The word protein by derivation means that which
is of first importance. They are complex nitrogenous substances
of great importance to us. They are composed of carbon, hydrogen,
oxygen, nitrogen and often sulfur in varying amounts. They differ
from carbohydrates mainly in that proteins contain nitrogen (about
15-20%) while carbohydrates lack nitrogen.
Proteins consist of about 20% of body weight in
Sources of proteins
Animal sources: Milk, meat, eggs, cheese, fish
and fowl. These proteins are rated superior to vegetable proteins
(read below). Egg protein is considered to be the best for humans.
Vegetable sources: Pulses (legumes), cereals, beans,
nuts, oilseed cakes etc. They are best taken in combination.
Functions of proteins
Body building: All our muscles are made up of a
protein called myoglobin.
Repair and maintenance of body tissues (e.g. proteins
are needed to repair a gap that a wound creates in the skin)
Maintenance of osmotic pressure: A blood protein
called albumin holds water in the blood. When albumin concentration
in blood decreases, the water leaks out of blood vessels to form
swelling (edema), most evident in legs.
Fighting infection: Proteins, called antibodies,
continually fight infection that we get from our environment. It
is because of these antibodies that we don't get infected despite
the presence of bacteria in the air we breathe, the food we eat
and the water we drink!
Blood clotting: When we get a cut in the skin,
special proteins in the blood called coagulation proteins (or coagulation
factors) make the blood clot, so that the blood loss can be minimized.
Catalyzing biochemical reactions: Proteins called
enzymes can increase or decrease the rates of biochemical reactions
inside the body according to the situation.
Hormones: Proteins (or their fragments) function
as hormones. Insulin is a hormone, and so are estrogen, testosterone,
growth hormone etc.
Supply energy: Proteins can also supply energy
(4 Calories per gram), but usually carbohydrates are used up first.
This is so fortunate, considering the functions proteins perform.
Really, proteins are too precious to be burnt up for calories!
Since proteins cannot be stored in the body, they
must be supplied daily in diet. Adults require 1 gram per kg body
weight/day. Children require double the amount.
Proteins are made up of smaller units called amino
acids. The amino acids join each other in a complex form to build
a protein molecule just like bricks join each other to form a house.
There are 24 amino acids needed by the human body.
Every protein in us can be built by various combinations of just
these 24 amino acids! Nine of these 24 amino acids cannot be synthesized
in the body, and must be supplies from diet. These amino acids are
called essential amino acids. The nine essential amino acids are:
Leucine, Isoleucine, Lysine, Methionine, Phenylalanine, Threonine,
Valine, Tryptophan and Histidine.
New tissue cannot be synthesized until all the
essential amino acids are present in diet.
In addition, premature babies also need Cysteine
and Tyrosine in diet.
Biologically complete proteins
A protein is said to be biologically complete if
it contains all the essential amino acids in amounts corresponding
to human needs. When one or more of the essential amino acids is
lacking, the protein is considered to be biologically incomplete.
From the nutritional standpoint, animal proteins
are considered superior to vegetable proteins because they contain
all the essential amino acids that we require. For example, milk
and egg proteins have a pattern of amino acids considered most suitable
Supplementary action of proteins
Vegetable proteins are usually deficient in one
or more essential amino acids. For example, cereals are deficient
in lysine and Threonine, and pulses lack Methionine. But when cereals
and pulses are eaten together, the mixed diet then provides all
the essential amino acids. This is known as supplementary action
Laboratory tests for protein nutrition status
At present, the best measure of the state of protein
nutrition is serum albumin concentration.
(Other tests that have been suggested are arm
muscle circumference, the creatinine-height index, serum transferrin
and total body nitrogen.)