Ross & Wilson Anatomy and Physiology in Health and Illness (125 page)

Biological value of protein

The nutritional value of a protein, its
biological value,
is measured by how well it meets the nutritional needs of the body. Protein of high biological value is usually of animal origin, easily digested and contains all essential amino acids in the proportions required by the body. A balanced diet, containing all the amino acids required, may also be achieved by eating a range of foods containing proteins of lower biological values, provided that deficiencies in amino acid content of any one of the constituent proteins of the diet is supplied by another. A balanced vegetarian diet, which consists primarily of proteins with lower biological values, e.g. vegetables, cereals and pulses, is based on this principle.

Functions of proteins

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 part 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 (
p. 37
).

Fats

Learning outcomes

After studying this section, you should be able to:

outline the main sources of dietary fat

list the functions of fats in the body.

Fats consist of carbon, hydrogen and oxygen, but they differ from carbohydrates in that the hydrogen and oxygen are not in the same proportions as in water. There are several groups of fats and lipids important in nutrition.

Fats (triglycerides)

Commonly known as ‘fats’, triglycerides consist of one glycerol and three fatty acid molecules (see
Fig. 2.9, p. 24
). 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 (
Ch. 5
)

Linoleic, linolenic and arachadonic acids are
essential fatty acids
, which cannot be synthesised by the body in significant amounts, but are needed for synthesis of prostaglandins, phospholipids and leukotrienes. These fatty acids are found in oily fish.

Cholesterol

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 synthesised by the body and is a constituent of full-fat dairy products, fatty meat and egg yolk. Cholesterol is needed for synthesis of steroid hormones, e.g. glucocorticoids and mineralocorticoids (
Ch. 9
), and is an important constituent of the cell membrane.

Functions of fats

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 (
p. 39
) and of sebum (
p. 356
)

•
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.

Vitamins

Learning outcomes

After studying this section, you should be able to:

outline the sources and functions of the fat-soluble vitamins: A, D, E and K

describe the sources and functions of the water-soluble vitamins: the vitamin B complex and C.

Vitamins are chemical compounds, required in very small quantities, which are essential for normal metabolism and health. They are in found 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.

Guidance from DoH (1991) is used in this section.

Fat-soluble vitamins

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

Vitamin A (retinol)

This vitamin is found in such foods as cream, egg yolk, liver, fish oil, milk, cheese and butter. It is absent from vegetable fats and oils but is added to margarine during manufacture. Vitamin A can be formed in the body from certain carotenes, the main dietary sources of which are green vegetables, fruit and carrots. Although some is synthesised in the body the daily dietary requirement is 600 to 700 μg. 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 defence against infection

•
promotion of growth, e.g. in bones.

The first sign of vitamin A deficiency is night blindness due to formation of abnormal retinal pigment. Other consequences include xerophthalmia, which is drying and thickening of the conjunctiva and, ultimately, ulceration and destruction of the conjunctiva. This is a common cause of blindness in developing countries. Atrophy and keratinisation of other epithelial tissues leads to increased incidence of infections of the ear, and the respiratory, genitourinary and alimentary tracts. Immunity is compromised and bone development may be abnormal and delayed.