Ross & Wilson Anatomy and Physiology in Health and Illness (124 page)

Learning outcomes

After studying this section, you should be able to:

describe the main mono-, di- and polysaccharides

list the nutritional function of digestible carbohydrates.

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. 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.

Monosaccharides

Carbohydrates are digested in the alimentary canal and absorbed as monosaccharides. Examples include glucose (see
Fig. 2.7, p. 23
), fructose and galactose. These are, chemically, the simplest form in which a carbohydrate can exist.

Disaccharides

These consist of two monosaccharide molecules chemically combined, e.g. sucrose (see
Fig. 2.7, p. 23
), maltose and lactose.

Polysaccharides

These consist of complex molecules made up of large numbers of monosaccharides in chemical combination, e.g. starches, glycogen, cellulose and dextrins.

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 (see NSP,
p. 273
).

Functions of digestible carbohydrates

These include:

•
provision of energy and heat; the breakdown of monosaccharides, preferably in the presence of oxygen, releases energy for metabolic work – glucose is the main fuel molecule used by body cells

•
‘protein sparing’; i.e. when there is an adequate supply of carbohydrate in the diet, protein does not need to be used to provide energy and heat

•
provision of a store of energy 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 (see
p. 307
)

    
–
fat and stored in the fat depots, e.g. under the skin.

Proteins (nitrogenous foods)

Learning outcomes

After studying this section, you should be able to:

list the sources of animal and vegetarian protein

list the nutritional functions of dietary proteins.

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

Amino acids (see
Fig. 2.8
)

These are composed of the elements carbon, hydrogen, oxygen and nitrogen. Some contain minerals such as iron, copper, zinc, iodine, sulphur and phosphate. They are divided into two categories:
essential
and
non-essential
.

Essential amino acids
cannot be synthesised in the body, therefore they must be included in the diet.
Non-essential amino acids
are those that can be synthesised in the body. The essential and non-essential amino acids are shown in
Box 11.2
.

Box 11.2
Essential and non-essential amino acids

Nitrogen balance

Excess amino acids are broken down. The amino group (~NH
₂
) 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 (see ketosis,
Ch. 12
), depending on the amino acid.
Negative nitrogen balance
occurs when amino acid supply does not meet body needs. This situation may arise either when intake is inadequate, e.g. dietary amino acids are absent or deficient, or protein requirement is increased, e.g. during growth spurts and following injury or surgery.