Foods originating from animals such as, meat, fish, egg, milk, cheese etc., from where the proteins are found. But some plants are also the source of protein, i.e. pulses, beans, nuts etc. Cereals like rice, wheat, maize also provide some amount of protein. Major proteins of our diet are—albumins and globulins that are found in both animal and plant products such as meat, fish, egg, milk, pulses etc. Animal proteins are nutritionally of much better quality and called the first class proteins whereas plant proteins are usually inferior in quality and called the second class proteins. Now we shall discuss about the protein function in human body.
It is known to us that proteins are major organic constituents of protoplasm as well as many extracellular materials. They form the fundamental structural and functional components of living organisms. Each organism has the ability to synthesize a large variety of proteins. For this, the proteins of food are digested and absorbed in the form of amino acids which are then assembled in a new sequence. This is why the body protein function differs from the dietary proteins. Amino acids consequent from dietary proteins take part in the formation of different types of proteins in the body and thereby help in the following protein function :-
Formation of structural proteins or tissue proteins is required for growth of the body and repair of the wear and tear of tissues.
Proteins are required for the synthesis of various important materials such as enzymes, hormones, pigments and plasma proteins.
Amino acids are important for the synthesis of antibody proteins that provide a mechanism for self defense against infections.
Proteins (amino acids) are required in the body for synthesis of various materials that contain nitrogen but are not proteins. These are called non-protein nitrogenous (NPN) substances. Some of these compounds work as neurotransmitters, hormones and coenzymes.
4 kcal of energy are liberated from each gram of protein when oxidized in the body. When sufficient carbohydrate and fat are present in the diet, very little protein is oxidized as fuel. But in the conditions of prolonged starvation or diabetes mellitus when food materials are not available to the tissues, the body proteins are catabolized for supplying energy.
It is another important protein function. The net result of metabolism of amino acids or proteins is expressed by 'nitrogen balance'. The term nitrogen balance denotes the difference between dietary nitrogen intake and excretion of nitrogen in urine and feces.
Normal healthy adults remain at a state of equilibrium in which the intake of nitrogen equals to its excretion. This is called normal nitrogen balance which indicates that there is no retention or loss of nitrogen so that the total quantity of nitrogen in the body remains unaffected. In this state, the rates of anabolism and catabolism of protein are equal. When the nitrogen intake exceeds its output, there is withholding of nitrogen in the body in the form of tissue proteins and the individual is said to be in positive nitrogen balance. This is seen during growth, recovery from illness, pregnancy, high protein diet etc. In this state, the rate of protein anabolism exceeds that of protein catabolism. On the other hand, if the nitrogen output exceeds its intake, the individual is in negative nitrogen balance. In this state, there is a loss of nitrogen from the body because the rate of protein catabolism is higher than that protein function of anabolism. This occurs during low protein diet, starvation, illness, old age etc.
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