Human Nutrition

A balanced diet: the nutrients you need

A balanced diet is a diet that provides all the nutrients the body needs, in the correct proportions, together with the right amount of energy. It is not one fixed menu: a growing teenager, a pregnant woman and an elderly person all need different amounts, because the balance depends on age, sex and how active you are.

The nutrients fall into seven groups: carbohydrates, proteins, lipids (fats and oils), vitamins, mineral ions, dietary fibre (roughage) and water. Each has a different job, and a shortage of any one of them causes a specific problem. Learn each nutrient as a trio - source, function and deficiency disease - because that is how the examiner asks for it.

Energy comes mainly from carbohydrates, with lipids as a concentrated store. Proteins are the only nutrient used for growth and repair. Vitamins and mineral ions are needed in tiny amounts, but they are not energy sources: they allow particular reactions to happen, which is why a small shortage causes such obvious disease.

Vitamins, mineral ions and the deficiency diseases

Vitamins and mineral ions are needed in very small amounts, but they are not optional. Most of them act as helpers in particular reactions, so when one is missing the reaction slows or stops and a deficiency disease develops. Cambridge fixes the vitamins at C and D and the mineral ions at calcium and iron, and expects the deficiency disease for each.

Vitamin C is needed to make collagen, the protein that gives structure to skin, gums, blood vessel walls and connective tissue. A shortage causes scurvy: the gums become swollen and bleed, teeth loosen, wounds heal slowly and joints ache. Sailors on long voyages without fresh fruit and vegetables were the classic victims.

Vitamin D is unusual because the body can make it in the skin when sunlight falls on the skin, so a shortage is often caused by lack of sunlight as well as by a poor diet. It helps the intestine absorb calcium and phosphate, and without it calcium cannot be deposited in bone. The result is rickets, in which the bones stay soft and bend under the body's weight, so the leg bones bow. A shortage of calcium itself also causes weak bones and teeth.

Iron is needed to make haemoglobin, the red pigment in red blood cells that carries oxygen. Without enough iron the body cannot make enough haemoglobin, and anaemia develops: the person looks pale, feels tired and becomes short of breath on exertion because less oxygen is delivered to the muscles.

Malnutrition: starvation, obesity and the wrong balance

Malnutrition means an unhealthy diet - not simply too little food. It covers starvation and obesity, and it also covers a diet that supplies enough energy but the wrong nutrients, such as a diet low in iron or vitamin C.

Starvation happens when the body takes in less energy than it uses for a long time. It has to break down its own fat stores, and then its own muscle, for energy, so the person loses body mass and becomes weak. Because the immune system needs protein to work, resistance to infection falls, and eventually organs such as the heart and kidneys stop working properly.

Obesity is the opposite problem: taking in more energy than is used, so the excess is stored as fat. The extra fat puts strain on the heart and on the joints, and raises the risk of coronary heart disease and type 2 diabetes. Coronary heart disease itself is often caused by a diet high in saturated fat and cholesterol, which leads to fatty deposits narrowing the coronary arteries, so less oxygen reaches the heart muscle; the person gets chest pain and may have a heart attack.

Too little fibre and too little water cause constipation: the gut contents move too slowly, so too much water is absorbed from them and the faeces become hard and dry and are difficult to pass. Fibre adds bulk, which stimulates the muscles of the gut wall to keep the contents moving.

The digestive system: the organs and what each one does

The alimentary canal is the tube that food passes through, from the mouth to the anus: mouth, oesophagus, stomach, small intestine (duodenum and ileum), large intestine (colon, rectum and anus). The associated organs are the salivary glands, pancreas, liver and gall bladder, which are not part of the tube but release substances into it.

Nutrition in the digestive system follows five processes. Ingestion is the taking of substances such as food and drink into the body. Digestion is the breakdown of food, which may be physical or chemical. Absorption is the movement of nutrients from the intestines into the blood. Assimilation is the uptake and use of those nutrients by cells. Egestion is the removal of undigested food from the body as faeces.

The liver is the largest associated organ and does more than make bile. It stores glycogen: after a meal, excess glucose is converted to glycogen and stored, and when blood glucose falls the glycogen is converted back to glucose. It also carries out deamination: excess amino acids are broken down, the amino group is removed and converted to urea, and the rest of the molecule is used for energy. Urea is carried in the blood to the kidneys and excreted in urine.

Physical digestion: teeth, peristalsis and churning

Physical digestion (also called mechanical digestion) is the breakdown of food into smaller pieces without chemical change to the food molecules. A piece of bread is still bread, just in smaller pieces. Its purpose is to increase the surface area of the food so that enzymes in chemical digestion can act on it faster - a crushed lump is digested far more quickly than a whole one.

In the mouth the teeth carry out this job. Incisors are flat and chisel-shaped for cutting; canines are pointed for tearing; premolars and molars have broad, bumpy surfaces for crushing and grinding. A tooth has enamel (the hard outer layer), dentine beneath it, a pulp cavity containing nerves and blood vessels, and cement holding it in place; the tooth is embedded in the bone of the jaw and surrounded by the gums.

The stomach continues physical digestion by churning the food with its muscular wall, mixing it with gastric juice to form a semi-liquid. Peristalsis then moves the contents along the whole canal: circular muscles contract behind the food and longitudinal muscles contract in front of it, pushing the food forward in waves. Peristalsis also moves food down the oesophagus, which is why you can swallow while lying down.

Chemical digestion: enzymes, hydrochloric acid and bile

Chemical digestion is the breakdown of large insoluble molecules into small soluble molecules that can be absorbed. It is carried out by enzymes, and each enzyme has its own substrate and its own product. You must know where each enzyme is produced and where it acts, because these are separate marks.

Amylase breaks starch down to maltose (a simple reducing sugar); it is secreted by the salivary glands into the mouth and by the pancreas into the duodenum. Proteases break protein down to amino acids; pepsin is secreted by the stomach lining and works in the acidic stomach, while trypsin is secreted by the pancreas and works in the alkaline small intestine. Lipase breaks fats and oils down to fatty acids and glycerol, and is secreted by the pancreas into the small intestine. On the membranes of the intestinal epithelium, maltase completes the job by breaking maltose down to glucose.

Hydrochloric acid in the gastric juice has two jobs: it kills harmful microorganisms that were swallowed with the food, and it provides an acidic pH that is the optimum for the protease pepsin. When the acidic mixture leaves the stomach, bile neutralises it. Bile is an alkaline mixture made by the liver and stored in the gall bladder; as well as neutralising the acid to give the enzymes of the small intestine the pH they need, it emulsifies fats, breaking large droplets into many small ones so that lipase has a much larger surface area to act on.

Absorption, villi and cholera

The small intestine is the region where nutrients are absorbed. Its inner surface is folded into millions of finger-like projections called villi, and each villus is covered in even smaller folds called microvilli. Together they give the small intestine an enormous internal surface area, so nutrients can be absorbed very rapidly. Most of the water in the gut is absorbed here too, but some is also absorbed from the colon, which is why faeces are solid.

Each villus is adapted in four ways. Its wall is only one cell thick, so the distance that substances must diffuse is very short. It has a large surface area, from the villi and microvilli themselves. It contains a dense network of capillaries, which carry away glucose and amino acids so that a steep concentration gradient is maintained. At its centre is a lacteal, a vessel of the lymphatic system that absorbs the fatty acids and glycerol, which are reassembled into fats before entering the blood.

Cholera shows what happens when the digestive system is attacked by disease. It is caused by a bacterium spread in water or food contaminated with the faeces of an infected person. The bacteria release a toxin in the small intestine that makes the lining secrete large amounts of water and salts into the gut, causing severe diarrhoea and vomiting. The body loses so much water and so many salts that the person becomes dehydrated, and without rapid treatment with oral rehydration salts or a drip, dehydration can be fatal.

• Large surface area, from the villi and microvilli, so more nutrients can be absorbed at once.
• Thin wall, only one cell thick, so the diffusion distance is short and absorption is fast.
• Dense capillary network, which removes glucose and amino acids and keeps the concentration gradient steep.
• A lacteal at the centre, which absorbs fatty acids and glycerol.
• Many mitochondria in the epithelium, supplying the energy for active transport of some nutrients.