Ross & Wilson Anatomy and Physiology in Health and Illness (127 page)

Minerals and trace elements

Minerals are inorganic substances needed in small amounts by all cells for normal functioning. 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.

Calcium

This is found in milk, cheese, eggs, green vegetables and some fish. An adequate supply should be obtained from a normal, well-balanced diet, although requirements are higher in pregnant women and growing children. 99% of body calcium is found in the bones and teeth, where it is an essential structural component. Calcium is also involved in coagulation of blood, and nerve and muscle function. Deficiency of calcium causes
rickets
in children and
osteomalacia
in adults (
p. 421
).

Phosphate

Sources of phosphate include cheese, oatmeal, liver and kidney. 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, see
Ch. 17
) and energy storage molecules inside cells as adenosine triphosphate (ATP,
Fig. 2.10
,
p. 24
).

Sodium

Sodium is found in most foods, especially fish, meat, eggs, milk, most processed foods and also added during cooking or as table salt. Intake of sodium chloride usually exceeds recommendations and excess is normally excreted in the urine. The recommended daily salt (sodium chloride) intake for adults should not exceed 6 g. In practice, food is usually labelled with sodium content, and to convert this to salt, the sodium content is multiplied by 2.5.

It is the most commonly occurring
extracellular cation
and is involved in muscle contraction, transmission of nerve impulses along axons and maintenance of water and electrolyte balance.

Potassium

This substance is found widely distributed in all foods, especially fruit and vegetables, and intake usually exceeds potassium requirements.

It is the most commonly occurring
intracellular cation
and is involved in many chemical activities inside cells including muscle contraction, transmission of nerve impulses and maintenance of water and electrolyte balance.

Iron

Iron, as a soluble compound, is found in liver, kidney, beef, egg yolk, wholemeal bread and green vegetables. In normal adults about 1 mg of iron is lost from the body daily. The normal daily diet contains more, i.e. 9 to 15 mg, but only 5–15% of intake is absorbed. Iron is essential for the formation of haemoglobin in red blood cells. It is also necessary for oxidation of carbohydrates and the synthesis of some hormones and neurotransmitters.

Iron deficiency is a relatively common condition, and causes anaemia (
p. 67
) if iron stores become sufficiently depleted. Menstruating and pregnant women have increased iron requirements, as do young people experiencing growth spurts. Iron deficiency anaemia may also occur in chronic bleeding, e.g. peptic ulcer disease.

Iodine

Iodine is found in salt-water fish and in vegetables grown in soil containing iodine. In some parts of the world where iodine is deficient in soil, very small quantities are added to table salt to prevent
goitre
(
p. 223
). Daily iodine requirement depends upon metabolic rate. Some people have a higher normal metabolic rate than others and their iodine requirements are greater.

It is essential for the formation of
thyroxine
and
tri-iodothyronine
, two hormones secreted by the thyroid gland (
p. 213
).

Water

Water is the most abundant constituent of the human body, it makes up about 60% of the body weight in men and about 55% in women. A man weighing 70 kg contains about 40 litres of water, 28 of which are intracellular and 12 extracellular. Extracellular water consists of 2 to 3 litres in plasma and the remainder, interstitial fluid (see
Fig. 2.14
,
p. 26
).

A large amount of water is lost each day in urine, sweat and faeces. This is normally balanced by intake in food and fluids, to satisfy thirst. Water requirements are increased following exercise and in high environmental temperatures. Dehydration, with serious consequences, may occur if intake does not balance loss.

Functions of water

These include:

•
provision of the moist internal environment required by all living cells

•
providing the aqueous (water-based) conditions in which all metabolic reactions take place

•
moistening of food for swallowing (see saliva,
p. 286
)

•
regulation of body temperature – as a constituent of sweat, which is secreted onto the skin, it evaporates, cooling the body surface (
Ch. 14
)

•
being the major constituent of blood and tissue fluid, it transports substances round the body and to and from body cells

•
dilution of waste products and poisonous substances in the body

•
providing the medium for the excretion of waste products, e.g. urine and faeces.

Non-starch polysaccharide (NSP)

Learning outcome

After studying this section, you should be able to:

describe the sources and functions of non-starch polysaccharide.

Non-starch polysaccharide (NSP) is the correct term for dietary fibre although the latter term continues to be more commonly used in the UK. It is the indigestible part of the diet and consists of bran, cellulose and other polysaccharides found in fruit and vegetables. It is widely distributed in wholemeal flour, the husks of cereals and in fruit and vegetables. Dietary fibre is partly digested by microbes in the large intestine with gas (flatus) formation. The daily requirement is at least 20 g.

Functions of NSP (dietary fibre)

Dietary fibre:

•
provides bulk to the diet and helps to satisfy the appetite

•
stimulates peristalsis (see
p. 281
), which prevents constipation

•
attracts water, increasing bulk and softness of faeces

•
increases frequency of defaecation, preventing constipation

•
prevents some gastrointestinal disorders, e.g. colorectal cancer and diverticular disease (
p. 320
).

Disorders of nutrition

Learning outcome

After studying this section, you should be able to:

describe the main consequences of malnutrition, malabsorption and obesity.