Protein in the Diet
We have introduced the macronutrients and micronutrients required in the diet. In this article we will take a closer look at proteins, their functions as a nutrient and their sources in the diet.
Protein is in every living cell in the body and it is important for a large number of functions in the body (Figure 1). Muscles, bones, connective tissues, blood cells, glands and organs all contain protein so it is an important nutrient for growth and repair. Protein can also be used as a secondary source of energy when insufficient carbohydrate and fat are available to meet the bodies needs. Although its not its primary role. 1g of protein contains 17kJ / 4 kcal of energy.
Protein in our body is not static but a dynamic constituent that is in the state of continuous flux. It is synthesised during growth, repaired, maintained and replaced throughout life. Protein requirements change throughout the lifecycle because body weight fluctuates and protein requirements increase with increased body weight. The number of cells in the body increases during periods of growth, therefore during childhood and adolescence proportionally more protein is required. During pregnancy and lactation woman need more protein in the diet to allow for growth of the baby and adequate breast milk production. Similarly, an injury or damage to body tissue, e.g. following surgery, results in increased protein requirements.
Figure 1: Functions of Protein (Click to expand)
Dispensable and Indispensable Amino Acids
Protein molecules consist of long chains of amino acids chemically combined. There are 20 amino acids commonly found in proteins, of which 12 are dispensable. This means that they can be synthesised by the body and therefore are not essential in the diet. The remaining 8 amino acids are indispensable (essential). These cannot be synthesised by the body and therefore must be supplied in the diet.
The distribution of the various essential amino acids in the food we eat is not necessarily the same as is required to synthesise protein in the cells. A food with protein which has a similar distribution of indispensable (essential) amino acids to the protein in the body is therefore of more use, or of better quality and higher biological value, than one which does not supply the essential amino-acids in the necessary quantities.
As a measure of protein quality, the biological value of a protein is used. Biological value is the percentage of absorbed protein which is converted into body protein. If a protein contains the indispensable amino acids in the approximate proportion required by humans, it is said to be of high biological value. For example, egg protein has an amino acid pattern similar to human protein and has a high biological value of 97%. If it is comparatively low in one or more of the essential amino acids, it is said to have low biological value e.g. rice and pulses. The amino-acid that is in shortest supply in relation to need, is termed the limiting amino-acid.
The limiting amino-acid tends to be different in different sources of protein, so when two or more foods providing protein are eaten together the limiting indispensable (essential) amino-acid of one is in excess in the other. The resultant mixture has a higher biological value than the average of its components.
For example, bread and cheese are complementary, because the lysine deficiency in bread is complemented by the excess in the cheese resulting in a combination of higher biological value. This is known as the complimentary action of protein. Other examples include breakfast cereals and milk; lentils and rice; pulses and grains; baked beans on toast; pasta and cheese; and milk and rice.
The term protein energy malnutrition (PEM) is used to describe a range of conditions from protein deficiency at one extreme to energy deficiency at the other. Protein energy malnutrition occurs mainly in children and in developing countries where diets are composed of mainly staple foods and where this food is low-protein food such as yams. The two most common forms of PEM are kwashiorkor and marasmus.
A severe protein deficiency causes a disease known as kwashiorkor. This is caused by a diet low in protein and high in starchy carbohydrate. The main symptoms include wasting of muscles, retarded growth and a distended abdomen caused by oedema, that is, fluid in the tissues.
Marasmus is a chronic condition that occurs in infants under 1 year who have been weaned off breast milk on a diet containing too little energy and protein. As a result, the children can become severely underweight, suffer from muscle wasting and an absence of subcutaneous fat. A lack of protein may also cause anaemia since protein is necessary for the formation of red blood cells.