Proteins: Food Sources, Functions, Digestion, Absorption and Utilization

Proteins: Food Sources, Functions, Digestion, Absorption and Utilization

Proteins like carbohydrates are organic compounds but are distinct from carbohydrates in certain respects. Besides carbon, hydrogen and oxygen, proteins also contain another element called nitrogen. The basic building blocks of proteins are nitrogen containing units called amino acids. Amino acids are joined or linked together in chains by linkages called peptide linkages. Several hundreds or even thousands of amino acids are strung together in specific strands and coils to form proteins.

You may be amazed to know that these proteins are built up of just 22 amino acids. Food has an almost endless variety of proteins and no two proteins have similar structure. This is because the twenty or more amino acids are present in varying proportions and are arranged in varying sequences in different kinds of proteins. Of these about 8 cannot be manufactured by the body while the rest can. Those which cannot be manufactured by the body must be supplied by the diet. These amino acids are called essential. The amino acids which can be manufactured by the body need not, of course, be supplied by the diet. They are hence called nun-essential. Here the terms "essential" and "non-essential" only refer to whether it is essential or not essential to provide them in the diet. We must remember that for the synthesis of body proteins both essential and non-essential amino acids are equally important.

Essential amino acids Non-essential amino acids

Isoleucine

Leucine

Lysine

Phenylalmine

Threonine

Tryptophan

Valine

Histidine*

Alanine

Asparagine

Aspartic acid

Cystine

Cysteine

Glutamic acid

Glutamine

Glycine

Hydroxyproline

Hydroxylysine

Proline

Serine

Tyrosine

* Essential for Infants only

The quality of food proteins depends on the number and the proportion of essential amino acids contained in them. Proteins are obtained from both animal and plant foods. The proteins in animal foods are of good quality because they contain all essential amino acids in the required amounts and proportions. On the other hand, the proteins in plant food generally lack one or two essential amino acids and, therefore, are not of good quality. For example, cereals are poor in lysine and rich in methionine. On the other hand, pulses are poor in methionine and rich in lysine. When cereals are combined with pulses in the same meal then the quality of protein improves. Protein quality of'plant foods can be thus improved by the combination of one or more kinds of foods of plant origin. If you look at the traditional Indian diets (whatever the region you may belong to), you will find many dishes which are prepared by using either a cereal-pulse combination as in dosa (rice, urad dal); dal-roti; rice-dal or cereal-animal food combinations as in dalia (milk, broken wheat); rice-fish. This is one of the ways of improving the quality of food proteins. By a judicious combination of foods of animal and plant origin, you can ensure that the protein consumed will be of good quality. You must remember that even small amounts of animal protein with vegetable protein in a meal greatly improves protein quality.

Food Sources

Here is the list of some of the rich sources of proteins. The list is vast and includes: milk, milk products (like curd, khoa, paneer), flesh foods (meat, fish, poultry), eggs, nuts and oilseeds (groundnuts, almond, cashewnut, walnut) and pulses (bengal gram, lentils, green gram, rajmah, soyabean). Among pulses, soyabean is particularly rich in protein. Protein content of some of these foods is given in Table.

Protein Rich Foods

Food Stuff

Amount of Protein (g)

(Per 100gm of edible portion of food)

Foods of Animal Origin
Flesh Foods
Goat meat 
Fish (Pomfret) 
Egg 

21.4

20.3

13.3

Milk and Milk Products:
Milk (cow) 
Khoa 
Paneer (Cottage cheese)

3.2

20.0

18.3

Foods of Plant origin
Nuts and Oilseeds
cashew nut 
Groundnut 

Pulses
Soyabean 
Rajmah 
Bengal gram (whole)

21.2

25.3

43.2

22.9

17.1

If you look at the current prices of foods of animal origin like meat, fish, poultry, you will find that most of these are very costly. The only animal foods which are relatively less expensive (though expensive as compared to plant foods) are milk and eggs. As the protein in foods of animal origin are of good quality, one should try to include small amounts of these foods in the daily diet. Milk is the only animal food used by both vegetarians and non-vegetarians. Although the protein content of 100gm milk is only 3.2 g yet it contains protein of very good quality and is rich in lysine. Hence, even a small amount of milk added to a basic Indian diet of dal-roti greatly enhances the protein quality of the whole diet. One should therefore try to include atleast a small amount of milk in the daily diet. Non-vegetarians, who cannot afford meat, fish and chicken can eat eggs which are cheaper and as nutritious as meat, fish or chicken.

Let us now take a look at the foods of plant origin. According to Table, pulses, nuts and oilseeds are rich sources of proteins. But these foods are also very expensive. Pulses are the major source of protein in Indian diets. One can try to improve the quality of cereal proteins by combining them with pulses. A small amount of milk, if it can be afforded, will further improve the quality of food protein.

Functions

Remember your childhood days when your parehts/elders forced you to drink milk. They always said that you need milk for the proper growth of you body. Now, as an adult you would realize that they were right. Milk is a good source of protein and proteins do play a role in growth and body-building. Let us now study the body-building and other functions of proteins in detail.

Other Functions of Proteins in detail

  • Body-building: Proteins supply amino acids for building new body tissues and for the replacement of worn out tissues. Thus they help in the growth and the maintenance of the body. For the constant growth of human beings from birth till adulthood, a regular supply of dietary protein is required. This does not mean that proteins are not needed when growth ceases. Even during adulthood worn out body tissues need continuous replacement. Thus, proteins are required throughout life for the growth and maintenance of the body.

  • Proteins as regulatory and protective substances: Proteins are also part of some chemical substances essential for the regulation of vital body processes. You are aware of the role of enzymes in the process of digestion of food. All enzymes are proteins in nature. Like enzymes, hormones are also chemical substances vital for the regulation of metabolism as well as some other body processes. Some of the hormones (like insulin) are proteins. Antibodies which protect the body from illness are also proteins.

  • Proteins as carriers: Some of the proteins act as carriers and help to transport certain substances from one place to another. One prominent example of a protein carrier is haemoglobin, the red coloured protein-containing substance present in the blood. Haemoglobin carries oxygen from the lungs to various body tissues and carbon dioxide from body tissues to the lungs.

  • Energy-giving function: Proteins can also be broken down in the body to provide energy. Each gram of protein yields about 4 Kcal. This, however, is not the major function of proteins and only takes place when the diet does not supply enough energy-giving nutrients such as carbohydrates and fats.

Digestion, absorption and utilization

Dietary proteins chiefly consist of proteins and small and large chains of amino acids. Digestion of proteins involves the breakdown of these amino acid chains to their constituent amino acids. Since saliva contains no proteolytic enzyme (enzymes which bring about breakdown of proteins), protein digestion mainly occurs in the stomach and the small intestine. Pepsin, a proteolytic enzyme, present in gastric juice breaks down proteins into smaller amino acid chains. But pepsin itself cannot complete the digestion proteins. Partly broken down proteins from the stomach are released into the small intestine where further digeation takes place in two Steps :

  1. Breakdown of partly digested protein to smaller amino acid chains: There are several proteolytic enzymes called proteases in the small intestine which act on partly digested proteins and convert them to even smaller amino acid chains.

  2. Breakdown of amino acid chains to amino acids: Finally other kinds of proteolytic enzymes called peptidases act on amino acid chains and convert them to their constituent amino acids.

The metabolism of proteins is essentially the metabolism of amino acids as these are the end products of the process of digestion of proteins. After digestion, amino acids are carried by the blood to the liver.

Here amino acids are used in three ways:

  1. Some of them are used for building of blood proteins;

  2. Some are retained in the liver and

  3. The rest enter the blood circulation as amino acids.

Some of the amino acids remain in circulation and others are taken up by body tissues for protein synthesis whenever needed. It must be emphasized here that only proteins of good quality are maximally utilized by the body for synthesis of its own proteins. Foods of animal origin like milk, meat have proteins of good quality. This is because the animal proteins have the right proportions and amounts of all essential amino acids. Food of plant origin, on the other hand (like wheat, rice and pulses), have protein of poor quality because they generally lack one or more essential amino acids.

Thus the amino acids present in proteins of good quality can be used optimally for body protein synthesis. On the other hand, proteins of poor quality are less likely to be used for tissue protein synthesis. These proteins (amino acids) are more likely to be used for other purposes including release of energy. Like glucose, amino acids can also be oxidized or burnt in the body to produce energy.


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