They cannot be synthesized in the body, and so must be provided by the diet. There is turnover of the vitamins, so there must be replacement of the losses.

1.3

Hunger and appetite

Human beings have evolved an elaborate system of physiological and psychological mechanisms to ensure that the body’s needs for metabolic fuels and nutrients are met. There are hunger and satiety centres in the brain, which stimulate us to begin eating (the hunger centres in the lateral hypothalamus), or to stop eating when hunger has been satisfied (the satiety centres in the ventromedial hypothalamus). A great deal is known about the role of these brain centres in controlling food intake, and there are drugs that modify responses to hunger and satiety. Such drugs can be used to reduce appetite in the treatment of obesity or stimulate it in people with anorexia.
The hypothalamic hunger and satiety centres control food intake very precisely. Without conscious effort, most people can regulate their food intake to match energy expenditure very closely; they neither waste away from lack of metabolic fuel for physical activity nor lay down excessively large reserves of fat. Even people who have excessive reserves of body fat, and can be considered to be so overweight or obese as to be putting their health at risk, balance their energy intake and expenditure relatively well, considering that the average intake is a tonne of food a year; the most obese people weigh about 250 kg (compared with average weights between 60 and 100 kg), and it takes many years to achieve such a weight. In addition to hunger and satiety, which are basic physiological responses, food intake is controlled by appetite, which is related not only to physiological need but also to the pleasure of eating: flavour and texture, and a host of social and psychological factors.

1.3.1

Taste and flavour

Taste buds on the tongue can distinguish five basic sensations: salt, savouriness, sweet, bitter and sour. Salt (correctly the mineral sodium) is essential to life, and wild animals will travel great distances to a salt lick. Like other animals, human beings have evolved a pleasurable response to salty flavours, which ensures that physiological needs are met. However, there is no shortage of salt in developed countries and, average intakes of salt are considerably greater than requirements and may pose a hazard to health.
The sensation of savouriness is distinct from that of saltiness and is sometimes called umami (the Japanese for savoury). It is largely due to the presence of free amino acids in foods. Stimulation of the umami receptors of the tongue is the basis of flavour enhancers such as monosodium glutamate, which is an important constituent of traditional oriental condiments, and is widely used in manufactured foods. The other instinctively pleasurable taste is sweetness. The evolutionary reason for this is less clear than the response to salt, but it can be argued that ripe fruits are sweet (the process of ripening is largely one of converting starches to sugars) and, in general, fruits are better sources of nutrients when they are ripe. Sourness and bitterness are instinctively unpleasant sensations. Learnt behaviour will overcome the instinctive aversion, but this is a process of learning or acquiring tastes, not an innate or instinctive response.
It is likely that the aversion to bitterness evolved as a protection against poisonous compounds found in some plants, many of which are bitter. The aversion to sourness is presumably the converse of the pleasurable reaction to sweetness—sour unripe fruit is a poorer source of nutrients. In addition to the sensations of taste provided by the taste buds on the tongue, a great many flavours can be distinguished by the sense of smell. Again, some flavours and aromas (fruity flavours, fresh coffee and, at least to a non-vegetarian, the smell of roasting meat) are pleasurable, tempting people to eat and stimulating their appetite. Other flavours and aromas are repulsive, warning us not to eat the food.
Again, this can be seen as a warning of possible danger; the smell of decaying meat or fish tells us that it is not safe to eat. Like the acquisition of a taste for bitter or sour foods, a taste for foods with what would seem at first to be an unpleasant aroma or flavour can also be acquired. Here things become more complex—a pleasant smell to one person may be repulsive to another. Some people enjoy the smell of cooked cabbage and sprouts; others can hardly bear to be in the same room. The durian fruit is a highly prized delicacy in South East Asia, yet to the uninitiated it smells of sewage or faeces—hardly an appetizing aroma.

1.3.2

Why do people eat what they do ?

People have different responses to the same taste or flavour. This may be explained in terms of childhood memories, pleasurable or otherwise. An aversion to the smell of a food may protect someone who has a specific allergy or intolerance (although sometimes people have a craving for the foods to which they are intolerant). Most often we simply cannot explain why some people dislike foods that others eat with great relish.

Various factors may influence why people choose to eat particular foods:

1.3.2.1

The availability and cost of food

In developed countries the simple availability of food is not likely to be a constraint on choice. There is a wide variety of foods available, and when fruits and vegetables are out of season at home they are imported; frozen, canned or dried foods are widespread. By contrast, in developing countries the availability of food is a major constraint on what people choose. Little food is imported, and what is available will depend on the local soil and climate. Even in normal times the choice of foods may be very limited, while in times of drought there may be little or no food available at all, and what little is available will be very much more expensive than most people can afford.
Even in developed countries, the cost of food may be important, and for the most disadvantaged members of the community, poverty may impose severe constraints on the choice of foods. In developing countries, cost is the major problem. Indeed, even in times of famine, food may be available, but it is so expensive that few people can buy it.

1.3.2.2

Religion, habit and tradition

Religious and ethical considerations are important in determining the choice of foods. Observant Jews and Muslims will eat meat only from animals that have cloven hooves and chew the cud. The terms kosher in Jewish law and hallal in Islamic law both mean clean; the meat of other animals, which are scavenging animals, birds of prey and detritus-feeding fish, is regarded as unclean (traife or haram). We now know that many of these forbidden animals carry parasites that can infect human beings, so these ancient prohibitions are based on food hygiene.
Hindus will not eat beef. The reason for this is that the cow is far too valuable, as a source of milk, dung (as manure and fuel) and as a beast of burden, for it to be killed just as a source of meat. Many people refrain from eating meat as a result of humanitarian concern for the animals involved.
Vegetarians can be divided into a variety of groups, according to the strictness of their diet:
• ovolactovegetarians will eat eggs and milk, but not meat
• lactovegetarians will eat milk, but not eggs
• vegans will eat only plant foods, and no foods of animal origin Perhaps the strictest of all vegetarians are the Jains (originally from Gujarat in India), whose religion not only prohibits the consumption of meat, but extends the concept of the sanctity of life to insects and grubs as well—an observant Jain will not eat any vegetable that has grown underground, lest an insect was killed in digging it up.
Foods that are commonly eaten in one area may be little eaten elsewhere, even though they are available, simply because people have not been accustomed to eating them. To a very great extent, eating habits as adults continue the habits learnt as children.
Haggis and oatcakes are rarely eaten outside Scotland, except as speciality items; black pudding is a staple of northern British breakfasts, but is rarely seen in the south-east of England. Until the 1960s, yogurt was almost unknown in Britain, apart from a few health food ‘cranks’ and immigrants from eastern Europe; many British children believe that fish is available only as rectangular fish fingers, whereas children in inland Spain may eat fish and other seafood three or four times a week. The French mock the British habit of eating lamb with mint sauce—and the average British reaction to such French delicacies as frogs’ legs and snails in garlic sauce is one of horror. The British eat their cabbage well boiled; the Germans and Dutch ferment it to produce sauerkraut.
This regional and cultural diversity of foods provides one of the pleasures of travel. As people travel more frequently, and become (perhaps grudgingly) more adventurous in their choice of foods, so they create a demand for different foods at home, and there is an increasing variety of foods available in shops and restaurants.
A further factor that has increased the range of foods available has been immigration of people from a variety of different backgrounds, all of whom have, as they have become established, introduced their traditional foods to their new homes. It is difficult to realize that in the 1960s there was only a handful of Tandoori restaurants in the whole of Britain, that Balti cooking was unknown until the 1990s, or that pizza was something seen only in southern Italy and a few specialist restaurants.
Some people are naturally adventurous, and will try a new food just because they have never eaten it before. Others are more conservative, and will try a new food only when they see someone else eating it safely and with enjoyment. Others are yet more conservative in their food choices; the most conservative eaters ‘know’ that they do not like a new food because they have never eaten it before.

1.3.2.3

Luxury status of scarce and expensive foods

Foods that are scarce or expensive have a certain appeal of fashion or style; they are (rightly) regarded as luxuries for special occasions rather than everyday meals. Conversely, foods that are widespread and cheap have less appeal. In the nineteenth century, salmon and oysters (which are now relatively expensive luxury foods) were so cheap that the Articles of apprentices in London specified that they should not be given salmon more than three times a week, and oysters were eaten by the poor. Conversely, chicken and trout, which were expensive luxury foods in the 1950s, are now widely available, as a result of changes in farming practice, and they form the basis of inexpensive meals. As farming practices change, so salmon is again becoming an inexpensive meal, and venison is no longer the exclusive preserve of the wealthy landed gentry or poachers.

1.3.2.4

The social functions of food

Human beings are essentially social animals, and meals are important social functions. People eating in a group are likely to eat better, or at least to have a wider variety of foods and a more lavish and luxurious meal, than people eating alone. Entertaining guests may be an excuse to eat foods that we know to be nutritionally undesirable, and perhaps to eat to excess. The greater the variety of dishes offered, the more people are likely to eat. As we reach satiety with one food, so another, different, flavour is offered to stimulate appetite. Studies have shown that, faced with only one food, people tend to reach satiety sooner than when a variety of different foods is on offer. This is the difference between hunger and appetite: even when we are satiated, we can still ‘find room’ to try something different.
Conversely, and more importantly, many lonely single people (and especially the bereaved elderly) have little incentive to prepare meals and no stimulus to appetite. Although poverty may be a factor, apathy (and often, in the case of widowed men, ignorance) severely limits the range of foods eaten, possibly leading to undernutrition. When these problems are added to the problems of infirmity, ill-fitting dentures (which make eating painful) and arthritis (which makes handling many foods difficult), and the difficulty of carrying food home from the shops, it is not surprising that we include the elderly among those vulnerable groups of the population who are at risk of undernutrition.

In hospitals and other institutions there is a further problem. People who are unwell may have low physical activity, but they have higher than normal requirements for energy and nutrients, as a part of the process of replacing tissue in convalescence or as a result of fever or the metabolic effects of cancer. At the same time, illness impairs appetite, and a side effect of many drugs is to distortthe sense of taste, depress appetite or cause nausea. It is difficult to provide a range of exciting and attractive foods under institutional conditions, yet this is what is needed to tempt the patient’s appetite.

By DAVID A.BENDER (Department of Biochemistry and Molecular Biology- University College London) in the book 'Introduction to Nutrition and Metabolism', Taylor & Francis, UK, 2005, p.1-8. Adapted and illustrated to be posted by Leopoldo Costa.