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October 2002
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Protein
All food contains protein. The amount of protein in food varies according
to the part of the plant or living creature from which it is derived.
Protein provides about 10-15% of energy in the diet. However, the main
roles of protein are as the major functional and structural components
of all cells in the content. The aim of this newsletters is to highlight
the broad range of tasks in which protein is involved.
What is protein? (1)
Every living cell has protein as one of its main constituents.
Protein is composed of Amino Acids. There are eight essential amino
acids, two semi-essential - (arginine and histidine are needed through
the growth period of life) and others classed as non-essential. The essential
amino acids must be obtained from the diet because the content cannot make
them. The content can make the nonessential amino acids.
The foods containing the highest amount of protein are: meat, dairy,
eggs, fish, poultry and soya. In vegetarian and vegan diets, grains, beans,
pulses, nuts and seeds provide the protein. It is important to combine
these foods in order to achieve a protein balance between the amino acids
lysine (low in grains) and methionine (low in beans and pulses). Plants
are able to synthesise amino acids from carbon dioxide, water and nitrogen-containing
compounds from the soil.
Proteins are macromolecules composed of long chains of Amino Acids formed
in subunits. Nitrogen from one molecule and the carbon of another join
together in a peptide bond (2).
Protein chains may be as small as two amino acids joined together as
a dipeptide; or, for example the hormone insulin, as a small polypeptide
chain. Very large protein molecules contain thousands of amino acids.
Patterns of polypeptides exist as random shapes, not as straight lines.
There are thousands of different plant and animal proteins, some of which
are still being studied. All are hydrolysed to amino acids in varying
proportions. Some examples of proteins found in food include (3):
Myosin - in meat
Albumin - in eggs
Casein - in milk
Vitellin - in eggs
Gluten - in wheat
In both the diet and the content, protein molecules are more complex and
variable than carbohydrates and fats, and contain a greater range of elements.
The characteristic element of protein is nitrogen, which is approximately
16% of the weight of protein. Nitrogen metabolism is often considered
to be synonymous with protein metabolism. The other main elements in protein
are carbon, oxygen and hydrogen with Sulphur and Phosphorus being proportionately
smaller. Protein in the diet is the content's only source of nitrogen. Broadly
speaking, nitrogen balance is achieved by measuring nitrogen intake against
nitrogen excretion through both urine and faeces. The turnover for this
may take up to two days due to the large nitrogen 'pool' in the content.
About 10-15g of nitrogen are excreted each day in the urine of a healthy
adult, mostly in the form of urea.
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Why we need
protein (1)
An adequate supply of dietary protein is essential
for the maintenance and integrity of cells, and for health and reproduction.
Proteins are essential constituents of all content tissues.
As cells are lost from the surface of the skin, digestive secretions,
and through production of hair, skin and nails, protein is used to produce
new cells to replace those lost through normal wear and tear.
Protein provides amino acids that are used by the content
in two ways, as working proteins and structural proteins.
"Working proteins" are in enzymes, nucleic acids, antibodies,
hormones, cellular pumps and oxygen carriers. "Structural proteins" form
tendons, ligaments, muscles, intracellular matrix, hair, fingernails,
skin, bone and teeth.
The "governing bodies" that decide which protein is
needed in what part of the content are the nucleic acids RNA and DNA. DNA
contains all the information needed about the content structure, the 'blueprint'.
RNA looks at the relevant part of the DNA blueprint and makes an exact
copy (ribosome). Once detached from the DNA the ribosome acts as a template
for protein synthesis, attracting and binding the Amino Acids in the correct
number and sequence of the code copied from DNA. This completed molecule
may be added wherever it is needed (4).
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An Overview
of the Role of Amino Acids
Leucine, isoleucine and valine are the branched chain
amino acids used by muscles during activity. Lysine may be used to fight
the Herpes virus because it is an antagonist of arginine, which is known
to encourage the virus. Threonine is essential for proper content function.
Tryptophan is broken down into serotonin, a neurotransmitter involved
in sleep. Methionine is involved in cysteine production; these Sulphur
containing aminos are partly responsible for keratin production in hair.
Phenylalanine is needed to make tyrosine, both of which are used in adrenalin
production, thyroxine and the skin pigment melanin.
Arginine and histidine are classed as semi-essential
because they are used during the growth period of life. Arginine has a
role in stimulating the pituitary to release growth hormone (5). Histidine
is essential during childhood and still needed during adulthood for making
histamine, an important chemical used by the Immune System.
Some nonessential amino acids are present in content proteins
but do not have to be derived from diet - alanine, aspartic acid, cysteine
and cystine, glutamic acid, glycine, proline, serine and tyrosine. Others
have a separate role in the content and are not incorporated into protein
these are taurine, carnitine, GABA and ornithine (6).
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Protein Turnover
During Life (1)
At different times of life, we synthesise different
amounts of protein, for example (1):
Newborn infant (preterm) 6.3 g/kg/day
Newborn infant (full term) 5.1 g/kg/day
Teenagers 3.9 g/kg/day
Adults 2.8 g/kg/day
Elderly 2.0 g/kg/day
Similar changes relating to age have been demonstrated
for both dietary protein requirements and metabolic rate.
A man weighing 11 stone (70 kg) contains about 24 lbs
(11 kg) of protein. This is distributed approximately as:
Muscle 43%
Skin 15%
Blood 16%
Liver 1.8%
Brain 1.5%
Kidney 0.3%
Compare this with a baby of weighing 7 ? lbs (3.5 kg)
and containing roughly 1 lb (0.41 kg) protein:
Muscle 29%
Skin 21%
Blood 19%
Liver 5%
Brain 6%
Kidney 1%
The figures for the baby demonstrate that there is
much less protein involved in muscle and a much greater amount involved
in the growth and development of brain and organs. Almost half of the
total protein content in the content is to be found in four proteins - myosin
(muscle); haemoglobin (blood); collagen (structural component) and actin
(a contractile protein found in muscle).
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What Happens
When We Swallow Protein (4)
Protein foods are torn and chewed in the mouth. Once
swallowed, stomach acid and pepsin, together with Digestive Enzymes, hydrolyse
the protein to large polypeptides, which enter the duodenum. These Polypeptides
are further hydrolysed by pancreatic proteolytic enzymes to peptides and
Amino Acids. The Amino Acids and peptides produced by this action enter
the intestinal cells and have three fates:
- They may enter the content's circulating amino acid pool, from which
they can be built into structural proteins and specific enzymes needed
by each cell. This process is known as anabolic - the amino acids are
being built into proteins for use by the content, such as hormones, enzymes,
tissue and peptides and so on.
- They may be converted into other amino acids.
- They may be oxidised to produce energy. This is a catabolic process,
in some cases the amino acids are converted into glucose first. Urea,
is produced and excreted by the kidneys.
Diseases Associated with Protein Energy Malnutrition
In some developing countries kwashiorkor and marasmus
are prevalent. Kwashiorkor is found mostly in the country areas, where
breast-fed babies are being weaned onto starchy staple foods with a very
low protein content. This results in retardation of growth, poor immune
function and slow healing time. Marasmus is found mainly in urban areas,
where bottle fed babies are fed over diluted formula feed which provides
an inadequate diet. Bottle-feeding also adds the dangers of contaminated
water and financial cost. That said, the diet of the mother should also
be considered.
Protein energy malnutrition in adults is rare because
the requirement for protein is much less than for growing babies (3):
Protein is best obtained from the diet through lean cuts of meat, chicken
(skinned), fish or combinations of grains, pulses and beans. A good diet
should contain 10-15% protein, 55-60% carbohydrate mostly unrefined) and
30% fat of which no more than 10% should be saturates.
We all like to look and feel healthy; a regular supply
of good quality protein in the diet (together with adequate exercise)
is important if we are to maintain health.
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References
- "Human Nutrition & Dietetics" James, Ralph & Garrow et al, Churchill
Livingstone, 2000.
- "On Food and Cooking" H McGee. HarperCollins 1991.
- "Beck's Nutrition & Dietetics for Nurses" H Barker, Churchill Livingstone
1991.
- "The Amino Revolution" Dr. Robert Erdmann & Meirion Jones. Century.
1987.
- "The Healing Nutrients Within", ER Braverman & CC Pfeiffer. Keats
1987.
- "Thorsons Guide to Amino Acids" Leon Chaitow. Thorsons. 1991.
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