IGCSE NOTES : Biology - Human nutrition and Diet
All living organisms need food. An important difference between plants and animals is that green plants can make food in their leaves but animals have to take it in ‘ready-made’ by eating plants or the bodies of other animals.
The need for food
All living organisms need food. An important difference between plants and animals is that green plants can make food in their leaves but animals have to take it in ‘ready-made’ by eating plants or the bodies of other animals. In all plants and animals, food is used as follows:
It provides the substances needed for making new cells and tissues.
As a source of energy
Energy is required for the chemical reactions that take place in living organisms to keep them alive. When food is broken down during respiration, the energy from the food is used for chemical reactions such as building complex molecules. In animals the energy is also used for activities such as movement, the heart beat and nerve impulses. Mammals and birds use energy to maintain their body temperature.
For replacement of worn and damaged tissues
The substances provided by food are needed to replace the millions of our red blood cells that break down each day, to replace the skin that is worn away and to repair wounds.
A balanced diet must contain enough carbohydrates and fats to meet our energy needs. It must also contain enough protein of the right kind to provide the essential amino acids to make new cells and tissues for growth or repair. The diet must also contain vitamins and mineral salts, plant fi bre and water.
Energy can be obtained from carbohydrates, fats and proteins. The cheapest energy-giving food is usually carbohydrate; the greatest amount of energy is available in fats; proteins give about the same energy as carbohydrates but are expensive. Whatever mixture of carbohydrate, fat and protein makes up the diet, the total energy must be sufficient:
- to keep our internal body processes working (e.g. heart beating, breathing action)
- to keep up our body temperature, and
- to meet the needs of work and other activities.
The amount of energy that can be obtained from food is measured in calories or joules. One gram of carbohydrate or protein can provide us with 16 or 17 kJ (kilojoules). A gram of fat can give 37 kJ. We need to obtain about 12 000 kJ of energy each day from our food. It is fairly obvious that a person who does hard manual work, such as digging, will use more energy than someone who sits in an office. Similarly, someone who takes part in a lot of sport will need more energy input than someone who doesn’t do much physical exercise. Females tend to have lower energy requirements than males. Two reasons for this are that females
have, on average, a lower body mass than males, which has a lower demand on energy intake, and there are also different physical demands made on boys and girls. However, an active female may well have a higher energy requirement than an inactive male of the same age.
As children grow, the energy requirement increases because of the energy demands of the growth process and the extra energy associated with maintaining their body temperature. However, metabolism, and therefore energy demands, tends to slow down with age once we become adults due to a progressive loss of muscle tissue.
The 2400 kJ used during 8 hours’ sleep represents the energy needed for basal metabolism, which maintains the circulation, breathing, body temperature, brain function and essential chemical processes in the liver and other organs. If the diet includes more food than is needed to supply the energy demands of the body, the surplus food is stored either as glycogen in the liver or as fat below the skin and in the abdomen. In 2006, the Food Standards Agency in Britain recommended that, for a balanced diet, 50% of our energy intake should be made up of carbohydrate, 35% of fat (with not more than 11% saturated fat) and the remaining percentage made up of fibre.
Proteins are an essential part of the diet because they supply the amino acids needed to build up our own body structures. Estimates of how much protein we need have changed over the last few years. A recent WHO/FAO/UNU report recommended that an average person needs 0.57 g protein for every kilogram of body weight. That is, a 70 kg person would need 70 × 0.57 = 39.9, i.e. about 40 g protein per day. This could be supplied by about 200 g (7 ounces) lean meat or 500 g bread but 2 kg potatoes would be needed to supply this much protein and even this will not contain all the essential amino acids.
Vegetarian and vegan diets
There is relatively less protein in food derived from plants than there is in animal products. Vegetarians and semi-vegetarians, who include dairy products, eggs and possibly fish in their diets, will obtain sufficient protein to meet their needs. However, some vegetarian foods now contain relatively high proportions of protein: Quorn products (made from mycoprotein – derived from fungi) typically contain 14.5 g protein per 100 g, compared with 18.0 g protein per 100 g for beef sausage, and they do not contain animal fats. Vegans, who eat no animal products, need to ensure that their diets include a good variety of cereals, peas, beans and nuts in order to obtain all the essential amino acids to build their body proteins.
A pregnant woman who is already receiving an adequate diet needs no extra food. Her body’s metabolism will adapt to the demands of the growing baby although the demand for energy and protein does increase. If, however, her diet is deficient in protein, calcium, iron, vitamin D or folic acid, she will need to increase her intake of these substances to meet the needs of the baby. The baby needs protein for making its tissues, calcium and vitamin D are needed for bone development, and iron is used to make the haemoglobin in its blood.
‘Lactation’ means the production of breast milk for feeding the baby. The production of milk, rich in proteins and minerals, makes a large demand on the mother’s resources. If her diet is already adequate, her metabolism will adjust to these demands. Otherwise, she may need to increase her intake of proteins, vitamins and calcium to produce milk of adequate quality and quantity.
Most children up to the age of about 12 years need less food than adults, but they need more in proportion to their body weight. For example, an adult may need 0.57 g protein per kg body weight, but a 6–11-month baby needs 1.85 g per kg and a 10-year-old child needs 1.0 g per kg for growth. In addition, children need extra calcium for growing bones, iron for their red blood cells, vitamin D to help calcify their bones and vitamin A for disease resistance.
Malnutrition is often taken to mean simply not getting enough food, but it has a much wider meaning than this, including getting too much food or the wrong sort of food. If the total intake of food is not sufficient to meet the body’s need for energy, the body tissues themselves are broken down to provide the energy to stay alive. This leads to loss of weight, muscle wastage, weakness and ultimately starvation. Extreme slimming diets, such as those that avoid carbohydrate foods, can result in the disease anorexia nervosa. Coronary heart disease can occur when the diet contains too much fat. Deposits of a fatty substance build up in the arteries, reducing the diameter of these blood vessels, including the coronary artery. Blood clots are then more likely to form. Blood supply to the heart can be reduced resulting in angina (chest pains when exercising or climbing stairs, for example) and eventually a coronary heart attack. If food intake is drastically inadequate, it is likely that the diet will also be deficient in proteins, minerals and vitamins so that deficiency diseases such as anaemia, rickets and scurvy also make an appearance. Scurvy is caused by a lack of vitamin C (ascorbic acid) in the diet. Vitamin C is present in citrus fruit such as lemons, blackcurrants, tomatoes, fresh green vegetables and potatoes. It is not unusual for people in developed countries who rely
on processed food such as tinned products, rather than eating fresh produce, to suffer from scurvy. Symptoms of scurvy include bleeding under the skin, swollen and bleeding gums and poor healing of wounds. The victims of malnutrition due to food deficiencies such as those mentioned above will also have reduced resistance to infectious diseases such as malaria or measles. Thus, the symptoms of malnutrition are usually the outcome of a variety of causes, but all resulting from an inadequate diet. The causes of malnutrition can be famine due to drought or flood, soil erosion, wars, too little land for too many people, ignorance of proper dietary needs but, above all, poverty. Malnourished populations are often poor and cannot afford to buy enough nutritious food.
The world population doubled in the last 30 years but food production, globally, rose even faster. The ‘Green Revolution’ of the 1960s greatly increased global food production by introducing high-yielding varieties of crops. However, these varieties needed a high input of fertiliser and the use of pesticides, so only the wealthy farmers could afford to use them. Moreover, since 1984, the yields are no longer rising fast enough to feed the growing population or keep pace with the loss of farmland due to erosion and urbanisation. It is estimated that, despite the global increase in food production, 15% of the world population is undernourished and 180 million children are underweight. There are no obvious, easy or universal solutions to this situation. Genetically modified crops may hold out some hope but they are some way off. There is resistance to their introduction in some countries because of concerns about their safety, gene transfer to wild plants or animals, the creation of allergies, the cost of seed and, with some GM seed, the necessity to buy particular pesticides to support them. Redistribution of food from the wealthy to the poorer countries is not a practical proposition except in emergencies, and the process can undermine local economies. The strategies adopted need to be tailored to the needs and climate of individual countries. Crops suited to the region should be grown. Millet and sorghum grow far better in dry regions than do rice or wheat and need little or no irrigation. Cash crops such as coffee, tea or cotton can earn foreign currency but have no food value and do not feed
the local population. There has been a surge in the production of palm oil due to world demand for the product as a biofuel as well as for food manufacture. This has resulted in deforestation to provide land to grow the crop and is putting endangered species at risk of extinction. Countries such as Indonesia and Malaysia have been particularly affected. Where cash crops are grown, it might be better to use the land, where suitable, to cultivate food crops.
The agricultural practices need to be sustainable and not result in erosion. Nearly one-third of the world’s crop-growing land has had to be abandoned in the last 40 years because erosion has made it unproductive. Over-irrigation can also cause a buildup in soil salinity, making the land effectively sterile due to the osmotic problems the salt creates Conservation of land, water and energy is essential for sustainable agriculture. A reduction in the growth of the world’s population, if it could be achieved, would have a profound effect in reducing malnutrition. Apart from the measures outlined above, lives could be saved by such simple and inexpensive steps as provision of regular vitamin and mineral supplements. It is estimated that about 30 million children are deficient in vitamin A. This deficiency leads to blindness and death if untreated.
In the affl uent societies, e.g. USA and Europe, there is no general shortage of food and most people can afford a diet with an adequate energy and protein content. So, few people are undernourished. Eating too much food or food of the ‘wrong’ sort, however, leads to malnutrition of a different kind. Refi ned sugar (sucrose) This is a very concentrated source of energy. You can absorb a lot of sugar from biscuits, ice-cream, sweets, soft drinks, tinned fruits and sweet tea without ever feeling ‘full up’. So you tend to take in more sugar than your body needs, which may lead to you becoming overweight or obese. The food industry has been urged to reduce the sugar content of its products to help curb the increase in obesity in countries like Great Britain and America. Sugar is also a major cause of tooth decay (see ‘Mechanical digestion’).
Fatty deposits, called ‘plaques’, in the arteries can lead to coronary heart disease and strokes. These plaques are formed from lipids and cholesterol combined with proteins (low density lipoproteins or LDLs). Although the liver makes LDLs, there is evidence to suggest that a high intake of fats, particularly animal fats, helps raise the level of LDLs in the blood and increase the risk of plaque formation. Most animal fats are formed from saturated fatty acids, so called because of their molecular structure. Plant oils are formed from unsaturated fatty acids (polyunsaturates) and are thought less likely to cause fatty plaques in the arteries. For this reason, vegetable fats and certain margarines are considered, by some nutritionists, to be healthier than butter and cream. However, there is still much debate about the evidence for this.
Many of the processed foods in Western diets contain too little fi bre. White bread, for example, has had the fi bre (bran) removed. A lack of fi bre can result in constipation (see ‘Classes of food’). Unprocessed foods, such as unskinned potatoes, vegetables and fruit, contain plenty of fi bre. Food rich in fi bre is usually bulky and makes you feel ‘full up’ so that you are unlikely to overeat. Fibre enables the process of peristalsis to move food through the gut more effi ciently and may also protect the intestines from cancer and other disorders. As explained later, fi bre helps prevent constipation.
Overweight and obesity
These are different degrees of the same disorder. If you take in more food than your body needs for energy, growth and replacement, the excess is converted to fat and stored in fat deposits under the skin or in the abdomen. Obese people are more likely to suffer from high blood pressure, coronary heart disease and diabetes. Having extra weight to carry also makes you reluctant to take exercise. By measuring a person’s height and body mass, it is possible to use a chart to predict whether or not they have an ideal body mass.
Why some people should be prone to obesity is unclear. There may be a genetic predisposition, in which the brain centre that responds to food intake may not signal when suffi cient food has been taken in; in some cases it may be the outcome of an infectious disease. Whatever the cause, the remedy is to reduce food intake to a level that matches but does not exceed the body’s needs. Taking exercise helps, but it takes a great deal of exercise to ‘burn off’ even a small amount of surplus fat.
Classes of food
There are three classes of food: carbohydrates, proteins and fats. In addition include salts, vitamins, water and vegetable fibre (roughage). These substances are present in a balanced diet and do not normally have to be taken in separately.
Sugar and starch are important carbohydrates in our diet. Starch is abundant in potatoes, bread, maize, rice and other cereals. Sugar appears in our diet mainly as sucrose (table sugar) which is added to drinks and many prepared foods such as jam, biscuits and cakes. Glucose and fructose are sugars that occur naturally in many fruits and some vegetables.
Although all foods provide us with energy, carbohydrates are the cheapest and most readily available source of energy. They contain the elements carbon, hydrogen and oxygen (e.g. glucose is C6H12O6). When carbohydrates are oxidised to provide energy by respiration they are broken down to carbon dioxide and water. One gram of carbohydrate can provide, on average, 16 kilojoules (kJ) of energy. If we eat more carbohydrates than we need for our energy requirements, the excess is converted in the liver to either glycogen or fat. The glycogen is stored in the liver and muscles; the fat is stored in fat deposits in the abdomen, round the kidneys or under the skin. The cellulose in the cell walls of all plant tissues is a carbohydrate. We probably derive relatively little nourishment from cellulose but it is important in the diet as fi bre, which helps to maintain a healthy digestive system.
Animal fats are found in meat, milk, cheese, butter and egg-yolk. Plant fats occur as oils in fruits (e.g. palm oil) and seeds (e.g. sunfl ower seed oil), and are used for cooking and making margarine. Fats and oils are sometimes collectively called lipids. Lipids are used in the cells of the body to form part of the cell membrane and other membrane systems. Lipids can also be oxidised in respiration, to carbon dioxide and water. When used to provide energy in this way, 1 g fat gives 37 kJ of energy. This is more than twice as much energy as can be obtained from the same weight of carbohydrate or protein.
Fats can be stored in the body, so providing a means of long-term storage of energy in fat deposits. The fatty tissue, adipose tissue, under the skin forms a layer that, if its blood supply is restricted, can reduce heat losses from the body.
Lean meat, fi sh, eggs, milk and cheese are important sources of animal protein. All plants contain some protein, but soybeans, seeds such as pumpkin, and nuts are the best sources.
Proteins, when digested, provide the chemical substances needed to build cells and tissues, e.g. skin, muscle, blood and bones. Neither carbohydrates nor fats can do this so it is essential to include some proteins in the diet. Protein molecules consist of long chains of amino acids. When proteins are digested, the molecules are broken up into the constituent amino acids. The amino acids are absorbed into the bloodstream and used to build up different proteins. These proteins form part of the cytoplasm and enzymes of cells and tissues.
The amino acids that are not used for making new tissues cannot be stored, but the liver removes their amino (—NH2) groups and changes the residue to glycogen. The glycogen can be stored or oxidised to provide energy. One gram of protein can provide 17 kJ of energy. Chemically, proteins differ from both carbohydrates and fats because they contain nitrogen and sometimes sulfur as well as carbon, hydrogen and oxygen.
All proteins are similar to each other in their chemical structure, as are all carbohydrates. Vitamins, on the other hand, are a group of organic substances quite unrelated to each other in their chemical structure. The features shared by all vitamins are:
- They are not digested or broken down for energy.
- Mostly, they are not built into the body structures.
- They are essential in small quantities for health.
- They are needed for chemical reactions in the cells, working in association with enzymes.
Plants can make these vitamins in their leaves, but animals have to obtain many of them ready-made either from plants or from other animals. If any one of the vitamins is missing or defi cient in the diet, a vitamin-defi ciency disease may develop. Such a disease can be cured, at least in the early stages, simply by adding the vitamin to the diet. Fifteen or more vitamins have been identifi ed and they are sometimes grouped into two classes: watersoluble and fat-soluble. The fat-soluble vitamins are found mostly in animal fats or vegetable oils, which is one reason why our diet should include some of these fats. The water-soluble vitamins are present in green leaves, fruits and cereal grains.
These are sometimes called ‘mineral salts’ or just ‘minerals’. Proteins, carbohydrates and fats provide the body with carbon, hydrogen, oxygen, nitrogen, sulfur and phosphorus but there are several more elements that the body needs and which occur as salts in the food we eat.
Red blood cells contain the pigment haemoglobin. Part of the haemoglobin molecule contains iron and this plays an important role in carrying oxygen around the body. Millions of red cells break down each day and their iron is stored by the liver and used to make more haemoglobin. However, some iron is lost and needs to be replaced through dietary intake. Red meat, especially liver and kidney, is the richest source of iron in the diet, but eggs, groundnuts, wholegrains such as brown rice, spinach and other green vegetables are also important sources. If the diet is deficient in iron, a person may suffer from some form of anaemia. Insufficient haemoglobin is made and the oxygen-carrying capacity of the blood is reduced.
Calcium, in the form of calcium phosphate, is deposited in the bones and the teeth and makes them hard. It is present in blood plasma and plays an essential part in normal blood clotting. Calcium is also needed for the chemical changes that make muscles contract and for the transmission of nerve impulses. The richest sources of calcium are milk (liquid, skimmed or dried) and cheese, but calcium is present in most foods in small quantities and also in ‘hard’ water. Many calcium salts are not soluble in water and may pass through the alimentary canal without being absorbed. Simply increasing the calcium in the diet may not have much effect unless the calcium is in the right form, the diet is balanced and the intestine is healthy. Vitamin D and bile salts are needed for efficient absorption of calcium. Dietary fibre (roughage) When we eat vegetables and other fresh plant material, we take in a large quantity of plant cells. The cell walls of plants consist mainly of cellulose, but we do not have enzymes for digesting this substance. The result is that the plant cell walls reach the large intestine (colon) without being digested. This undigested part of the diet is called fibre or roughage. The colon contains many bacteria that can digest some of the substances in the plant cell walls to form fatty acids. Vegetable fibre, therefore, may supply some useful food material, but it has other important functions. The fibre itself and the bacteria, which multiply from feeding on it, add bulk to the contents of the colon and help it to retain water. This softens the faeces and reduces the time needed for the
undigested residues to pass out of the body. Both effects help to prevent constipation and keep the colon healthy. Most vegetables and whole cereal grains contain fibre, but white flour and white bread do not contain much. Good sources of dietary fibre are vegetables, fruit and wholemeal bread.
About 70% of most tissue consists of water; it is an essential part of cytoplasm. The body fluids, blood, lymph and tissue fluid are composed mainly of water. Digested food, salts and vitamins are carried around the body as a watery solution in the blood and excretory products such as excess salt and urea are removed from the body in solution by the kidneys. Water thus acts as a solvent and as a transport medium for these substances. Digestion is a process that uses water in a chemical reaction to break down insoluble substances to soluble ones. These products then pass, in solution, into the bloodstream. In all cells there are many reactions in which water plays an essential part as a reactant and a solvent.
Since we lose water by evaporation, sweating, urinating and breathing, we have to make good this loss by taking in water with the diet.
Kwashiorkor (roughly = ‘deposed child’) is an example of protein–energy malnutrition (PEM) in the developing world. When a mother has her second baby, the fi rst baby is weaned on to a starchy diet of yam, cassava or sweet potato, all of which have inadequate protein. The fi rst baby then develops symptoms of kwashiorkor (dry skin, pot-belly, changes to hair colour, weakness and irritability). Protein defi ciency is not the only cause of kwashiorkor. Infection, plant toxins, digestive failure or even psychological effects may be involved. The good news, however, is that it can often be cured or prevented by an intake of protein in the form of dried skimmed milk.
The term ‘marasmus’ is derived from a Greek word, meaning decay. It is an acute form of malnutrition. The condition is due to a very poor diet with inadequate carbohydrate intake as well as a lack of protein. The incidence of marasmus increases in babies until they reach the age of 12 months. Sufferers are extremely emaciated with reduced fat and muscle tissue. Their skin is thin and hangs in folds. Marasmus is distinguished from kwashiorkor because kwashiorkor is due to lack of protein intake, while energy intake is adequate. Treatment involves provision of an energy-rich, balanced diet, but the complications of the disorder, which may include infections and dehydration, also need attention to increase chances of survival and recovery.
Causes and effects of mineral and vitamin deficiencies
Iron is present in red meat, eggs, nuts, brown rice, shellfi sh, soybean fl our, dried fruit such as apricots, spinach and other dark-green leafy vegetables. Lack of iron in the diet can lead to iron-defi ciency anaemia, which is a decrease in the number of red blood cells. Red blood cells, when mature, have no nucleus and this limits their life to about 3 months, after which they are broken down in the liver and replaced. Most of the iron is recycled, but some is lost as a chemical called bilirubin in the faeces and needs to be replaced. Adults need to take in about 15 mg each day. Without suffi cient iron, your body is unable to produce enough haemoglobin, the protein in red blood cells responsible for transporting oxygen to respiring tissues. Iron is also needed by the muscles and for enzyme systems in all the body cells. The symptoms of anaemia are feeling weak, tired and irritable.
Vitamin D is the only vitamin that the body can manufacture, when the skin is exposed to sunlight. However, for 6 months of the year (October to April), much of western Europe does not receive enough UV rays in sunlight to make vitamin D in the skin. So, many people living there are at risk of not getting enough vitamin D unless they get it in their diet. Also, people who have darker skin, such as people of African, African-Caribbean and South Asian origin, are at risk because their skin reduces UV light absorption. Foods that provide vitamin D include oily fish such as sardines and mackerel, fi sh liver oil, butter, milk, cheese and egg-yolk. In addition, many manufactured food products contain vitamin D supplements. Vitamin D helps in the absorption of calcium and phosphorus through the gut wall. Bone is made of the mineral calcium phosphate. A lack of the vitamin therefore results in poor calcium and phosphorus deposition in bones, leading to softening. The weight of the body can deform bones in the legs, causing the condition called rickets in children. Adults defi cient in vitamin D can suffer from osteo-malacia; they are very vulnerable to fracturing bones if they fall.