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What is food?

Though food is taken obviously to satisfy hunger and to prevent hunger, the main functions of food in the body are as follows:

1. To supply calories or energy.
2. To provide materials for tissue building and growth.
3. To help in replenishing the worn-out cells and tissues.

{N.B. – The first of the above-mentioned functions is subserved primarily by carbohydrate and fat while the latter two are served by protein}.

What is meant by ‘Energy’ in respect of health and nutrition?

The term ‘Energy’ means work performance and the unit of ‘energy’ is called ‘jule’(J). However, the ‘energy’ received from dietary food is still, referred in terms of ‘calorie’ though calorie is the measuring unit of heat.

Ordinarily, one calorie(designated by small ’c’) is the amount of heat required to raise the temperature of 1 gram of water by one degree (1) centigrade. But, in medical science, by one unit of Calorie (designated by capital ‘C’) 1000 ordinary calories are meant. This Calorie unit now is designated as Kcal. (i.e., Kilo-calorie or thousand ordinary calories).

What are the essential dietary constituents (i.e., ‘nutrients’) indispensable for life ?

There are six such essential dietary constituents :
  • (a) Energy-producing factors viz.
    • (i) Carbohydrate
    • ii) Protien
    • (iii)Fat
  • (b) Non energy-producing factors viz.
    • (i) Vitamins
    • (ii) Minerals
    • (iii) Water
The energy-producing food factors i.e., carbohydrate, protein and fat are called the Macronutrients (previously called ‘proximate principles’) because these are not only essential for the survival and health but also required to be supplied in considerably large amounts. Vitamins and Minerals are now called the Micronutrients (previously called ‘accessory food factors’) because though essential for the human metabolism, these are needed in small quantities. What is the Calorie value of different food factors?

Actual amount of Calories available to the body from the different food factors (‘Macronutrients’) are as follows:

Carbohydrate=4 Kcal. per gram.
Protein = 4 Kcal. per gram.
Fat = 9 Kcal. per gram.

What is meant by balanced diet?
A balance diet should fulfil the following criteria :

1. It should be adequate in Calories (which varies according to the age, sex, nature of work, climatic condition etc.).
2. It should have the right proportion amongst the main food factors (i.e., Carbohydrate : Protein ; Fat).
3.It should contain sufficient amounts of essential vitamins and minerals.

That is, to sum- up, the diet should be adequate in quantity and quality (in respect of the essential food factors).
The guidelines for the ‘balanced diet’ for an active adult person must emphasise on the following key principles :

(i) Carbohydrate food is taken in the polysaccharide form, primarily as cereals (viz, rice or wheat flour ) in bulk quantity. Carbohydrates should provide 50 to 60 percent of the total calorie requirement.
(ii) Protein in the daily diet must be adequate in quantity and quality for the reasons mentioned above. The daily diet should contain 1 gm. per kg. of the ideal weight of the person concerned . And, there should be sufficient proportion of protein of higher biological value (i.e., the ones rich in so-called essential amino acids).
(iii) Fats and oils are primarily required for the cooking of food and make the make the food tasty and appetitising. Considering the rich calorie value (and certain other factors), the intake of fat/oil should be limited to 30 to 35 percent of the total daily calorie requirement of an average adult person.

[ It is customary to refer the daily calorie requirement of an average active adult as 3000 kcal. in general. ]
Natural sources of essential macronutrients
Food as such and the various macronutrients must be suitable for assimilation in the human system.

Assimilable Carbohydrate food is taken in three forms according to the chemical structures. ‘Polysaccharides’{starch} form the staple food (e.g., rice, wheat flour), tuberous root vegetables (e.g., potato, beet etc.). ‘Disaccharides’ are taken as sucrose (the common sugar), lactose (the mik sugar), fructose(as present in fruits, honey). ‘Monosaccharide’ generally used is glucose. All carbohydrate food must be digested to the monosaccharide levels before the assimilation.

Protein rich foods of animal source are milk, egg, meat, fish and of vegetable source are pulses or dal, nuts, soyabean etc. The elementary building-blocks in the protein are called ‘amino acids’ and the digestion of protein breaks them down to the aminoacid level for the assimilation.

Fat / oil of the diet is essentially ‘triglycerides’ according to the chemical structure i.e., a compound made of one molecule of ‘glycerol’ and three molecules of ‘fatty acids’. All edible fat or oil are to be digested liberating the fatty acids for the assimilation. The edible oils are derived from the varions oil seeds. The fat is available naturally in solid or semi-solid form e.g., butter, ghee etc. Mergarine or vanaspati are produced from edible oils by artificial solidification by the process of hydrogenation.

Metabolism and its significance
Metabolism can be defined as intricate chemical processes occurring continuously inside the body.

Metabolism of food starts with the process of ‘digestion’ with subsequent absorption of the nutrients in the intestine and shunting those to the liver via the ‘portal venous system’. Liver is generally considered the ‘metabolic headquarters’ because the nutrients are further processed in the liver cells (hepatocytes) before allowing them to be dispatched for the use of the individual body cells through the general blood circulation. (Liver also deals with the re-processing and de-toxicating of various by-products of general metabolism that reach there via the systemic venous channels).

In general, the process of metabolism involves building up of substances required for the proper functioning of the body cells as well breaking down of some. When the metabolic process results in growth and development of tissues, the effects is known as anabolism, On the other hand, when there occurs significant break-down of tissues of the body, the effects is called catabolism. In an healthy adult, both these processes are evenly balanced resulting in a steady state of normal health. In a growing child, the anabolism is predominant. But, in an ageing person (as well as in debilitating diseases) the catabolic process becomes more prominent resulting in wasting, loss of weight, lethergy etc.

‘Free radicals’ as metabolic by products and their role of harmful effects on body tissues and health :

Certain chemicals are produced as by-products of metabolism that are called the ‘Oxidative free radicals’ because of their chemical structure (with upaired ’electron’) and reactive nature (with powerful ’oxidant’ property).:

These chemicals are extremely harmful for the body tissues and can not be ordinarily eliminated or detoxicated. The accumulation of the so-called ‘free radicals’ in the circulation causes various damages e.g., capillary endothelial cell damage increased tendency to platelet aggregation. And, thereby, produce tendency to augment the ageing process, coronary heart diseases and even cancerous change in tissues etc.

[‘Antioxidant’ vitamins and minerals are therefore gaining increased importance in nutrition and therapeutics inorder to neutralize and inacftivate these harmful chemicals.]
What is meant b y the BMR or Basal Metabolic Rate ?
BMR refers to the energy spent by a person while in a state of absolute physical and mental rest. Hence, the energy supply of person (in the form of daily diet) should be such as to supply the energy requirement for the BMR plus the energy required for the type or nature of his/her work.

Normal BMR in an adult male is 37.5 Kcal per square metre of the body surface. It is slightly less in the female viz. 35.2 Kcal. The BMR is raised in the latter part of pregnancy, fever thyrotoxicosis i.e., hyperthyroidism and certain other conditions.
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