Book: Nutrition: A Very Short Introduction (Very Short Introductions)

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A healthy adult eats about a tonne of food a year. This book attempts to answer the question why?—and it does this by exploring the need for food and the uses to which that food is put in the body. Clearly, we eat because we are hungry. However, why have we evolved such complex physiological and psychological mechanisms to control not only hunger and satiety, but also our appetite for different types of food? Why do meals form such an important part of our life?

There is an obvious need for energy from food 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. As will be discussed in , the energy used in various activities can be measured, as can the energy yield of the foods that provide the fuel for that work. Fats, carbohydrates, protein, and alcohol all provide metabolic fuels.

Apart from its role as a metabolic fuel, 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, because there is a continual turnover of body proteins, which have to be replaced. Protein nutrition will be discussed in .

In addition to metabolic fuels and protein, the body has a requirement for two groups of nutrients that are required in very much smaller amounts—the micronutrients, minerals and vitamins. If a metal or ion has a function in the body, it must be provided by the diet, since it is not possible to convert one chemical element into another. The vitamins are organic compounds with a variety of functions. They cannot be synthesized in the body, and so must be provided by the diet. Micronutrients will be discussed in .

Other compounds in the diet (especially from fruit and vegetables) are not dietary essentials, but they may have beneficial effects in reducing the risk of developing a variety of chronic diseases. These compounds will also be discussed in .

The need for water

The body’s first need is for water. The human body contains about 60 per cent water—a total of 42 litres in a 70 kg (11 st) person. We excrete water in our urine as a way of ridding the body of the end-products of metabolism, and so we obviously need an intake of water to balance losses from the body. It is possible to survive for several weeks without any food, by using body reserves of fat and protein, but without water, death from dehydration occurs within a few days. There is no storage of water in the body; if water intake is in excess of what is required to maintain the normal levels in the blood stream, cells, and tissues any excess is rapidly lost in the urine. Daily fluid balance for adults is shown in .

Average daily output of urine is often said to be 1.5 litres (although the figures in show that this is an over-estimate), and advertisements for bottled water suggest that we should drink at least this much water per day. At first glance it might seem obvious that we would need an intake of the same amount of fluid to replace the loss in urine. However, as shown in , total daily fluid output from the body is about 3 litres for an adult man and about 2.1 litres for a woman; urine accounts for less than half of this. Equally, fluid consumption in beverages accounts for only about two-thirds of total fluid intake.

Table 1. Daily fluid balance

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In addition to the obvious water in beverages, food provides a significant amount of water: around 22 per cent of total intake, and more if you eat the recommended five servings of fruit and vegetables per day. Most fruits and vegetables contain 60–90 per cent water.

A further source of water is metabolic water—the water produced when fats, carbohydrates, and proteins are oxidized to yield energy. This accounts for about 12 per cent of total water ‘intake’, and more on a high fat diet, or when metabolizing fat reserves. The camel is able to survive for a considerable time in desert conditions without drinking because it metabolizes the fat stored in its hump; the water produced in fat oxidation meets its needs.

Urine accounts for less than half the total fluid output from the body; as shown in , the remainder is made up of sweat, water in exhaled air, so-called insensible losses through the skin (this is distinct from the loss in sweat produced by sweat glands), and a relatively small amount in faeces. This last will also increase on a diet rich in fruit and vegetables, because of their content of dietary fibre. Part of the beneficial effect of a high fibre diet is that the fibre retains water in the intestinal tract, so softening the faeces.

Sweat losses obviously depend on the environmental temperature and the intensity of physical activity; we do indeed need to drink more in a hot environment or after strenuous exercise. Losses in exhaled air, faeces, and other insensible losses are relatively constant; urine output varies widely, depending on how much fluid has been consumed. Although average urine volume is 1–1.4 litres per day, this reflects average fluid intake; the output of urine required to ensure adequate excretion of waste material and maintain fluid balance without becoming dehydrated is no more than about 500 ml. Put simply, the more you drink, the more urine you will produce.

A final consideration is whether water is the most appropriate liquid to drink to balance large losses in sweat after vigorous exercise or in a hot climate. The answer is probably no. Sweating involves loss of mineral salts as well as water, and these losses have to be made good. Sports drinks contain balanced mixtures of mineral salts in the same proportions as they are lost in sweat, together, usually, with glucose or another carbohydrate as a source of metabolic fuel and to increase the absorption of mineral salts. Milk and fruit juices also provide mineral salts.

Hunger, satiety, and appetite

Human beings have evolved an elaborate system of physiological mechanisms to ensure that the body’s needs for metabolic fuels and nutrients are met, and to balance food intake with energy expenditure. The physiological systems for the control of appetite interact with psychological, social, environmental, and genetic factors, all of which have to be understood in order to understand eating behaviour.

There are hunger centres in the brain (in the hypothalamus) that stimulate us to begin eating, and satiety centres that signal us to stop eating when hunger has been satisfied. Damage to, or destruction of, the hunger centres leads to more or less complete loss of appetite, while electrical stimulation leads to feeding even if the person has eaten enough. Similarly, destruction of the satiety centres leads to uncontrolled eating, and electrical stimulation leads to cessation of feeding, even in someone who is physiologically hungry and in the fasting state.

These appetite control centres have links to other brain regions. The amygdala controls learnt food behaviour—in other words, knowing that something is a food, as opposed to non-food. A young child will put almost anything into its mouth, and gradually learns what is, and what is not, food. Another structure deep in the brain, the nucleus acumbens is part of the reward system of the brain, and is concerned with the pleasure of eating and rewards from food. The appetite control centres also have connections to the cortex and other higher brain centres, which mean that psychological factors (including individual likes and dislikes) can override physiological control of appetite.

The appetite centres respond to the different patterns of metabolic fuels in the bloodstream in the fed and fasting states, and also to hormones such as insulin (which is secreted by the pancreas as blood glucose rises) and glucagon (which is secreted by the pancreas when blood glucose falls), as well as to a number of hormones secreted by the gastro-intestinal tract. One of these hormones, ghrelin, which is secreted by the stomach, acts to increase appetite and stimulate feeding; the others, which are secreted mainly by the small intestine, act as satiety signals, telling us we have eaten enough.

The appetite centres control food intake remarkably precisely. Without conscious effort, most people 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. The average intake is a tonne of food a year, while the record obese people weigh about 250–300 kg (39–47 st), compared with average weights between 60–100 kg (9–16 st), and it takes many years to achieve such a weight. A gain or loss of 5 kg (11 lb) body weight over 6 months would require only a 1 per cent daily mismatch between food intake and energy expenditure.

In addition to the immediate control of feeding by sensations of hunger and satiety, there is long-term regulation of food intake and energy expenditure. This is a function of the hormone leptin, which is secreted mainly by adipose tissue, where the fat reserves of the body are contained. The circulating concentration of leptin is determined mainly by the amount of adipose tissue (fat) in the body, so that leptin acts as a signal of the size of body fat reserves. In women, low levels of leptin, reflecting adipose tissue reserves that are not adequate to permit a normal pregnancy, both increase food intake and lead to cessation of ovulation and menstruation. This happens when body weight falls to below about 45 kg (7 st).

In addition to its role in appetite control, leptin also acts to increase energy expenditure and body temperature. It does this by increasing metabolic rate (so-called non-shivering thermogenesis), rather than through shivering and physical activity, and so promotes the loss of adipose tissue.

Appetite

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, texture, and a variety of social and psychological factors. In addition, we become accustomed to eating at a set time, and the clock can provide a cue to eat.

Taste buds on the tongue can distinguish five basic tastes: salt, savoury, sweet, bitter, and sour, as well as a less well understood ability to taste fat. The ability to taste sweetness, savouriness, and fat permits detection of nutrients; the ability to taste sourness and bitterness permits avoidance of toxins in foods.

Salt (more correctly, the mineral sodium) is essential to life, and wild animals will travel great distances to find a salt lick. Like other animals, human beings have evolved a pleasurable response to salty flavours—this ensures that physiological needs are met. However, there is no shortage of salt in developed countries; indeed average intakes of salt are considerably greater than requirements, and pose a hazard to health.

The sensation of savouriness is distinct from that of saltiness, and is sometimes called umami (the Japanese word for ‘savoury’). It is largely due to the presence of free amino acids in foods, and permits detection of protein-rich foods. The stimulation of the umami receptors of the tongue is the basis of flavour enhancers such as monosodium glutamate, an important constituent of traditional oriental condiments that is often used in manufactured foods.

The other instinctively pleasurable taste is sweetness, which permits the detection of carbohydrates, and hence energy sources. While it is only sugars (and artificial sweeteners) that taste sweet, human beings (and some other animals) secrete the enzyme amylase in saliva, which catalyses the breakdown of a small amount of starch, the major dietary carbohydrate, to sweet-tasting sugars, while the food is being chewed.

The tongue is also sensitive to the taste of free fatty acids, and secretes an enzyme (lipase) that catalyses the breakdown of a small amount of fat in the food in the mouth to yield free fatty acids. This permits the detection of fat-rich foods as an energy source.

Sourness and bitterness are instinctively unpleasant sensations; many of the toxins that occur in foods have a bitter or sour flavour. Learnt behaviour will overcome the instinctive aversion, but this is a process of learning or acquiring tastes, not an innate or instinctive response.

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. 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 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 a repulsive one to another. Some people enjoy the smell of cooked cabbage and Brussels sprouts, while others can hardly bear to be in the same room. The durian fruit is a highly prized delicacy in Southeast Asia, yet to the uninitiated it has the unappetizing aroma of sewage or faeces.

Disorders of appetite

Extremely rarely, severe obesity may be due to a genetic failure of the secretion of leptin, insensitivity of the leptin receptor, or a defect in a part of the signalling pathway in response to leptin. However, more commonly, it is the interactions between the higher brain centres and the appetite control centres of the hypothalamus that explain why people eat more than they need, and so become overweight or obese. The physiological control of hunger and satiety can be over-ridden by signals from the reward centres of the brain that increase the pleasure of eating. Obesity will be discussed in .

The more serious eating disorders of anorexia nervosa and bulimia are also due to interactions between higher brain centres and the hunger and satiety centres of the hypothalamus, so that a variety of psychological factors can override the normal sensations of hunger.

Anorexia nervosa is characterized by a very severe restriction of food intake, often coupled with excessive exercise, in a desperate attempt not to gain weight. The anorexic patient is commonly very clever about hiding the condition, eating very little apart from salad stuff, playing with food on the plate rather than eating it, sometimes hiding food under the table to pretend it has been eaten. Most sufferers are adolescent girls, although boys can also develop anorexia, and in some cases it develops for the first time in later life. The anorexic patient frequently has a distorted self-image of her or his body, especially as the physical changes of puberty develop, and many welcome the failure of menstruation that occurs as leptin levels fall (due to a body weight below about 45 kg (7 st)). It is estimated that up to 2 per cent of adolescent girls go through a phase of anorexia. In most cases they can be treated successfully by appropriate sensitive psychological help, but a small number continue to have intermittent problems in their eating behaviour throughout life.

Like anorexia nervosa, the underlying cause of bulimia nervosa is psychological. In addition to severe restriction of food intake much of the time, bulimic patients binge eat, consuming a very large amount of food in a very short time, followed by desperate attempts to lose the food they have consumed by inducing vomiting and taking laxatives and diuretics. Again, adolescent girls are most commonly affected, but unlike anorexic patients, their body weight is often within the normal range. Like anorexia, treatment is by psychological help, although in some cases antidepressant medication is successful.

Why do people eat what they do?

People have different responses to any given 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 of which they are intolerant). Most often we simply cannot explain why some people dislike foods that others eat with great relish.

A number of factors influence why people choose either to eat or not to eat particular foods. These include: the availability and cost of foods; the time taken to prepare and consume food; space to prepare and store food; disability and infirmity; personal likes and dislikes; intolerance or allergy; whether you are eating alone or in company; marketing pressure and advertising; religious and ethical taboos; perceived or real health benefits and risks; having to consume a modified diet for the control of a disease; and illness or medication.

In developed countries the easy availability of food means there is little 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 widely available. By contrast, in developing countries the availability of food may be a major constraint on what people choose. Even in developed countries, the cost of food is important, and for the more disadvantaged members of the community, poverty may impose severe constraints on their choice of foods.

Religious and ethical considerations are important in determining the choice of foods. Observant Jews and Muslims will only eat meat 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, that of scavenging animals, birds of prey, and detritus-feeding fish, is regarded as unclean (traife or haram, respectively). We now know that many of these forbidden animals carry parasites that can infect human beings, so these ancient prohibitions can be seen to be 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 and dung (as manure and fuel), and as a beast of burden, for it to be killed as a source of meat.

Many people refrain from eating meat as a result of humanitarian concern for the animals involved, or because of real or perceived health benefits. Vegetarians can be divided into various groups, according to the strictness of their diet: some avoid red meat, but will eat poultry and fish; some specifically avoid beef because of the potential risk of contracting variant Creutzfeld-Jacob disease from BSE-infected animals; pescetarians eat fish, but not meat or poultry; ovo-lacto-vegetarians will consume eggs and milk, but not meat or fish; lacto-vegetarians will consume milk, but not eggs; Vegans will eat only plant foods, and no foods of animal origin.

Many people choose to eat organically produced foods in preference to those produced by conventional or intensive farming methods. Organic foods are plants grown without the use of (synthetic) pesticides, fungicides, or inorganic fertilizers and prepared without the use of preservatives. Foodstuffs must be grown on land that has not been treated with chemical fertilizers, herbicides, or pesticides for at least three years. Organic meat is from animals fed on organically grown crops without the use of growth promoters, with only a limited number of medicines to treat disease, and commonly maintained under traditional, non-intensive, conditions. Within the European Union (EU), foods may be labelled as organic if they contain at least 95 per cent organic ingredients and not more than 0.9 per cent genetically modified ingredients.

People who wish to avoid pesticide, fungicide, and other chemical residues in their food, or genetically modified crops, will choose organic produce. Other people choose organic foods because they believe they are nutritionally superior to conventional produce, or because they have a better flavour. There is little evidence that organic produce is nutritionally superior to that produced by conventional farming, although if organic fruits and vegetables are also slower growing, and possibly lower yielding varieties, they may have a higher nutrient content. Many of the older, slower growing and lower yielding fruits and vegetables have a better flavour than more recently introduced varieties that are grown for their rapid yield of a large crop of uniform size and shape. Flavour does not depend on whether or not they are grown organically, but many organic farmers do indeed grow traditional, more flavourful, varieties.

The nutrient content of the same variety of a fruit or vegetable may vary widely, depending not only on the soil (and any fertilizers used), but also on how much sunlight the plant has received and how frequently it has been watered. The apples from one side of a tree may vary in nutrient content from those on the other side of the same tree. The yield, flavour, and nutrient content of the same crop may vary along the length of a field.

While organic produce is indeed free from chemical residues that may be harmful, there is still a potential hazard. Animal manure is used in organic farming to a very much greater extent than in conventional farming, and unless salad vegetables are washed well, there is a potential risk of food poisoning from bacteria in the manure that remains on the produce.

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 of adults are the continued habits learnt in childhood. Haggis and oat cakes travel south from Scotland as specialty items; black pudding is a staple of northern British breakfasts, but is rare in the southeast of England. Until the 1960s yoghurt was almost unknown in Britain, apart from among a few health food ‘cranks’ and immigrants from eastern Europe. Many British children believe that fish comes as rectangular fish fingers, while 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 American or British reaction to such French delicacies as frogs’ legs and snails is one of horror. The British eat their cabbage well boiled; the Germans and Dutch ferment it to produce sauerkraut. American cuisine reflects the rich cultural heritage of immigrants to the USA, and while many American foods such as hamburgers and hot dogs are now common throughout the world, others such as succotash and crullers remain, for the present, (regional) American specialties.

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 homelands. It is difficult to realize that in the 1960s there was only a handful of tandoori restaurants in the whole of Britain, that pizza was something seen only in southern Italy and a few specialist restaurants, or that Balti cooking and sushi were unknown until the 1990s.

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.

Foods that are scarce or expensive have a certain appeal that is to do with fashion or style; they are (rightly) regarded as luxuries for special occasions rather than everyday meals. Conversely, foods that are widespread and inexpensive have less appeal. In the 19th century, salmon and oysters were so cheap that the articles of apprentices in London specified that they should not be given salmon more than three times a week, while oysters were eaten by the poor. Through much of the 20th century, salmon was scarce and a prized luxury food; however, fish farming has increased the supply of salmon to such an extent that it is again an inexpensive food. Chicken, turkey, guinea fowl, 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. By contrast, fish such as cod, herring, and skate, once the basis of cheap meals, are now becoming scarce and expensive as a result of depletion of fish stocks by overexploitation.

Human beings are social animals, and meals have important social functions. People eating in a group are likely to eat better, or at least have a wider variety of foods and a more lavish and luxurious meal, than people eating alone. 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 our appetite. A number of studies have shown that, faced with only one food, people tend to reach satiety sooner than when a variety of 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. This may have been important in evolutionary terms: to ensure a mixed diet providing a variety of nutrients (and especially vitamins and minerals), rather than relying on a single food to simply meet energy needs.

Conversely, and more importantly, many lonely single people (and especially the bereaved elderly) have little incentive to prepare meals, and little stimulus to appetite. While poverty may be a factor, apathy (and frequently, in the case of widowed men, ignorance) severely limits the range of foods eaten, possibly leading to under-nutrition. When these problems are added to by the problems of ill-fitting dentures (which make eating painful), 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 the vulnerable groups of the population who are at risk of under-nutrition.

Food intolerance is a physiological reaction, not just a dislike of a food. The commonest is intolerance of lactose, the sugar in milk. Lactose intolerant people suffer from painful bloating, abdominal cramps, and diarrhoea when they consume more than a small amount of milk. Food allergy is more complex, and involves the formation of antibodies against a protein in a specific food or group of foods. Allergy to the protein gluten in wheat and other cereals is the basis of coeliac disease, and allergy to peanut proteins can lead to serious, and possibly life-threatening, reactions when nuts are eaten. It is now common for labels on manufactured foods to contain ‘allergy information’, such as whether or not they contain nuts, or may be contaminated with traces of nuts used elsewhere in the factory. Most supermarkets have an aisle of gluten-free foods suitable for people with coeliac disease.

Sometimes people change their diet to avoid certain foods, because of advice from their doctor or a dietitian, as a means of controlling a disease. A more difficult problem is to persuade healthy people to change their diet (e.g., by reducing fat, sugar, and salt intake, as will be discussed in ) in order to reduce their risk of developing heart disease and cancer in later life.

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