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Components Of Food


The main components of our food are nutrients, roughage and water.

 are the elements present in food which provide energy to the body and enable it to grow, repair and maintain. Nutrition is a multistep process involving ingestion, digestion, absorption, assimilation and egestion. Food can be classified according to their function as follows:

Energy-rich foods:
 These foods, when consumed, are broken down to release energy,

e.g. carbohydrates (rice, wheat, potato and sugar) and fats (oil and ghee).

Body-building foods:
 These foods provide nutrients to build mass and repair tissues,

e.g. proteins (milk, meat, egg white and pulses).

Protective foods:
 These foods are vital for body functions and the absence of these nutrients result in deficiency diseases. Some of the examples of protective foods are vitamins and minerals.

The energy-rich and body-building foods are called essential food and the protective food is called welfare food.


  • Calorie (C): The amount of energy obtained from food is called food energy. This food energy is expressed in calories or joules. 1 food calorie = 1 kilocalorie (kcal) or 1000 calories (= 4.18 kJ).
    If a particular food is said to contain so much calories, it means how much energy our body could get by consuming it. Most food types have at least some calories, but some foods have high calories and some have low calories. Calorie requirement varies with respect to the person’s age, physiological condition, gender, activity level etc. However, the minimum calorie intake should be 1200 calories per day for women and 1800 calories per day for men. If a person eats more calories than the body needs, the leftover calories are converted into fat, and too much fat can lead to health hazards.
    Calorie (or food calorie or kilocalorie) is equal to the amount of energy required to raise the temperature of 1 kg of water by 1°C at 1 atmospheric pressure.
    BMR (basal metabolic rate): It is the number of calories that the body burns at rest. This energy is used for the functioning of vital organs of the body such as the heart, kidney, lungs, and brain. BMR helps to know the number of calories required for gaining, loosing or maintaining the weight. The various factors that affect BMR include genetics, age, sex, body composition, stress, external temperature and malnutrition.
    BMI (body mass index): Body mass index helps to determine the health risk of weight.
    The value of BMI with 25–29.9 indicates overweight and a value of 30 and above indicates obesity that leads to severe health hazards.


Let us now discuss the various components of food in greater detail.


Carbohydrates are the dominant component of our food. Carbohydrates are the compounds of carbon, hydrogen and oxygen. They are oxidised in the cells to release energy.

Carbohydrates include sugars and starch.


Sugars are soluble in cold water and taste sweet. Broadly, sugars in our foods are divided into the following major categories:
  • Monosaccharides or simple sugars with a general chemical formula C6H12O6. These need no digestion and are absorbed into the body. Three types of such simple sugars are glucose, fructose and galactose.
  • Disaccharides or double sugars have a general chemical formula C12H22O11. These include sucrose, maltose and lactose.
  • Oligosaccharides (in Greek Oligos = few, saccharon = sugar) contain small numbers (3–10) of simple sugars (monosaccharides). Examples include raffinose (galactose, glucose and fructose) and stachyose (galactose, glucose and fructose).
  • Polysaccharides (in Greek Poly = many, saccharon = sugar) contain repeating sugar units. Some of them show extensive branching. The number of sugar units ranges between 200 and several thousands. Examples include starch, glycogen and cellulose.
  • Starch is an insoluble carbohydrate. It is in this form that plants commonly store carbohydrates. Potatoes, grains and bread are chief sources of starch. Starch has a chemical formula (C6H10O5)n and is called a polysaccharide.

Two other insoluble polysaccharide carbohydrates are cellulose and glycogen.

  • Cellulose forms the cell wall in plants. The food that we eat consists of cellulose. Cellulose is not digested in our body as we have no cellulose digesting enzyme. The undigested cellulose, being fibrous in nature, acts as roughage.
    Being fibrous, cellulose absorbs a lot of water and retains it, thus keeping the faecal matter soft and prevents constipation by stimulating muscle contraction in the intestinal wall, making the movement of faecal matter easy.
  • Glycogen is the form in which carbohydrates are stored in animals.
  • One gram of carbohydrate gives 4.2 kcal of energy. We meet around 60% of our energy requirement from carbohydrates.


Proteins are large chemical molecules containing carbon, hydrogen, oxygen and nitrogen. Nitrogen is the most essential element in proteins. Some proteins may also contain sulphur and phosphorus. Proteins are formed from chemical units called amino acids.

Amino acids are of two types, namely essential and non-essential amino acids.

  1. Essential amino acids are those which cannot be synthesised by the animal body and must be supplied through food in adequate amounts.
  2. Non-essential amino acids are synthesised in the animal body.

Proteins in our food are digested and broken down into constituent amino acids. These amino acids are absorbed in the small intestine and transported by blood to different parts of the body. They are then re-joined in specific sequences to form different proteins in the cells. Hair, muscles, enzymes, haemoglobin, antibodies and hormones are all proteins synthesised by the cells. Proteins can also be a source of energy. Amino acids can be processed to yield carbohydrates, and the latter can provide energy. But the body uses proteins mainly for other important activities such as building and repairing of tissues.

One gram of protein
gives 4.2 kcal of energy. We need 1 g of protein for every kilogram of body weight per day. We meet around 10% of our energy requirement from proteins.

Protein–energy Deficiency
A diet lacking in nutrients, such as proteins, required for the growth and repair of the body results in degeneration of muscles and body weight.

Two common forms of protein–energy malnutrition include kwashiorkor and marasmus.

  1. Kwashiorkor: Symptoms of this disease include dry and cracked skin, pot belly, thin and curved legs (matchstick legs), bulging eyes and reddish hair (due to hair losing its lustre) and swelling in the body due to the accumulation of water in the tissues. Children become restless and irritable.
    Control and prevention: A protein-rich diet cures as well as prevents kwashiorkor. The patient should be given a combination of protein-rich food items such as milk, meat, eggs, groundnuts, gram, soya bean, jaggery etc.
  2. Marasmus: Marasmus affects children aged about a year old. It is caused by the deficiency of proteins and energy-giving nutrients in the diet. This causes the body to use stored fats and body proteins as sources of energy. The child loses weight and muscles start shedding. If not cured, this disease can cause mental retardation and stunted growth. The symptoms of this disease include retarded physical and mental growth, gradual shedding of subcutaneous fat and muscles; dry, loose and wrinkled skin, thin arms and muscles, clearly visible ribs and diarrhoea.
    Control and prevention: Marasmus can be cured or prevented by a diet rich in proteins, carbohydrates and fats along with vitamins and minerals.

Fats Fats are members of a group of organic compounds called lipids. Fats are compounds of carbon, oxygen and hydrogen made of fatty acids and glycerol. The digestion of fats and oils yield fatty acids and glycerol. These are transported to various parts of the body, where they recombine to form fats and are deposited. Whenever energy is required to be obtained from fats, both glycerol and fatty acids are utilised. One gram of fat gives about 9.3 kcal of energy. We meet 30% of our energy requirement from fats and oils.

 is a type of lipid produced by the liver and also found in certain foods. Liver produces about 1000 mg of cholesterol a day. It is needed to make vitamin D and some hormones.

Functions of fats

  • Fats provide energy by acting as a reserve food material.
  • Excess of fat gets accumulated underneath the skin. Because of their location beneath the skin, they are called subcutaneous fat. This acts as an insulator.
  • Fats are used to build membranes of cells and organelles such as those of the Golgi complex and mitochondria.
  • Fats around organs such as kidneys, ovaries and eyes protect them.
  • In the blood, they dissolve vitamins A, D, E and K and transport them from the intestine to different parts of the body.
  • They serve as a solvent for fat-soluble vitamins.



Vitamins are chemical substances which help to maintain a healthy body. Most vitamins act as catalysts or enzymes in essential chemical changes in the body but each vitamin has also some special

function in our body. The name ‘vitamin’ was suggested by Dr. Casmir Funk. Vitamins are contained in foods naturally but a couple of them are also synthesised in our body. Vitamins are neither chemically similar nor functionally related. Vitamins can be divided into two main groups, namely fat-soluble and water-soluble. Vitamins A, D, E and K are fat-soluble.
Vitamin C and the B group of vitamins are water-soluble. A number of water-soluble vitamins are traditionally grouped as the B group of vitamins because of their similar properties and functions in the body. They constitute vitamin B complex. Vitamin B complex includes vitamins B1, B2, B3, B6, B12, pantothenic acid, folic acid and biotin.

Vitamins are compounds of various chemical natures which are easily destroyed on storage (especially in daylight) or prolonged boiling. Table shows the types of vitamins and their properties.




Dietary Sources


Deficiency Disorders


Retinol, retinal, four carotenoids (including beta carotene)

Liver, orange, carrots, muskmelons, dark green leafy vegetables such as spinach


Immune function


Healthy skin

Night blindness




B1 (thiamine)

Yeast, pork, liver, eggs, cereal grains (whole) oat meal, flax, sunflower seeds, brown rice, whole green rye, asparagus, cauliflower, potatoes, oranges

Helps in converting blood sugar into

Essential for nervous system, cardiovascular system and muscle function


Korsakoff syndrome

B2 (riboflavin)

Milk, cheese, liver, kidney, leafy green vegetables, legumes, tomatoes, yeast, mushrooms, almonds

Energy metabolism

Metabolism of fats, ketone bodies, carbohydrates and proteins

Ariboflavinosis (it is characterised by sore throat with redness, swelling of mouth and throat mucosa, inflammatory lesions at corners of mouth, cracking of lips and mouth corners, moist scaly skin, decreased RBCs and haemoglobin content)

B3 (niacin)

Liver, chicken, beef, fish, cereals, peanut, legumes, mushrooms

Essential for energy release from carbohydrates, fats and proteins

Helps in DNA synthesis

Necessary for healthy skin, nerves, digestive system



(pantothenic acid)

Meat, cold-water fish ovaries, royal jelly, whole grains, vegetables such as broccoli and avocados

Assists in metabolism of carbohydrates, proteins and fats

Essential for cell metabolism

Required for cholesterol, metabolism, hormone production and haemoglobin

Paraesthesia (it is characterised by sensation of tingling, pricking or numbness of skin)



Meat, whole grain products, vegetables, nuts, bananas

Macronutrient metabolism

Neurotransmitter synthesis

Haemoglobin synthesis and function

Gene expression


Peripheral neuropathy

B7 (biotin)

Egg yolk, liver, kidney

Helps in converting food to energy

Helps to maintain healthy hair, skin and nail

Assists in cell growth



B9 (folic acid)

Leafy vegetables such as spinach and asparagus, legumes such as dried, fresh beans, peas and lentils, liver and liver products, baker’s yeast, sunflower seeds.

Synthesises and repairs DNA

Required during rapid cell growth and division

Produces healthy RBC

Anaemia, skin pigmentation, fatigue, mental depression Deficiency during pregnancy is linked with birth defects

B12 (cobalamin)

Lean meat, eggs, dairy products like milk, cheese etc.

Maintaining a healthy nervous system

Development of RBC’s

Affects DNA synthesis and fatty acid synthesis

Energy production

Megaloblastic anaemia

(ascorbic acid)

Citrus fruits such as oranges, muskmelon, strawberries, tomatoes, broccoli, cabbage, kiwi fruit, sweet red pepper

Keeps body tissues such as gums and muscles in good shape

Helps to heal wound.

Resists infection



(ergocalciferol) D3 

Liver, egg yolk, fish, fortified cereal, milk fortified with vitamin D

Regulates body levels of calcium and phosphorus

Supports healthy immune system

Rickets, Osteomalacia


Whole grains such as wheat and oats, wheat germ, green leafy vegetables such as spinach, nuts and seeds

Maintains body tissues in eyes, skin and liver

Protects lungs from damage by pollutants

Formation of RBC’s

Haemolytic anaemia in new born




Green leafy vegetables, dairy products such as milk and yogurt, liver

Helps in blood clotting.

Protects bone from fracture

Prevents calcification of arteries

Bleeding disorders



Many metals and non-metals are required for various reactions taking place in our body. These are collectively called minerals. Just seven minerals comprise 60–80% of all the minerals needed by our body. These are calcium, magnesium, sodium, potassium, phosphorus, sulphur and chlorine. 

These are required in amounts exceeding 10 grams per day. They are called macro-elements. The remaining minerals include iron, iodine, copper, zinc, manganese, cobalt, molybdenum, selenium, chromium and fluorine. These are microelements or trace elements required in almost negligible quantities. Minerals are needed in the diet in small quantities and are not destroyed by cooking. Table 7.2 shows the types of minerals and their properties.




Dietary Sources



Calcium (Ca)

1000 mg

Dairy products—milk, cheese, yogurt. Canned fish with bones. Green leafy vegetables like broccoli.

• Constituent of bone and teeth.

• Required for nerve function, muscle contraction and blood clotting.

Osteomalacia, osteoporosis, rickets, tetany.

Magnesium (Mg)

420 mg

Eggs, milk and dairy products, fish (shell fish) nuts, legumes, whole grains, green vegetables

• Constituent of muscles, soft tissues and bones.

• Functions in many enzyme processes.

Muscle weakness, abnormal heart rhythm tiredness, cramps, fits.

Phosphorus (P)

700 mg

Meat, poultry, fish, dairy products, cereal products, green leafy vegetables

• Essential for bone formation and maintenance.

• Energy metabolism, nerve function and acid balance.

Anaemia, demineralisation of bones, nerve disorders, respiratory problems, weight loss.

Sodium (Na)

1500 mg

Table salt, milk products, eggs, sea foods.

• Regulate body fluid volume.

• Acid base balance.

• Nerve and muscle activity.

Low blood pressure, weakness, fatigue, thirst, cramps.

Potassium (K)

4700 mg

Legumes, potatoes, tomatoes, bananas

• Essential for nerve function, muscle contraction, maintenance of normal blood pressure.

Muscle paralysis, heart problems

Chloride (Cl)

2300 mg

Table salt, sea foods, eggs, milk products

• Regulates body fluid volume.

• Acid base balance.

Respiratory disorders, decreased blood acidity.

Sulphur (S)


(as sulphur is found in wide variety of foods, there is no RDA for sulphur)

Cheese, eggs, fish, cauliflower, nuts, onions, broccoli, cucumber, wheat germ, corn

• Healthy skin and nails

• Detoxification.

Skin disorders, muscle pain, nerve disorders, arthritis, circulatory trouble, inflammation, wrinkles

Iron (Fe)

8 mg

Red meat, liver, eggs, legumes, green leafy vegetables, broccoli, whole grains

• It is important for formation of Hb of RBCs that carries oxygen throughout the body.

Anaemia, susceptible to infections.

Iodine (I)

150 mg

Sea foods, iodised salt and food products with iodised salt

• Essential for production of thyroid hormones.





Water is indispensable. It is the most abundant substance in the body. It constitutes 65% by weight of protoplasm and 92% by weight of blood plasma. The water we drink meets most of the water requirement of the body. Some comes from other beverages, some from food and some are formed in the body during respiration. Some of the functions of water in the body are as follows:

  • It is the best solvent and provides medium in which most reactions in the body take place.
  • Water regulates the temperature of the body by sweating and evaporation.

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