Chapter 11: Introduction to nutrition

Question 1

Carbohydrates

Answer 1

Carbohydrates are mainly sugars and starches, which are found in a wide variety of foods, e.g., sugar, jam, cereals, bread, biscuits, pasta, convenience foods, fruit, and vegetables. Chemically, they consist of carbon, hydrogen, and oxygen, the hydrogen and oxygen being in the same proportion as in water. Carbohydrates are classified according to the complexity of the chemical substances from which they are formed.

Question 2

Monosaccharides

Answer 2

Carbohydrates are digested in the alimentary canal and absorbed as monosaccharides. Examples include glucose, fructose, and galactose. These are, chemically, the simplest form of carbohydrates.

Question 3

Disaccharides

Answer 3

These consist of two monosaccharide molecules chemically combined, e.g., sucrose, maltose, and lactose.

Question 4

Polysaccharides

Answer 4

• These are complex molecules made up of large numbers of monosaccharides in chemical combinations, e.g., starches, glycogen, and cellulose.
• Not all polysaccharides can be digested by humans, e.g., cellulose and other substances present in vegetables, fruit and some cereals pass through the alimentary canal almost unchanged

Question 5

Functions of digestible carbohydrates

Answer 5

•These include:
-provision of energy and heat; the breakdown of monosaccharides, preferably in the presence of oxygen, releases heat and chemical energy for metabolic work – glucose is the main fuel molecule used by body cells
•providing energy stores when carbohydrate is eaten in excess of the body’s needs as it is converted to:
–glycogen – as a short-term energy store in the liver and skeletal muscles
–fat, which is stored in adipose tissue, e.g., under the skin.

Question 6

Proteins

Answer 6

During digestion, proteins are broken down into their constituent amino acids and it is in this form that they are absorbed into the bloodstream. A constant supply of amino acids are needed to build new proteins, e.g., structural proteins, enzymes, and some hormones.

Question 7

Amino acids

Answer 7

• These are composed of the elements carbon, hydrogen, oxygen, and nitrogen. Some contain minerals such as iron, copper, zinc, iodine, sulfur, and phosphate. Amino acids are divided into two categories: essential and non-essential.
• Essential amino acids cannot be synthesized in the body; therefore, they must be included in the diet. Non-essential amino acids are those that can be synthesized in the body.

Question 8

Nitrogen balance

Answer 8

Excess amino acids are broken down. The amino group (~NH2) is converted to the nitrogenous waste product urea and excreted by the kidneys. The remainder of the molecule is converted to either glucose or a ketone body, depending on the amino acid. Negative nitrogen balance occurs when the amino acid supply does not meet body needs. This situation may arise either when dietary protein intake is inadequate, e.g., deficiency or absence of amino acids or protein requirement is increased, e.g., during growth spurts and following injury or surgery.

Question 9

Functions of proteins

Answer 9

•Amino acids are used for:
-growth and repair of body cells and tissues
-synthesis of enzymes, plasma proteins, antibodies (immunoglobulins), and some hormones
-provision of energy. Normally a secondary function, this becomes important only when there is not enough carbohydrate in the diet and fat stores are depleted.
•When protein is eaten in excess of the body’s needs, the nitrogenous amino group is detached, i.e., it is deaminated, and excreted by the kidneys. The remainder is converted to fat for storage in the fat depots, e.g., in the fat cells of adipose tissue

Question 10

Fats (triglycerides)

Answer 10

• Commonly known as ‘fats’, a triglyceride molecule consists of three fatty acids liked a glycerol molecule. Depending on the type and relative amounts of fatty acids they contain, fats are classified as saturated or unsaturated. In general, saturated fats are solid at room temperature and originate from animal sources, while unsaturated fats are oils, usually derived from vegetables or plants. A high intake of saturated fat can predispose to coronary heart disease.
• Fats consist of carbon, hydrogen, and oxygen, but they differ from carbohydrates in that hydrogen and oxygen are not in the same proportions as in water. There are several groups of fats and lipids important in nutrition.

Question 11

Cholesterol

Answer 11

•, Unlike other lipids whose molecules are composed of chains of atoms, this molecule contains four rings, which give it the characteristic steroid structure. It can be synthesized by the body (around 20%) with the remainder coming from saturated fats in the diet as a constituent of full-fat dairy products, fatty meat, and egg yolk. Cholesterol is needed for the synthesis of steroid hormones, e.g., glucocorticoids and mineralocorticoids, and is an important constituent of cell membranes.
•Cholesterol is transported in the blood combined with proteins, forming lipoproteins. Two examples are:
-low-density lipoprotein (LDL): this carries cholesterol from the liver to the body cells.
-high-density lipoprotein (HDL): this carries cholesterol back from body cells to the liver, where it is either broken down or excreted. This may be referred to as ‘good cholesterol’ and raised HDL levels are cardioprotective.
High blood cholesterol levels are associated with an increased risk of atherosclerosis, hypertension (high blood pressure), and diabetes mellitus.

Question 12

Functions of fats

Answer 12

•These include:
-provision of the most concentrated source of chemical energy and heat
-support of some organs, e.g., the kidneys, the eyes
-transport and storage of the fat-soluble vitamins: A, D, E, K
-constituent of myelin sheaths and of sebum
-formation of steroid hormones from cholesterol
-storage of energy as fat in adipose tissue under the skin and in the mesentery, especially when eaten in excess of requirements
-insulation – as a subcutaneous layer it reduces heat loss through the skin
-satiety value – the emptying time of the stomach is prolonged after eating food that is high in fat, postponing the return of hunger.
•As the body stores excess fat, it is important not to eat too much as this will lead to weight gain and becoming overweight or obese.

Question 13

Vitamins

Answer 13

Vitamins are chemicals required in very small quantities for essential metabolic processes. As most cannot be made by the body, they are an essential part of the diet and insufficiency may lead to a deficiency disease. They are in founding a wide range of foods and are divided into two groups:

• fat-soluble vitamins: A, D, E, and K
• water-soluble vitamins: B complex and C

Question 14

Fat-soluble vitamins

Answer 14

Bile is needed for the absorption of these vitamins from the small intestine. The presence of mineral oils in the intestine and malabsorption impair their absorption.

Question 15

Vitamin A

Answer 15

Vitamin A can be formed in the body from certain carotenes, the main dietary sources of which are green vegetables, orange-colored fruit (e.g., mangoes, apricots), and carrots. The main roles of vitamin A in the body are:

• generation of the light-sensitive pigment rhodopsin (visual purple) in the retina of the eye.
• cell growth and differentiation; this is especially important in fast-growing cells, such as the epithelial cells covering both internal and external body surfaces
• promotion of immunity and defense against infection
• promotion of growth, e.g., in bones.

Question 16

Vitamin D

Answer 16

• Vitamin D is found mainly in animal fats such as eggs, butter, cheese, fish liver oils. Humans can synthesize this vitamin by the action of the ultraviolet rays in sunlight in a form of cholesterol (7-dehydrocholesterol) in the skin.
• Vitamin D increases calcium and phosphate absorption from the gut and stimulates their retention by the kidneys. It, therefore, promotes the calcification of bones and teeth.

Question 17

Vitamin E

Answer 17

Also known as tocopherol, this is found in nuts, egg yolk, wheat germ, whole cereal, milk, and butter.
Vitamin E is an antioxidant, which means that it protects body constituents such as membrane lipids from being destroyed in oxidative reactions caused by free radicals. Recently, vitamin E has been shown to protect against coronary heart disease.

Question 18

Vitamin K

Answer 18

The sources of vitamin K are liver, some vegetable oils, and leafy green vegetables. It is also synthesized by bacteria in the large intestine and. significant amounts are absorbed. A small amount is stored in the liver. Vitamin K is required by the liver to produce prothrombin and factors VII, IX, and X, all essential for blood clotting

Question 19

Vitamin B complex

Answer 19

This is a group of water-soluble vitamins that promote the activity of enzymes involved in the chemical breakdown (catabolism) of nutrients to release energy.

Question 20

Vitamin B1 (Thiamin)

Answer 20

This is present in nuts, yeast, egg yolk, liver, legumes, meat, and the germ of cereals. It is rapidly destroyed by heat. Thiamin is essential for the complete aerobic release of energy from carbohydrates. Absence or deficiency causes accumulation of lactic and pyruvic acids, which may lead to accumulation of tissue fluid (edema) and heart failure. Thiamin is also important for nervous system function because of the dependency of these tissues on glucose for fuel.

Question 21

Vitamin B2 (Riboflavin)

Answer 21

Riboflavin is found in yeast, green vegetables, milk, liver, eggs, cheese, and fish roe. Only small amounts are stored in the body, and it is destroyed by light and alkalis. It is involved in carbohydrate and protein metabolism, especially in the eyes and skin. Deficiency leads to cracking of the skin, commonly around the mouth (angular stomatitis), and inflammation of the tongue (glossitis).

Question 22

Vitamin B3 (Niacin)

Answer 22

• This is present in the liver, cheese, yeast, whole cereals, eggs, and dairy products; in addition, the body can synthesize it from the amino acid tryptophan. It is central to energy release from carbohydrates in cells. In fat metabolism, it inhibits the production of cholesterol and assists in fat breakdown. Deficiency is rare and occurs mainly in areas where maize is the chief constituent of the diet because niacin in maize is in an unusable form. Pellagra develops within 6 to 8 weeks of severe deficiency. It is characterized by:
• Dermatitis – sunburn-like skin sensitivity affecting areas exposed to sunlight
• Delirium and dementia.

Question 23

Vitamin B6 (Pyridoxine)

Answer 23

This stable vitamin is found in egg yolk, peas, beans, soya beans, yeast, poultry, white fish, and peanuts. Dietary deficiency is very rare. Vitamin B6 is associated with amino acid metabolism, including the synthesis of non-essential amino acids and important molecules such as haem and nucleic acids.

Question 24

Vitamin B12 (Cobalamin)

Answer 24

Vitamin B12 is essential for DNA synthesis, and deficiency leads to megaloblastic anemia, which is correctable with supplements. However, vitamin B12 is also required for the formation and maintenance of myelin, the fatty substance that surrounds and protects some nerves. Deficiency accordingly causes irreversible damage such as peripheral neuropathy and/or subacute spinal cord degeneration and usually affects older adults. The presence of intrinsic factors in the stomach is essential for vitamin B12 absorption, and deficiency is usually associated with insufficient intrinsic factors.

Question 25

Folic acid

Answer 25

A nutrient in the vitamin B complex that the body needs in small amounts to function and stay healthy. Folic acid helps to make red blood cells. It is found in whole-grain bread and cereals, liver, green vegetables, orange juice, lentils, beans, and yeast.

Question 26

Pantothenic acid

Answer 26

This is found in many foods and is associated with energy-yielding carbohydrate metabolism; no deficiency diseases have been identified. It is destroyed by excessive heat and freezing.

Question 27

Biotin

Answer 27

This is found in a wide range of foods including yeast, egg yolk, liver, kidney, and tomatoes, and is synthesized by microbes in the intestine. It is associated with the metabolism of carbohydrates, lipids, and some amino acids; deficiency is very rare.

Question 28

Vitamin C

Answer 28

Vitamin C is associated with protein metabolism, especially the laying down of collagen fibers in connective tissue. Vitamin C, like vitamin E, acts as an antioxidant, protecting body molecules from damaging oxidative reactions caused by free radicals. When scurvy affects collagen production there is the fragility of blood vessels, delayed wound healing, and poor bone repair. Gums become swollen and spongy, and the teeth loosen in their sockets. Systemic effects include fatigue, weakness, and aching joints and muscles

Question 29

Minerals and trace elements

Answer 29

Minerals are inorganic substances needed in small amounts for normal cellular functioning. Some minerals, e.g., calcium, phosphate, sodium, and potassium are needed in larger amounts than others. Those required in only tiny quantities are known as trace elements or trace minerals, e.g., iron, iodine, zinc, copper, cobalt, selenium, and fluoride. The main minerals and trace elements are outlined below.

Question 30

Calcium

Answer 30

This is found in milk, cheese, eggs, green vegetables, and some fish, e.g., sardines. An adequate supply should be obtained from a normal, well-balanced diet, although requirements are higher in pregnant women and growing children. The most abundant of the minerals, 99% of calcium (about 1 kg in adults) is found in the bones and teeth, where it is an essential structural component. Calcium is also involved in blood clotting, and nerve and muscle function. The deficiency of calcium causes rickets in children and osteocalcin in adults.

Question 31

Phosphate

Answer 31

• Sources include milk and dairy products, red meat, fish, poultry, bread, and rice. If there is sufficient calcium in the diet it is unlikely that there will be phosphate deficiency.
• It is associated with calcium and vitamin D in the hardening of bones and teeth; 85% of body phosphate is found in these sites. Phosphates are an essential part of nucleic acids (DNA and RNA), cell membranes, and energy storage molecules such as adenosine triphosphate (ATP)

Question 32

Potassium

Answer 32

• This is found widely distributed in all foods, espe­cially fruit and vegetables, and intake usually exceeds requirements.
• It is the most common intracellular cation and is involved in muscle contraction and transmission of nerve impulses.

Question 33

Iron

Answer 33

• Iron is essential for the formation of hemoglobin in red blood cells. It is also necessary for carbohydrate metabolism and the synthesis of some hormones and neuro­transmitters. Menstruating and pregnant women have increased iron requirements, as do young people experiencing growth spurts.
• Iron deficiency anemia is relatively common and occurs when iron stores become depleted. Iron deficiency anemia may also occur arise from chronic bleeding, e.g., peptic ulcer disease.

Question 34

Iodine

Answer 34

• Iodine is found in seafood, and vegetables are grown in soil rich in iodine. In parts of the world where iodine is deficient in the soil, very small quantities are added to table salt to prevent goiter.
• It is essential for the formation of thyroxine and triiodothyronine, two hormones secreted by the thyroid gland which regulate metabolic rate, and physical and mental development.

Question 35

Water

Answer 35

• Water is the most abundant constituent of the human body, accounting for around 60% of the body weight in an adult.
• A large amount of water is lost each day in urine, sweat, and feces. This is normally balanced by intake of food and fluids, to satisfy thirst. Water requirements are increased following exercise and in high environmental temperatures. Dehydration, with serious consequences, may occur if the intake does not balance loss. Water balance is finely regulated by the action of hormones on the kidney tubules.

Question 36

Function of water

Answer 36

These include:

• providing the moist internal environment required by all living cells, e.g., for metabolic reactions
• dilution of waste products and toxins in the body
• regulation of body temperature – as a constituent of sweat, which is secreted onto the skin, it evaporates, cooling the body surface
• being the major constituent of blood and tissue fluid, it transports substances around the body and allows exchange between the blood, tissue fluid, and body cells