Nutritional Roles of Food Constituents | Human Physiology

The following points highlight the three important roles of nutrition’s in food constituents. The nutrition’s are: 1. Carbohydrates 2. Proteins 3. Fats. 

Food Constituent: Nutrition # 1. Carbohydrates:

(i) These are found in sugar, jam, cereals, bread, biscuits, potatoes, fruits and vegetables.

(ii) Carbohydrates are classified as monosaccharide’s (e.g., glucose, fructose, and galac­tose), disaccharides (e.g., sucrose, maltose, lactose) and polysaccharides (e.g., starches, glycogen, cellulose, dextrins).

(iii) Carbohydrates are absorbed from the alimentary canal as monosaccharide’s.

(iv) All polysaccharides can not be digested by human beings, for example, cellulose present in vegetables passes through the alimentary canal almost unchanged.

(v) Carbohydrates are used primarily as sources of chemical energy to be either metabolised immediately as glucose or stored as glucogen. The synthesis of glycogen is called glycogenesis.

(vi) If carbohydrate is eaten in excess it is converted into fat and stored in the body. Carbohydrates may be changed into amino acids. When there is an adequate supply of carbohydrate in diet, protein does not need to be used to provide energy and heat.

(vii) Carbohydrates are more suitable for the production of energy in the body than proteins and fats because carbohydrate molecules contain relatively more oxygen than the others, hence require less molecular oxygen for oxidation. So athletes, labourers doing heavy work and mountaineers should take high carbohydrate diets.

(viii) Carbohydrates are also stored in the body cells as glycogen and are used for the production of energy whenever required.

(ix) The liver can store enough glycogen to maintain blood glucose level for several hours. Under acute starved conditions, the liver cells begin to convert amino acids and the glycerol into glucose. Such production of new glucose is called gluconeogenesis.

Food Constituent: Nutrition # 2. Proteins:

(i) Proteins are made up of a number of units, called amino acids. 22 amino acids have been identified so far. Amino acids are divided into two categories: essential and non-essential amino acids.

Essential amino acids are so called because they cannot be synthesized in the body and therefore, must be included in the diet. Eight amino acids (ten in children) are considered essential for human nutrition. These are isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine.

Non-essential amino acids are those which can be synthesized in the body. These are alanine, arginine, asparagine, aspartic acid, cysteine, cystine, glutamic acid, glutamine, glycine, histidine, hydroxyproline, proline, serine and tryosine. Arginine and histidine are considered semi-indispensable amino acids.

Amino acids can be classified into three groups depending on their reaction in solution:

(a) Neutral amino acids, e.g., glycine, alanine, serine, threonine, valine, leucine, isoleucine, phenylalanine, tryosine, tryptophan, histidine, pro-line, hydrox­yproline, cysteine, cystine, methionine (last three are sulphur containing amino acids),

(b) Acidic amino acids, e.g., aspartic acid, glutamic acid,

(c) Basic amino acids, e.g., arginine, lysine.

(ii) Proteins can be classified as first class proteins and second class proteins. First class proteins contain all the essential amino acids in the correct proportions. They are derived almost entirely from animal sources and include meat, fish, milk and eggs. Second class proteins do not contain all the essential amino acids in the correct proportions.

They are considered nutritionally inferior to animal proteins with respect to essential amino acids. They are mainly of vegetable origin. Examples of second class proteins are peas, beans and lentils, which are known as pulses. A small proportion of second class protein is to be found in other vegetables and in some of the mainly carbohydrate foods, such as bread and potatoes.

(iii) Proteins are used as structural components of tissues, as channels, transporters, regulatory molecules and enzymes.

(iv) Amino acids, the units of proteins, are required for the formation growth and repair of body cells. Therefore, the protein requirement rises during pregnancy and lactation. Infants and children, also need good quantity of proteins.,

(v) If proteins are not provided properly, two deficiency diseases named Marasmus and Kwashiorkor are caused in children.

(vi) Amino acids are required for the formation of some secretions of the cells, i.e., hormones (adrenaline and thyroxine).

(vii) Amino acids are also needed for the formation of blood proteins, i.e., albumin, globulin, fibrinogen and prothrombin.

(viii) Amino acids are also required for the formation of products like heme of haemo­globin, the skin pigment melanin and purines and pyrimidine’s of nucleic acids.

(ix) Proteins can also be utilized as energy sources. Normally, this is a secondary function and becomes important only when there is not enough carbohydrate in the diet and stored fat is in less quantity.

(x) Some amino acids give rise to carbohydrates in the body.

(xi) When protein is eaten in excess, the nitrogenous part is detached and excreted by the kidneys and the remaining portion is converted into fat which is stored in the body.

Food Constituent: Nutrition # 3. Fats (Lipids):

(i) Fats are divided into two group; animal and vegetable. Animal fat is found in milk, cheese, butter, eggs and meat and oily fish such as Cod. Vegetable fat is found in margarine (butter substitute made from animal or vegetable fats) and in vegetable oils. Nuts of various kinds are the best natural source of vegetable oil.

(ii) The fat cells of adipose tissue can store up to 95% of their volume of triglycerides (fats), and for this reason adipose tissues are often called the fat depot of body. Triglyc­erides are used as fuel. Fat is used for long term energy storage by animals.

(iii) Since fat oxidation gives about 2¼ times the energy yielded by the same weight of glycogen, so fat is more suitable as stored food. In man, 10-25 per cent of total calorie requirement should be met with fat. Athletes, weight lifters and manual labourers should take more than 40 per cent of their food calorie from fats as it will fulfil their high caloric need.

(iv) The ratio of saturated and unsaturated fats should be low because an excess intake of saturated and unsaturated fats such as ghee, butter and hydrogenated vegetable fats increase cholesterol in the blood. Excess of fats, particularly saturated fats and cholesterol should be avoided by sedentary obese, old persons and patients of heart disease and high blood pressure.

(v) Some polyunsaturated fatty acids (with more than one double bond) are not syn­thesized in the body hence they must be supplied with food to avoid their deficiency. They are called essential fatty acids. The latter are present in many unsaturated vegetable oils like groundnut oil, sunflower oil and safflower oil. Linoleic, linolenic and arachidonic acids are most essential fatty acids (EFA).

(vi) Excess of carbohydrates are converted largely into fats for storage. Lambs and pigs store large amounts of fat in their body if they are kept on starch rich foods such as maize, cereals or gram.

(vii) Fat is useful in transport of the fat-soluble vitamins A, D, E and K.

(viii) It is present in the nerve sheaths and in the secretions of the sebaceous glands in the skin.

(ix) Fat is used in the formation of cholesterol and certain hormones.

(x) Fat provides support to certain organs, for example, the kidneys and the eyes.

(xi) Fats are important constituents of cell organelle.