Protein in Human Nutrition

Role of Protein in Human Nutrition (sources, excess, deficiency)
Proteins are large, complex molecules that have many critical roles in the body. They are made up of smaller units called amino acids, which are attached to one another in long chains via peptide bonds.

In the gastrointestinal tract proteins undergo hydrolysis via proteolytic enzymes resulting in amino acids.

Amino acids are classified into three groups:


 * Essential
 * Non-essential
 * Conditional

Essential amino acids cannot be made by the body, and must be supplied by the diet. They do not need to be eaten at one meal. The balance over the whole day is more important. The nine essential amino acids are:


 * Histidine
 * Isoleucine
 * Leucine
 * Lycine
 * Methionine
 * Phenylalanine
 * Threonine
 * Tryptophan
 * Valine

Non-essential amino acids are made by the body from essential amino acids or in the normal breakdown of proteins.

They include:
 * Alanine
 * Asparagine
 * Aspartic acid
 * Glutamic acid

Conditional amino acids are usually not essential, except in times of illness and stress.

They include:
 * Arginine
 * Cysteine
 * Glutamine
 * Glycine
 * Ornithine
 * Proline
 * Serine
 * Tyrosine

Proteins are used in many different ways throughout the body, such as:
 * part of DNA!
 * part of haemoglobin
 * creating neurotransmitters
 * creating antibodies in blood
 * part of keratin in hair and nails;
 * creating enzymes throughout the body

Complete proteins: usually of animal origin and contain all essential amino acids (which the body cannot synthesise itself.

Incomplete proteins: mostly of plant origin, they are deficient in one or more of the essential amino acids. The capacity of proteins is called their supplementary value.

Food sources from animals (meat, fish, eggs, dairy) usually score highly on the amino acid profile and are subsequently regarded as “high-quality proteins”. Eggs are often regarded as the basic standard for comparison of protein value. There are some vegetables which are pretty good sources of protein, such as beans, peas, lentils, and seaweed. For vegetarians and vegans, the most complete protein comes from soya beans. These beans are excellent sources of protein, and are available in various different forms, such as tofu, soy milk (and other dairy substitutes).

Requirement The recommended daily allowance for protein for adult females is 46g/day and for males is 56g/day. Approximately 10-35% of daily calories should be from protein.

Excess
The human body is unable to store extra protein. Protein consumed in excess of the body's needs is not used to build muscle; rather, it is used for non-protein bodily functions.

An upper limit of safe protein intake has not yet been established but an intake about 2g/kg body weight leads to increased levels of nitrogenous substances, increased glomerular filtration and changes in liver functions.

Deficiency
Protein deficiency is a state of malnutrition in which insufficient amounts of protein is taken in for the body to utilize in order to produce energy. This condition is largely responsible for the high incidence of starvation and disease in many Third World countries, causing death to millions of people each year. However, protein deficiency also occurs in developed countries, primarily due to poverty. Certain individuals may also become prone to protein deficiency, such as crash dieters and vegetarians who neglect to properly balance their diet.

Symptoms of protein deficiency include
 * weight loss
 * diarrhoea
 * accumulation of fluids (oedema) in the legs and abdomen
 * hair loss, scaly skin, and lethargy