HUMAN NUTRITION Flashcards
Balanced diet
This is a diet that contains all the essential nutrients in the correct proportions to maintain good health. The nutrients needed are carbohydrate, fat, protein, vitamins, minerals, fibre and water
Ingestion
The taking of substances such as food and drink into the body through the mouth
Mechanical digestion
The breakdown of food into smaller pieces without chemical change to the food molecules
Chemical digestion
The breakdown of large insoluble molecules into small soluble molecules
Absorption
The movement of small food molecules and ions through the wall of the intestine into the blood
Assimilation
The movement of digested food molecules into the cells of the body where they are used, becoming part of the cells
Egestion
The passing out of food that has not been digested or absorbed, as faeces, through the anus
Dietary requirements
Your dietary requirements depend on your age, sex and levels of physical activity. The amount of energy needed is provided mainly by our carbohydrate and fat intake.
Generally, males use up more energy
than females, and energy demand increases until we stop growing. Someone doing physical work will use up more energy than an office worker. While children are growing, they need more protein per kilogram of body weight than adults do.
Pregnant women need
extra nutrients for the development of the fetus. Once the baby has been born, a breast-feeding mother will need more protein and minerals, e.g. calcium, in her diet to satisfy the baby’s requirements.
Effects of malnutrition Malnutrition is the result of an unbalanced diet:
• Too much food – or too much carbohydrate, fat or protein – can lead to obesity. This can lead to coronary heart disease and diabetes (which can cause blindness). • Too much animal fat in the diet results in high cholesterol levels. Cholesterol can stick to the walls of arteries, gradually blocking them. If coronary arteries become blocked, the result can be angina and coronary heart disease. • Too little food can result in starvation. Extreme slimming diets, such as those that avoid carbohydrate foods, can result in the disease anorexia nervosa. • Constipation is caused by a lack of fibre in the diet. It can lead to bowel cancer. • Vitamin and mineral deficiency diseases are all the result of malnutrition. Scurvy is caused by a lack of vitamin C.
Carbohydrate
Source of energy Rice, potato, sweet potato, cassava, bread, millet, sugary foods (e.g. cake, jam, honey
Fat/ oil (oils are liquid at room temperature but fats are solid)
Source of energy (twice as much as carbohydrate); used as insulation against heat loss, for some hormones, in cell membranes, for insulation of nerve fibres Butter, milk, cheese, egg yolk, animal fat, groundnuts (peanuts)
Protein
Growth, tissue repair, enzymes, some hormones, cell membranes, hair, nails. Can be broken down to provide energy Meat, fish, eggs, soya, groundnuts, milk, meat substitute (e.g. Quorn), cowpeas
Vitamin C
Needed to maintain healthy skin and gums Citrus fruits, blackcurrants, cabbage, tomato, guava, mango
Vitamin D
Needed to maintain hard bones. Helps in absorption of calcium from small intestine Milk, cheese, egg yolk, fish liver oil. Can be made in the skin when exposed to sunlight
Calcium
Needed to form healthy bones and teeth and for normal blood clotting Milk, cheese, fish
Fibre
This is cellulose. It adds bulk to undigested food passing through the intestines, maintaining peristalsis Vegetables, fruit, wholemeal bread Water Formation of blood, cytoplasm, as a solvent for transport of nutrients and removal of wastes (as urine). Enzymes work only in solution Drinks, fruit, vegetables
Iron
Needed for formation of haemoglobin in red blood cells Red meat, liver, kidney, eggs, green vegetables (spinach, cabbage, cocoyam, groundnut leaves), chocolate
Water
Formation of blood, cytoplasm, as a solvent for transport of nutrients and removal of wastes (as urine). Enzymes work only in solution Drinks, fruit, vegetables
Vitamins and minerals,
although needed in only small quantities, are important for maintaining good health. A shortage can result in a deficiency disease. You only need to know vitamins C and D, and the minerals calcium and iron. Fibre (roughage) is needed in much larger quantities. Do not forget that water is also a vital part of our dietary requirements.
Vitamin D and iron deficiencies
A shortage of vitamin D can lead to a deficiency disease called rickets. The symptoms are soft bones that become deformed. Sufferers may become bow legged. Exposure to moderate sunlight helps the body make vitamin D. Thus, a lack of exposure (because of climate or season or wearing clothing that acts as a barrier to sunlight) can result in the development of a deficiency. A deficiency of iron in the diet can lead to anaemia. The symptoms are constant tiredness and a lack of energy. Normally, as red blood cells in the body are broken down, the iron in the haemoglobin is recycled to make new red blood cells. However, if a woman has a heavy period (see Chapter 16) there is a lot of blood (and therefore iron) loss, which can result in anaemia.
Sometimes the balance of food in the diet
is wrong, e.g. too much carbohydrate and too little protein, such as when the bulk of the diet is starchy food, such as sweet potato or cassava. This can lead to kwashiorkor in young children. They lack protein, but other problems such as plant toxins can also play a role. The symptoms of kwashiorkor are dry skin, pot-belly, changes to hair colour, weakness and irritability.
Marasmus
Marasmus is an acute form of malnutrition. The condition is caused by a very poor diet with inadequate carbohydrate intake and a lack of protein. The symptoms are emaciation, with reduced fat and muscle tissue. The skin is thin and hangs in folds.
alimentary canal
Mouth Food
Mouth Food is ingested here. It is mechanically digested by cutting, chewing and grinding of teeth. Saliva is added
Salivary glands
Salivary glands Produce saliva, containing the enzyme amylase to begin the chemical digestion of starch. The water in saliva helps lubricate food and makes small pieces stick together
Oesophagus
Boluses (balls) of food pass through by peristalsis, from mouth to stomach
Stomach
Muscular walls squeeze on food to make it semi-liquid. Gastric juice contains protease to chemically digest protein and hydrochloric acid to maintain an optimum pH (1– 2.5). The acid also kills bacteria
Duodenum
This is the first part of the small intestine. It receives pancreatic juice containing protease, lipase and amylase. The juice also contains sodium hydrogencarbonate, which neutralises acid from the stomach, producing a pH of 7– 8
Ileum
The second part of the small intestine. Enzymes in the epithelial lining chemically digest maltose and peptides. Its surface area is increased by the presence of villi, which allow the efficient absorption of digested food molecules
Pancreas
Secretes pancreatic juice into the duodenum for chemical digestion of proteins, fats and starch
Liver
Makes bile, which is stored in the gall bladder. Bile contains salts that emulsify fats, forming droplets with a large surface area to make digestion by lipase more efficient. Digested foods are assimilated here. For example, glucose is stored as glycogen; surplus amino acids are deaminated (see Chapter 13)
Gall bladder
Stores bile, made in the liver, to be secreted into the duodenum via the bile duct
Colon
The second part of the large intestine. Its main function is the reabsorption of water from undigested food. It also absorbs bile salts to pass back to the liver
Rectum
Stores faeces until they are egested
Anus
Has muscles to control when faeces are egested from the body
Diarrhoea
Diarrhoea is the loss of watery faeces. It is sometimes caused by bacterial or viral infection, for example from food or water, resulting in the intestines being unable to absorb fluid from the contents of the colon or too much fluid being secreted into the colon.
Undigested food then moves through the large intestine too quickly, resulting in insufficient time to absorb water from it. Unless the condition is treated, dehydration can occur.
The treatment is called oral hydration therapy – drinking plenty of fluids (sipping small amounts of water at a time) to rehydrate the body.
cholerra is caused by a bacterium that causes acute diarrhoea.
Effects of the cholera bacteria
When cholera bacteria, Vibrio cholerae, are ingested they multiply in the small intestine and invade its epithelial cells.
As the bacteria become embedded, they release toxins (poisons) that irritate the intestinal lining and lead to the secretion of large amounts of water and salts, including chloride ions.
The salts decrease the osmotic potential of the gut contents, drawing more water from surrounding tissues and blood by osmosis (see ‘Osmosis’ in Chapter 3).
This makes the undigested food much more watery, leading to acute diarrhoea, and the loss of body fluids and salt leads to dehydration and kidney failure.
incisor
canine
premolar
molar
structure of teeth
Causes of dental decay
Bacteria are present on the surface of our teeth. Food deposits and bacteria form a layer called plaque. Bacteria in the plaque respire sugars, producing acid. This acid dissolves enamel and dentine, forming a hole. Dentine underneath the enamel is softer – it dissolves more rapidly. If the hole reaches the pulp cavity, bacterial infection can get to the nerve. This results in toothache and, possibly, an abscess (an infection in the jaw).
Chemical digestion
Food that we ingest is mainly made up of large, insoluble molecules that cannot be absorbed through the gut wall. It needs to be changed into small, soluble molecules. Chemical digestion involves breaking down large, insoluble molecules into small, soluble ones. Enzymes speed up the process. They work efficiently at body temperature (37 ° C) and at a suitable pH. The main places where chemical digestion happens are the mouth, stomach and small intestine.
amylase
protease
lipase
functions of hydrochloric acid in gastric juice (in the stomach):
- It gives an acid pH, which protease needs to work at its optimum.
- It kills bacteria that may be present in food that has been ingested.
Digestion of starch
Starch is digested in two places in the alimentary canal:
by salivary amylase in the mouth and by pancreatic amylase in the duodenum. Amylase works best in a neutral or slightly alkaline pH and converts large, insoluble starch molecules into smaller, soluble maltose molecules. Maltose is a disaccharide sugar and is still too big to be absorbed through the wall of the intestine. Maltose is broken down to glucose by the enzyme maltase, which is present in the membranes of the epithelial cells of the villi.
Digestion of protein
There are actually several proteases that break down proteins. One protease is pepsin, which is secreted in the stomach. Pepsin acts on proteins and breaks them down into soluble compounds called peptides. These are shorter chains of amino acids than proteins. Another protease is called trypsin. Trypsin is secreted by the pancreas in an inactive form, which is changed to an active enzyme in the duodenum (part of the small intestine). It has a similar role to pepsin, breaking down proteins to peptides.
Hydrochloric acid in gastric juice
The hydrochloric acid that is secreted by cells in the wall of the stomach creates a very acid pH of 2. This pH is important because it denatures enzymes in harmful organisms in food, such as bacteria (which may otherwise cause food poisoning), and it provides the optimum pH for the protein-digesting enzyme pepsin to work.
Bile
Bile is made in the liver, stored in the gall bladder and transferred to the duodenum by the bile duct (Figure 7.2). It has no enzymes but does contain bile salts, which act on fats in a similar way to a detergent.
The bile salts emulsify the fats, breaking them up into small droplets with a large surface area, which are more efficiently digested by lipase. Bile is slightly alkaline, as it contains sodium hydrogencarbonate, and has the function of neutralising the acidic mixture of food and gastric juices as it enters the duodenum. This is important because enzymes secreted into the duodenum need alkaline conditions to work at their optimum rate.
The small intestine has a very
rich blood supply. Digested food molecules are small enough to pass through the wall of the intestine into the bloodstream. The small intestine and the colon are both involved in the absorption of water, but the small intestine absorbs the most.
Villi are present
in the small intestine – these are finger-like projections that increase the surface area for absorption. If a section of small intestine was turned inside out, its surface would be like a carpet. The surface area of a villus is further increased by the presence of microvilli.
Inside each villus are blood
capillaries that absorb amino acids and glucose. There are also lacteals – these absorb fatty acids and glycerol. Food molecules are absorbed mainly by diffusion. Figure 7.6 shows the features of a villus that increase the efficiency of diffusion. Molecules can also be absorbed by active transport (see Chapter 3). Epithelial cells contain mitochondria to provide energy for absorption against the concentration gradient.
villi
