Chapter 11: Nutrition
In the 1740s, long sea voyages had a grim reputation. Sailors who had been at sea for months began to weaken. Their gums swelled and bled, old wounds opened again, and many died. The disease was called scurvy, and nobody knew what caused it.
In May 1747, a ship’s surgeon named James Lind tried something unusual aboard the naval vessel Salisbury. He selected twelve sailors who were already ill with scurvy. He housed them together and gave them all the same basic food. Then he divided them into six pairs, and gave each pair one extra item — cider to one pair, vinegar to another, sea water to another, and so on. One pair received two oranges and a lemon every day.
Within about a week, the difference was impossible to miss. The pair eating the citrus fruit recovered so well that one of them was fit to nurse the others. The rest stayed ill.
Lind published his findings in 1753. He could not say what the oranges contained, because nobody yet knew. The substance responsible, vitamin C, was only identified in the 1930s. But his simple comparison had already shown something important: food is not just fuel. A diet can supply plenty of energy and still be missing something the body cannot do without.
Diagram
Fig. 11.0-1 — Lind’s Comparison Aboard The Salisbury Simple line illustration of the deck of an eighteenth-century sailing ship with six pairs of sailors seated in a row; each pair labelled with the one extra item it received — cider, elixir of vitriol, vinegar, sea water, spiced paste with barley water, two oranges and a lemon; the citrus pair drawn upright and recovering, the others drawn seated and unwell; caption line beneath reading: the same basic food for all twelve, one item different for each pair.
11.1 Nutrition, Food And Diet
Everyone eats. Very few people stop to ask what eating actually delivers to the body. Before we can talk about what a good diet contains, four everyday words need to be pinned down, because in Biology they do not mean quite the same thing as they do at the table.
11.1.1 Nutrition And Nutrients
Your body is built out of chemical substances, and it uses up chemical substances every moment you are alive. It cannot manufacture most of them from nothing. They have to be supplied from outside, and supplying them is what nutrition means — the supply of the chemical compounds the body needs in order to live and work.
The compounds themselves are called nutrients. Some are organic, meaning they are carbon-based compounds made by living things — proteins, fats, carbohydrates and vitamins. Others are inorganic, such as mineral salts and water. Both kinds are needed, and neither can stand in for the other.
11.1.2 Food And Diet
A nutrient is a chemical compound. Food is the ordinary material we eat or drink that carries those compounds into us. Milk, bread, an egg, fish and an apple are all food, and each is a mixture of several nutrients rather than a single one.
No single food carries every nutrient in the right amount. That is why what matters is not one item but the whole set of items a person eats. That set is the diet — the combination of food items taken in the meals of a day.
A meal is simply the food taken at one time. Meals make up the diet, foods make up a meal, and nutrients make up the foods. FC 11.1 sets out that ladder from the top down.
Flowchart
FC 11.1 — From Nutrient To Diet Nutrient (a single chemical compound the body needs) → food (an item we eat, containing several nutrients) → meal (the food taken at one time) → diet (all the food items taken over the day)
When A Diet Is Balanced
A diet that supplies every class of nutrient, in the quantity that particular person needs, is called a balanced diet. The phrase ‘that particular person’ is doing real work here. The same plate of food may be well balanced for one person and quite wrong for another.
11.1.3 What A Balanced Diet Depends On
Because the requirement changes from person to person, four things have to be known before any diet can be called balanced.
- Age. A growing child is building new tissue and needs proportionally more protein than an adult, who is mainly maintaining what already exists. An older person, less active, needs fewer energy-giving foods.
- Sex. The requirements of males and females differ, partly because average body size differs and partly because certain needs, such as iron, differ.
- Lifestyle and nature of work. A farmer or an athlete uses far more energy in a day than someone doing seated work, and the diet has to replace what is spent.
- Special needs. A person who is ill, a pregnant woman or a mother who is breastfeeding has requirements above the ordinary, because the body is repairing itself or supporting another life.
Notice that all four are about quantity and proportion, not about which nutrients are needed. The classes of nutrient are the same for everybody. How much of each is not.
11.1.4 Meal, Appetite And Hunger
Two more everyday words are worth separating, because they describe two different signals.
Appetite is the desire to eat. It can be switched on by the smell of food or the sight of a favourite dish, even when the body is not short of anything.
Hunger is the uneasy sensation produced by an actual lack of food. It is a warning from the body, not a preference. A person can lose appetite while still being hungry in this sense — which is one reason loss of appetite during illness is taken seriously.
11.2 Why The Body Needs Food
Every living organism, from a bacterium to a blue whale, requires food. That is easy to state and easy to accept. It is more useful to ask what the food is actually spent on once it is inside, because the answer explains why a diet short in one nutrient can fail even when there is plenty to eat.
Food is put to six main uses in the body. FC 11.2 gathers them together.
Flowchart
FC 11.2 — The Six Uses Of Food In The Body Food taken in → branches into: (1) growth — building new protoplasm and new cells; (2) repair — replacing worn-out and injured cells; (3) energy — powering all life processes; (4) maintenance — keeping the chemical composition of cells steady; (5) raw materials — for making secretions such as enzymes, hormones, sweat and milk; (6) protection — resisting disease and infection
Building And Rebuilding
Growth means an increase in the living material of the body. New cells are made, and each new cell has to be filled with protoplasm — the living substance inside a cell, made largely of protein, water and other compounds. That material cannot appear from nowhere; it is assembled from nutrients brought in as food.
Repair is the same job done on a smaller scale, and it never stops. Cells wear out, get damaged, and die every day. The lining of your gut, your skin and your blood cells are all replaced continuously, and every replacement is built out of nutrients you ate.
Powering The Body
The third use is energy. Breathing, pumping blood, moving a muscle, sending a nerve impulse and even thinking all cost energy, and the body draws that energy by oxidising nutrients inside its cells. Unlike growth and repair, this demand never pauses — it continues through sleep, and it continues in an adult who has stopped growing altogether.
The fourth use is maintenance of the chemical composition of cells. A cell has to keep a particular balance of salts, water and other substances inside it. Food replaces what is constantly lost, so that this internal composition stays steady rather than drifting.
Raw Materials And Protection
The fifth use is the provision of raw materials. The body manufactures a long list of secretions — substances made by cells and released for a particular purpose. Enzymes, hormones, sweat and, in a nursing mother, milk are all secretions, and every one of them is built from nutrients supplied by food.
The sixth use is protection from disease and infection. A well-fed body resists infection far better than a poorly fed one, because the cells and substances that defend it are themselves made from nutrients. This is why undernutrition and repeated infection so often appear together.
11.3 The Six Classes Of Nutrients
Everything we eat, however varied the plate looks, is made of just six classes of nutrients. They are carbohydrates, fats, proteins, mineral salts, vitamins and water. Three of them are needed in large amounts, three in very small amounts, and no class can do another’s job.
11.3.1 Carbohydrates
Carbohydrates are compounds of carbon, hydrogen and oxygen, in which hydrogen and oxygen are always present in the ratio \(2:1\) — the same proportion as in water. Inside the cells they are oxidised, and the energy locked in their chemical bonds is released for the body to use.
This is why carbohydrates are described as the principal energy source of the body. They are cheap to obtain, easy to digest and easy to break down, so the body turns to them first and keeps its other nutrients for the jobs only those can do.
Carbohydrates are not a single substance but a family, and its members differ mainly in the size of the molecule. FC 11.3 sets out the family.
Flowchart
FC 11.3 — The Carbohydrate Family Carbohydrates → branch 1: sugars (soluble, sweet) → monosaccharides or single sugars (glucose, fructose, galactose) and disaccharides or double sugars (sucrose, maltose, lactose); branch 2: polysaccharides (insoluble, not sweet) → starch, cellulose, glycogen
11.3.1.1 Sugars
Sugars are the small carbohydrates. They dissolve in water and taste sweet, and these two properties go together — a substance can only reach the taste buds if it dissolves. Broadly, the sugars in our food fall into two categories.
A. Monosaccharides, or single sugars, are the simplest of all. The ones we meet in food have the formula \(C_6H_{12}O_6\). Their molecules are already small enough to pass through the wall of the intestine, so they need no digestion at all and are absorbed straightaway. Three simple sugars matter here.
- Glucose, popularly known as grape sugar, is the commonest and simplest sugar found in organisms. When we speak of blood sugar, we mean the level of glucose in the blood.
- Fructose is common in plants and is popularly called fruit sugar.
- Galactose is another simple sugar, found in milk.
Glucose is the form in which sugar actually travels in the blood and the form the cells oxidise. The energy it holds is considerable: one mole of glucose releases 686 kilocalories when it is completely oxidised.
B. Disaccharides, or double sugars, have the general formula \(C_{12}H_{22}O_{11}\). Each molecule is built from two single sugars joined together, which makes it too large to be absorbed as it is. A double sugar must first be digested — split back into its two halves — before it can enter the body. There are three to know.
- Sucrose is our commercial sugar, obtained in India chiefly from sugarcane and also from sugar-beet. It is made up of two simple sugars, glucose and fructose.
- Maltose, or malt sugar, is made up of two glucose molecules.
- Lactose, or milk sugar, is made up of glucose and galactose.
Fig. 11.3-1 shows why the size of the molecule decides whether digestion is needed.
Diagram
Fig. 11.3-1 — Single, Double And Complex Sugars Three-part schematic line drawing using identical small hexagons as sugar units. Part (a): one hexagon, labelled monosaccharide (glucose) — absorbed as it is. Part (b): two hexagons joined by a short bond, labelled disaccharide (sucrose) — must be split before absorption. Part (c): a long unbranched chain of many hexagons labelled starch, and beside it a branched chain of many hexagons labelled glycogen — polysaccharides. Label each part clearly with straight, non-crossing lead lines.
11.3.1.2 Starch
Starch is an insoluble carbohydrate, and it is in this form that plants commonly store carbohydrate. Being insoluble is exactly what makes it a good store — it stays where it is put instead of dissolving away into the plant’s sap.
Potatoes, grains such as rice, maize, wheat and barley, and bread made from them are the chief sources of starch in our food. Its chemical formula is written \((C_6H_{10}O_5)_n\), where the \(n\) shows that the same small unit is repeated many times over. A carbohydrate built in this way, from many sugar units, is called a polysaccharide.
11.3.1.3 Cellulose And Glycogen
Two other insoluble polysaccharides deserve attention, and they are strikingly different in what they do for us.
Cellulose is found in the cell walls of plants. Our body cannot use it at all and it passes through undigested — but it is far from useless, because it provides the roughage that keeps the gut working properly.
Glycogen is the form in which carbohydrate is stored in animals, mainly in the liver and to a smaller extent in the muscles. When blood glucose falls, the liver breaks glycogen down and releases glucose again, so the store works as a buffer between meals.
11.3.1.4 Roughage In The Diet
The cells of all plants have cell walls made of cellulose. Fruits, vegetables, and the outer coats of seeds and grains — the bran, or ‘chokar’, separated from wheat flour — are particularly rich in it.
None of this cellulose is digested in our food canal, for a simple reason: we produce no cellulose-digesting enzyme. The undigested cellulose is fibrous, and this fibrous undigested material passing through the gut is called roughage.
What Roughage Does
Roughage supplies no nutrients and no energy, yet a diet without it soon runs into trouble. It works in four ways.
- Being fibrous, cellulose absorbs a large amount of water and retains it. This keeps the faecal matter soft and prevents constipation.
- It makes the movement of undigested food through the intestine easier.
- It stimulates the muscular contraction of the intestinal wall, so that faecal matter is moved along.
- It stimulates secretion from the digestive tract, and helps in the removal of cholesterol — a fatty excretory substance — and of toxins.
Salads, green vegetables, fruits such as guava and banana, corn and half-crushed wheat (dalia) are the chief sources of roughage in our food. Cabbage is one vegetable that provides a great deal of it.
11.3.2 Fats
Fats are also composed of carbon, hydrogen and oxygen, but the proportion of oxygen in them is much lower than in carbohydrates. That single difference explains their most useful property. A molecule with little oxygen already in it has more still to gain by being oxidised, and so releases more energy.
The difference is large. Oxidising one gram of fat releases about 9 kilocalories of energy, against about 4 kilocalories from one gram of carbohydrate — more than twice as much from the same weight.
Butter, clarified butter (ghee), cheese, vegetable oils, and beef and fish liver oils are common foods rich in fats. Chemically, fats and oils are built from two kinds of smaller molecule: fatty acids and glycerol, arranged as shown in Fig. 11.3-2.
Diagram
Fig. 11.3-2 — How A Fat Molecule Is Built Simple line diagram of one fat molecule: a short vertical backbone labelled glycerol, with three long zig-zag chains attached to it, each labelled fatty acid; a bracket spanning the whole structure labelled fat (or oil). Straight, non-crossing lead lines.
The Four Functions Of Fat
- Fat produces energy in the body, as carbohydrates do.
- It is an important storage form of food. Because it packs the most energy into the least weight, the body stores its energy reserve as fat rather than as carbohydrate.
- It serves as a solvent for the fat-soluble vitamins. These vitamins will not dissolve in water, so a diet with no fat at all makes them difficult to absorb.
- Fat stored under the skin protects the body against rapid loss of heat, acting as insulation.
11.3.3 Proteins
Proteins are large chemical molecules. Like carbohydrates and fats they contain carbon, hydrogen and oxygen, but they also contain a fourth element that the other two lack — nitrogen. Nitrogen is the most essential element in proteins, and some proteins contain sulphur and phosphorus as well.
A protein molecule is far too large to be absorbed whole. It is built from smaller units called amino acids, and it is these that the body absorbs and then reassembles into the particular proteins it needs.
That reassembly is the whole point of eating protein. Proteins provide the chemical material for the growth and repair of body cells and tissues. Only in an emergency, when carbohydrate and fat have run out, are they oxidised to release energy — an expensive last resort.
Lean meat, fish, eggs, milk, cheese, nuts, beans and peas are foods rich in protein. A mild shortage of protein causes weakness. A severe shortage causes two serious diseases, kwashiorkor and marasmus, which are described next.
11.3.3.1 Kwashiorkor
Kwashiorkor is a severe protein-deficiency disease that usually affects young children, from about one to five years of age. The name comes from a West African language and means roughly the sickness the older child gets when a new baby comes.
That name describes the cause exactly. Mother’s milk is rich in protein. When breastfeeding is stopped early, often because a younger baby has arrived, the older child is put on a diet made up mainly of carbohydrates and poor in proteins. Growth and repair then have almost nothing to work with, and a child who is not treated may not survive to the age of five.
Signs Of Kwashiorkor
- Underweight, with stunted growth.
- Belly protruding out.
- Skin becoming dark and scaly.
- Loss of appetite.
- Repeated diarrhoea.
- Enlarged liver, and anaemia.
- Oedema — swelling of the feet and the face, caused by the accumulation of water in the tissues.
Control and treatment. A child with kwashiorkor should be given a protein-rich diet of pulses, milk, egg, fish and meat. A mixed diet of wheat, gram, peanuts, soybean and jaggery is also useful, and is often more affordable.
11.3.3.2 Marasmus
Marasmus usually affects infants below one year of age. Here the diet is short not only of protein but of carbohydrates and fats as well — the food supplies too little of everything.
It is typically caused by the sudden stoppage of breastfeeding, followed by a diet poor in the energy-giving foods, that is, in carbohydrates, fats and proteins together. The body, receiving no fuel, begins to consume its own tissues.
Signs Of Marasmus
- Very low body weight.
- Degeneration of the tissues, leaving a very weak body that appears to be made of muscle, skin and bone only.
- Skin becoming loosely folded.
- Thin face, and thinning of the limbs.
- Ribs appearing prominent.
- Retarded physical and mental growth.
Control and treatment. A child with marasmus should be given a diet rich in proteins and carbohydrates, since both the building material and the fuel have to be replaced.
The two diseases are compared in Table 11.1, and their outward differences are drawn in Fig. 11.3-3.
Diagram
Fig. 11.3-3 — Outward Signs Of The Two Deficiency Diseases Two simple, non-identifiable outline drawings of a child, side by side, drawn respectfully and without facial detail. Left figure labelled kwashiorkor, with lead lines to: swollen feet and face (oedema), protruding belly, dark scaly patches of skin, muscles retained. Right figure labelled marasmus, with lead lines to: very thin limbs, prominent ribs, loosely folded skin, wasted muscles, no swelling. Straight, non-crossing lead lines.
11.3.4 Mineral Salts
Mineral salts are inorganic substances needed in the diet in small quantities. Table salt, which is mainly sodium chloride, is the most familiar of them, but most mineral elements reach us through ordinary foodstuffs such as green vegetables and fruits.
Small quantity does not mean small importance. A mineral element is often part of a single structure or a single reaction that nothing else can carry out, so even a slight shortage shows up as a definite disease.
Four Minerals To Know Well
Calcium and phosphorus together strengthen the bones and the teeth. Calcium is also required for the clotting of blood, and phosphorus takes part in many chemical processes, including the production of ATP — the energy currency of the cell — during cell respiration. Rich sources are milk, meat, eggs, fish, pulses and vegetables.
Iron has a role in the formation of haemoglobin, the red pigment that carries oxygen in the blood. Its deficiency leads to anaemia. Green leafy vegetables and liver are rich sources.
Iodine is required for the proper working of the thyroid gland, which uses it to make the hormone thyroxine. Its deficiency leads to goitre, a swelling of the thyroid gland in the neck. Iodised salt, sea foods and drinking water are the usual sources.
Potassium and sodium are needed for cell permeability, especially in nerve cells, where they make the passage of nerve impulses possible. Most foods and table salt supply them.
A fuller list of the essential mineral elements, with their sources, functions and related deficiency diseases, is given in Table 11.2. Those needed in relatively large quantity are called macronutrients; those needed in very small quantity are called micronutrients.
11.3.5 Vitamins
Vitamins are chemical substances needed in minute amounts, which help to keep the body healthy. Most of them work as co-enzymes — helpers that an enzyme must have before it can carry out a chemical change — and each vitamin also has some special function of its own.
Vitamins are naturally contained in foods, and a couple of them are also synthesised within our own body. In low concentration they have a catalytic and regulatory function in cell metabolism, which is why such small quantities are enough.
Vitamins fall into two groups according to what they dissolve in, and this single property decides whether the body can keep a store of them. FC 11.4 sets out the division.
Flowchart
FC 11.4 — The Two Groups Of Vitamins Vitamins → branch 1: fat-soluble (A, D, E, K) → dissolve in fat → can be stored in the body for a longer period → need not be eaten every single day; branch 2: water-soluble (B complex, including \(B_1\) and \(B_2\), and C) → dissolve in water → cannot be stored for long → excess excreted in urine → must be supplied regularly
Absence or shortage of a vitamin in the diet, continued over a period of time, causes a deficiency disease. Note the phrase ‘over a period of time’ — one poor meal does nothing, but months of the same poor diet will show.
An excess of vitamins may also be harmful. Excess water-soluble vitamins are easily excreted out, but the fat-soluble ones accumulate in the body, so taking them in large doses is not harmless.
The essential vitamins, their sources, functions and deficiency diseases are collected in Table 11.3.
11.3.6 Water
Water is not usually thought of as a nutrient, because it supplies no energy and builds no tissue. Yet about two-thirds of the body weight is water, and a person deprived of it dies far sooner than a person deprived of food.
The reason is that almost every process in the body happens in solution. Water serves five main functions.
- It acts as a solvent in the body for thousands of organic and inorganic substances.
- It is used to produce the digestive juices.
- It helps in the transport of digested foods and of oxygen throughout the body.
- It is used in the excretion of soluble wastes, which must be dissolved before they can be removed.
- It is involved in the maintenance of body temperature.
Loss And Replacement
Water is lost from the body continuously — through sweat, through urine, and as water vapour in the breath. The last of these goes unnoticed, but it goes on with every exhalation, day and night.
Because the loss never stops, the supply must be constant. The liquids we drink and the food we eat, particularly fruits and vegetables, both supply water. Even so, about seven to eight glasses of water a day are needed to keep the balance.
11.4 Balanced Diet
We met the idea of a balanced diet at the start of this chapter. Now that all six classes of nutrients have been described, it can be stated properly: a balanced diet is one which contains all the principal constituents of food, in proper quantity, for a particular person.
Two conditions are doing the work in that sentence. All the constituents must be present — a diet missing one class is not balanced however generous it is with the rest. And they must be present in proper quantity, which means neither too little nor too much.
Different nutrients are supplying quite different needs at the same time, and a balanced diet is really an arrangement that keeps all three needs met at once. FC 11.5 sets out which nutrients serve which purpose.
Flowchart
FC 11.5 — The Overall Role Of Nutrients In The Body Carbohydrates and fats → oxidation, release of energy as heat and ATP; proteins, water and mineral salts → building and repair of cells and tissues, and production of hormones, enzymes and other secretions; mineral salts, vitamins and water → regulation of body processes
Table 11.4 shows what such a diet looks like on an actual plate, worked out for one adult doing moderate physical work. Two versions are given, one vegetarian and one non-vegetarian, and both are balanced.
Read the two columns side by side and one pattern stands out. Where the non-vegetarian menu takes protein from meat, fish and egg, the vegetarian menu takes more of it from pulses and milk. The nutrients required are identical; only the foods carrying them change. This is why a carefully planned vegetarian diet is fully adequate. Fig. 11.4-1 shows one day’s menu assembled on a single plate.
Diagram
Fig. 11.4-1 — A Balanced Meal On One Plate Line drawing of a round thali with small bowls, showing chapati or rice, a bowl of dal, a cooked green leafy vegetable, a portion of salad with a wedge of lemon, a glass of milk or a bowl of curd, and a whole seasonal fruit placed beside the plate. Label each item with the nutrient class it chiefly supplies — carbohydrate, protein, vitamins, mineral salts, roughage, fat, water. Straight, non-crossing lead lines.
11.4.1 Calories And Energy Requirement
One special aspect of a balanced diet is the number of calories it will provide. Energy is measured as heat, and the unit used is the calorie — the amount of heat required to raise the temperature of one gram of water by one degree Celsius.
That unit is far too small for measuring meals. In considering the energy value of foods we therefore use the kilocalorie, written ‘kcal’, which is 1000 times the calorie. When a food is said to give 100 kcal, this is the unit meant.
How many kilocalories a person needs in a day is not a fixed number. It depends on age, on body size and above all on how much physical work is being done, as Table 11.5 shows.
If the daily intake of food does not provide sufficient calories, the person loses weight. This happens rapidly during fasting. The body, receiving less fuel than it spends, oxidises its existing food stores instead, and as those stores are used up the capacity for work is reduced.
11.4.2 Malnutrition And Undernutrition
A diet can go wrong in two directions, and both have a name. When a person suffers because the diet is unbalanced, the condition is called malnutrition. The word covers both a deficiency of nutrients and an excess of them.
That last point is easily missed. A person eating far more than the body can use is also malnourished, because the diet is not in proper quantity — even though there is no shortage of food at all.
Undernutrition is the narrower term. It refers to the condition in which a person suffers from poor nutrition caused by not having enough food containing the essential nutrients necessary for growth and health. Undernutrition is therefore one part of malnutrition — the deficiency half of it.
Kwashiorkor and marasmus, described earlier in this chapter, are both severe forms of undernutrition, and so both are forms of malnutrition.
11.5 Summary
- Nutrition is the supply of the essential organic and inorganic chemical compounds, called nutrients, to the body.
- Food is any substance we eat or drink that contains nutrients; a meal is the food taken at one time; and the diet is the combination of food items taken in our meals.
- A balanced diet contains all the principal constituents of food in proper quantity for a particular person. It depends on four factors — age, sex, lifestyle or nature of work, and any special needs such as illness, pregnancy or nursing.
- Appetite is the desire to eat, while hunger is the uneasy sensation caused by an actual lack of food.
- Food is needed for six purposes: growth, repair of worn-out or injured cells, energy, maintenance of the chemical composition of cells, provision of raw materials for secretions such as enzymes, hormones, sweat and milk, and protection from disease and infection.
- There are six classes of nutrients: carbohydrates, fats, proteins, mineral salts, vitamins and water.
- Carbohydrates contain carbon, hydrogen and oxygen, with hydrogen and oxygen always in the ratio \(2:1\). They are oxidised in the cells to release energy and are the principal energy source of the body.
- Sugars are soluble and sweet. Monosaccharides, or single sugars, have the formula \(C_6H_{12}O_6\), need no digestion and are absorbed directly; they are glucose, fructose and galactose.
- Disaccharides, or double sugars, have the formula \(C_{12}H_{22}O_{11}\) and must be digested before absorption. Sucrose is made of glucose and fructose, maltose of two glucose molecules, and lactose of glucose and galactose.
- One mole of glucose releases 686 kilocalories of energy on complete oxidation.
- Starch, \((C_6H_{10}O_5)_n\), is an insoluble polysaccharide and is the form in which plants store carbohydrate. Cellulose and glycogen are the two other insoluble polysaccharides; cellulose forms plant cell walls, and glycogen is the storage form in animals, in the liver and muscles.
- We produce no cellulose-digesting enzyme, so cellulose passes through undigested and acts as roughage. Roughage absorbs and retains water, keeps the faecal matter soft, prevents constipation, eases the movement of undigested food, stimulates contraction of the intestinal wall, and helps in removing cholesterol and toxins.
- Fats contain carbon, hydrogen and oxygen, with much less oxygen than carbohydrates, and therefore release more than twice as much energy per gram. They are made of fatty acids and glycerol.
- Fat produces energy, is an important storage form of food, serves as a solvent for the fat-soluble vitamins, and insulates the body against rapid heat loss.
- Proteins are large molecules containing carbon, hydrogen, oxygen and nitrogen, and sometimes sulphur and phosphorus. Nitrogen is their most essential element, and amino acids are their smaller units.
- Proteins provide the chemical material for the growth and repair of body cells and tissues, and are oxidised for energy only in an emergency.
- Kwashiorkor is a severe protein-deficiency disease of children from one to five years, caused by early stoppage of breastfeeding followed by a diet poor in protein. Its signs include underweight, a protruding belly, dark scaly skin, stunted growth, loss of appetite, repeated diarrhoea, enlarged liver, anaemia and oedema of the feet and face. It is treated with a protein-rich diet.
- Marasmus affects infants below one year and is due to a deficiency of carbohydrates, fats and proteins together. Its signs include very low body weight, wasted muscles, loosely folded skin, a thin face and limbs, prominent ribs and retarded physical and mental growth. It is treated with a diet rich in proteins and carbohydrates.
- Mineral salts are needed in small quantities. Calcium and phosphorus strengthen bones and teeth, calcium is also needed for blood clotting and phosphorus for ATP production; iron is needed for haemoglobin and its shortage causes anaemia; iodine is needed by the thyroid gland and its shortage causes goitre; potassium and sodium are needed for cell permeability, especially in nerve cells.
- Vitamins are needed in minute amounts and act mainly as co-enzymes, with a catalytic and regulatory role in cell metabolism.
- Vitamins A, D, E and K are fat-soluble and can be stored in the body. The B complex and vitamin C are water-soluble, cannot be stored for long, and the excess is excreted. Prolonged shortage of any vitamin causes a deficiency disease, and an excess may also be harmful.
- Water forms about two-thirds of the body weight. It acts as a solvent, produces the digestive juices, transports digested food and oxygen, carries out soluble wastes, and maintains body temperature. It is lost through sweat, urine and the breath, and about seven to eight glasses a day are needed to replace it.
- A calorie is the heat needed to raise the temperature of one gram of water by one degree Celsius; the kilocalorie, written kcal, is 1000 times the calorie. Daily requirement rises with body size, growth and physical work.
- If the diet does not supply sufficient calories, the body oxidises its existing food stores, weight falls and the capacity for work is reduced.
- Malnutrition is the condition caused by an unbalanced diet, and includes both a deficiency and an excess of nutrients. Undernutrition, caused by not having enough food containing the essential nutrients, is one part of malnutrition.
Flowchart
FC 11.6 — The Argument Of The Chapter Body needs materials and energy → nutrition supplies them as nutrients → six classes of nutrients (carbohydrates, fats, proteins, mineral salts, vitamins, water) → each class does work no other class can do → all classes in proper quantity for that person = balanced diet → any class short or in excess = malnutrition → prolonged shortage = deficiency disease (kwashiorkor, marasmus, anaemia, goitre, rickets, scurvy, beriberi)
11.6 Key Terms
- Amino acids — the small units from which protein molecules are built, and the form in which protein is absorbed.
- Anaemia — a condition in which the blood is deficient in haemoglobin or in red blood cells; caused by a shortage of iron.
- Appetite — the desire to eat.
- Balanced diet — a diet containing all the principal constituents of food in proper quantity for a particular person.
- Calcium — a mineral element that strengthens bones and teeth and is required for the clotting of blood.
- Calorie — the amount of heat required to raise the temperature of one gram of water by one degree Celsius.
- Carbohydrates — compounds of carbon, hydrogen and oxygen, with hydrogen and oxygen in the ratio \(2:1\), oxidised in the cells to release energy.
- Cellulose — an insoluble polysaccharide forming the cell walls of plants; not digested by humans.
- Deficiency disease — a disease caused by the prolonged absence or shortage of a particular nutrient in the diet.
- Diet — the combination of food items taken in our meals.
- Disaccharide — a double sugar, of formula \(C_{12}H_{22}O_{11}\), made of two single sugars and needing digestion before absorption.
- Energy — the capacity to do work, released in the cells by the oxidation of nutrients.
- Fats — nutrients of carbon, hydrogen and oxygen with a low oxygen content, made of fatty acids and glycerol, releasing about twice as much energy per gram as carbohydrates.
- Fatty acids — the long-chain molecules that, with glycerol, make up a fat.
- Food — any substance we eat or drink which contains nutrients.
- Fructose — a single sugar common in plants, popularly called fruit sugar.
- Galactose — a single sugar found in milk.
- Glucose — the commonest and simplest sugar in organisms, known as grape sugar; the sugar carried in the blood.
- Glycerol — the small molecule to which fatty acids are attached in a fat.
- Glycogen — the insoluble polysaccharide in which animals store carbohydrate, mainly in the liver and muscles.
- Goitre — a swelling of the thyroid gland caused by a shortage of iodine.
- Growth — the building up of new protoplasm and new cells, increasing the living material of the body.
- Hunger — the uneasy sensation produced by an actual lack of food.
- Iodine — a mineral element required by the thyroid gland for making its hormone.
- Iron — a mineral element required for the formation of haemoglobin.
- Kilocalorie — a unit of energy equal to 1000 calories, written kcal, used for the energy value of foods.
- Kwashiorkor — a severe protein-deficiency disease of children from about one to five years of age.
- Lactose — the double sugar of milk, made of glucose and galactose.
- Maintenance — the keeping of the chemical composition of cells steady by replacing what is lost.
- Malnutrition — the condition in which a person suffers due to an unbalanced diet, whether from a deficiency or an excess of nutrients.
- Maltose — malt sugar, a double sugar made of two glucose molecules.
- Marasmus — a deficiency disease of infants below one year, due to a shortage of carbohydrates, fats and proteins together.
- Meal — the food taken at one time.
- Mineral salts — inorganic substances needed in the diet in small quantities for particular structures and reactions.
- Monosaccharide — a single sugar, of formula \(C_6H_{12}O_6\), absorbed without digestion.
- Nutrients — the organic and inorganic chemical compounds the body needs and obtains from food.
- Nutrition — the supply of the essential organic and inorganic chemical compounds to the body.
- Oedema — swelling caused by the accumulation of water in the tissues, seen in the feet and face in kwashiorkor.
- Phosphorus — a mineral element needed for bones and teeth and for the production of ATP in cell respiration.
- Polysaccharide — a carbohydrate built from many sugar units joined together, such as starch, cellulose or glycogen.
- Potassium — a mineral element needed for cell permeability and for nerve and muscle activity.
- Protection — the resistance of the body to disease and infection, supported by an adequate diet.
- Proteins — large nitrogen-containing molecules built from amino acids, supplying material for the growth and repair of cells and tissues.
- Protoplasm — the living substance inside a cell.
- Repair — the replacement of worn-out or injured cells using nutrients from food.
- Roughage — the fibrous indigestible part of food, mainly cellulose, which assists the working of the food canal.
- Secretion — a substance made by cells and released for a particular purpose, such as an enzyme, a hormone, sweat or milk.
- Sodium — a mineral element needed for cell permeability, especially in nerve cells, and for acid–base balance.
- Starch — the insoluble polysaccharide, \((C_6H_{10}O_5)_n\), in which plants store carbohydrate.
- Sucrose — commercial sugar, a double sugar of glucose and fructose, obtained from sugarcane and sugar-beet.
- Sugars — the small, water-soluble, sweet-tasting carbohydrates.
- Undernutrition — the condition in which a person suffers from poor nutrition through not having enough food containing the essential nutrients; a part of malnutrition.
- Vitamins — chemical substances needed in minute amounts, acting mainly as co-enzymes in the body’s chemical changes.
11.7 Points To Ponder
Cellulose and starch are both built from glucose units, yet we can digest one and not the other. If a human enzyme for cellulose existed, how would our diet, our farming and our idea of ‘waste’ food change?
Fat stores more than twice the energy of carbohydrate for the same weight. Why, then, does the body keep any store of carbohydrate at all instead of converting everything to fat?
A person can survive many weeks without food but only a few days without water, although water supplies no energy and builds no tissue. What does this tell you about the difference between a nutrient that is used up and one that is worked in?
Kwashiorkor and marasmus both begin at the same moment — when breastfeeding stops — yet they produce almost opposite appearances. What does this suggest about how the body decides which tissue to sacrifice first?
Vitamin C is destroyed by heat, and much of the food we eat is cooked. How might a community whose staple diet is entirely cooked still meet its vitamin C requirement?
Table 11.5 gives calorie requirements by category of person rather than by body weight. What is gained by presenting the figures this way, and what is lost?
11.8 Objective Questions
Multiple choice questions
- In carbohydrates, hydrogen and oxygen are always present in the ratio: (1 mark)
- \(1:1\)
- \(2:1\)
- \(1:2\)
- \(3:1\)
- The sugar that needs no digestion and is absorbed straightaway is: (1 mark)
- sucrose
- maltose
- lactose
- glucose
- The element present in proteins but absent in carbohydrates and fats is: (1 mark)
- carbon
- oxygen
- nitrogen
- hydrogen
- A person with a diet lacking iodine is most likely to develop: (1 mark)
- rickets
- goitre
- anaemia
- scurvy
- Which of the following vitamins cannot be stored in the body for a long period? (1 mark)
- Vitamin A
- Vitamin D
- Vitamin C
- Vitamin K
- The storage carbohydrate of the animal body is: (1 mark)
- starch
- cellulose
- glycogen
- maltose
Assertion–reason
- For each of the following, select the correct option:
- Both A and R are true, and R is the correct explanation of A.
- Both A and R are true, but R is not the correct explanation of A.
- A is true but R is false.
- A is false but R is true.
- Assertion (A): Cellulose is an important part of our diet even though it supplies us with no energy. Reason (R): Being fibrous, cellulose retains water and keeps the faecal matter soft. (1 mark)
- Assertion (A): A child with kwashiorkor shows swelling of the feet and face. Reason (R): Kwashiorkor is caused by a diet short of carbohydrates, fats and proteins together. (1 mark)
- Assertion (A): Fats are the body’s preferred form for storing energy. Reason (R): One gram of fat releases more than twice as much energy as one gram of carbohydrate. (1 mark)
Match the following
- Match each item in Column A with the correct item in Column B. (4 marks)
| Column A | Column B |
|---|---|
| a. Iron | i. Component of the thyroid hormone |
| b. Calcium | ii. Deficiency causes beriberi |
| c. Iodine | iii. Formation of haemoglobin |
| d. Vitamin \(B_1\) | iv. Clotting of blood |
Fill in the blanks
A double sugar made up of two glucose molecules is called ____________. (1 mark)
Fats are made up of fatty acids and ____________. (1 mark)
The unit which is 1000 times the calorie is called the ____________. (1 mark)
The smaller units from which proteins are built are called ____________. (1 mark)
True or false
Water forms about two-thirds of the body weight. (1 mark)
Marasmus affects children between one and five years of age. (1 mark)
Excess of a fat-soluble vitamin is easily excreted from the body. (1 mark)
Name the following
The sugar popularly known as grape sugar. (1 mark)
The mineral element needed for cell permeability, especially in nerve cells, obtained from table salt. (1 mark)
The disease caused by a prolonged shortage of vitamin C. (1 mark)
The insoluble carbohydrate of plant cell walls that we cannot digest. (1 mark)
Odd one out — give a reason for your choice
Glucose, fructose, sucrose, galactose (1 mark)
Vitamin A, vitamin D, vitamin C, vitamin K (1 mark)
Rice, potato, egg, wheat (1 mark)
11.9 Check Your Concepts
Define nutrition and name the six classes of nutrients. (2 marks)
Distinguish between appetite and hunger, stating the basis of difference. (2 marks)
State two functions of water in the human body. (2 marks)
Give a reason for each of the following: (3 marks)
- Sucrose must be digested before it can be absorbed, but glucose need not be.
- A person who eats no fat at all may become short of vitamin A.
- Cellulose is included in the diet although it supplies no nutrient.
Differentiate between the following pairs, stating the basis of difference in each case: (3 marks)
- Starch and glycogen
- Fat-soluble and water-soluble vitamins
- Malnutrition and undernutrition
Explain, with one example each, why mineral elements are needed even though the quantities required are very small. (3 marks)
Why does the diet of a growing child need proportionally more protein than that of an adult? (3 marks)
List the four factors on which a balanced diet for a particular person depends, and explain how any two of them change the requirement. (4 marks)
Describe the four ways in which roughage helps the working of the food canal. (4 marks)
What would happen if a person’s diet supplied fewer calories than the body spends each day? Explain the changes that follow. (4 marks)
Give an account of kwashiorkor under the following headings: cause, any four symptoms, and treatment. (5 marks)
Differentiate between kwashiorkor and marasmus with respect to age affected, cause, oedema, condition of the skin and condition of the muscles. (5 marks)
Explain why food is needed by a fully grown adult who has stopped growing altogether. Refer to at least three of the six uses of food. (5 marks)
11.10 Figure-Based Questions
- Study Fig. 11.3-1. (4 marks)
- Name the type of sugar shown in part (a) and give one example of it.
- Why must the molecule shown in part (b) be digested before absorption?
- Name the two carbohydrates shown in part (c) and state where each is stored.
- Refer to Fig. 11.3-2. (3 marks)
- Name the parts labelled in the diagram.
- Name the class of nutrient the molecule belongs to.
- State one function of this nutrient other than supplying energy.
- Fig. 11.3-3 shows the outward signs of two deficiency diseases. (4 marks)
- Name the disease shown on the left and the one shown on the right.
- Name the nutrient chiefly lacking in each case.
- State one visible sign that occurs in one of these conditions but not in the other.
- Look at Fig. 11.4-1. (3 marks)
- Name the item on the plate that is the chief source of roughage.
- Name one item that supplies protein and one that supplies vitamin C.
- State what would be missing from the meal if the fruit and the salad were removed.
- Redraw a simple labelled diagram of a fat molecule as shown in Fig. 11.3-2, labelling glycerol and fatty acids on straight, non-crossing lead lines. (3 marks)