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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 an adult.

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!

Daily requirements

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.

Amino Acids

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 for humans.

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 of proteins.

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.)

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This page last updated on:
October 15, 2003