Classification of food nutrients sources and functions.

WEEK: 5 -6


Subject: Food and nutrition

Topic: Basic food nutrients.

Content: Classification of food nutrients sources and functions.



Classification of food nutrients

Food nutrients are chemical substances that are found in the foods. Nutrients are used for the performance of body functions and those that are required in large quantities by the body are called macro nutrients while those that are required by the body in a very little quantity are called micro nutrients. Each nutrient performs a specific function in the body and the lack or shortage of any in our diet results in deficiency diseases.



Food nutrients include the following:

  1. Carbohydrates
  2. Protein
  3. Fats and oil
  4. Vitamins
  5. Minerals
  6. Water


Carbohydrates are organic compounds whose molecules contain carbon, hydrogen and oxygen atoms. Hydrogen and oxygen atoms are always in the ratio 2:1 as in water. Carbohydrates serve a significant function in nature because they are the chief source of immediate energy for animals (including man).

Classification of carbohydrates

Carbohydrates are classified into three: namely

  1. Monosaccharide
  2. Oligosaccharides
  3. Polysaccharides



These are the basic units of carbohydrates which cannot be further broken down except for the release of energy. They are often referred to as simple sugars. They include: glucose, fructose, galactose, mannose, ribose, erythrose, etc. they are the basic building blocks of other carbohydrates. Glucose is the end product of carbohydrate digestion in human and it is the form in which carbohydrate occurs in the blood stream. Glucose is converted to glycogen or stored as fat in human tissues. The chemical formula is glucose is C₆H₁₂O₆.

Fructose: this is the sweetest form of sugar and it is found in fruits.

Galactose: this is the sugar that is present in fresh milk.








  1. Oligosaccharides:

These are carbohydrates which are referred to as double sugars because they are formed by the joining of two monosaccharide molecules together. When two monosaccharide units are joined, the resulting carbohydrate is a disaccharide. When three monosaccharide are joined, they are called trisaccharides and so on. Oligosaccharides include:

Disaccharides:These are maltose, sucrose and lactose.

Maltose: this is made up of two glucose units joined together. It is found in malt and fermented cereal product like corn-drink and fermented corn dough dishes.

Sucrose: this occurs naturally in sugar cane and sugar beet. It consists of one unit of glucose joined to another unit of fructose.

Lactose: it occurs naturally in milk. It consists of one unit of glucose joint to another unit of galactose.


Refines: this is a trisaccharide which consists of three units of monosaccharide joined together. The three combining units are glucose, fructose and galactose. Raffinose is found in molasses (brown sugar) and legumes.


Stachyose: it is a tetrasaccharide made up of four monosaccharide units joined together. It contains two glucose units and one unit each of fructose and galactose all linked together. It is found in beans and is not easily digested.

  1. Polysaccharides:

These are called complex sugars. They are made up of ten or more monosaccharide units joined together. Starch is an example of a polysaccharide which occurs naturally in most foods such as cassava, yams,maize, etc.

Also cellulose is another form of polysaccharide which is the main component of plant cell walls and plant fibers.

Glycogen is the only polysaccharide that occurs in animal tissues; hence it is also called animal starch. They are formed and stored in the liver and muscles. Glycogen is therefore an energy reserve for animals in the same way starch provides for plants.


Functions of carbohydrates

  1. The main function of carbohydrate is to supply energy for the functioning of the human body.
  2. Carbohydrate is stored as glycogen to help the liver and the muscles. The rest is converted to fat and stored as dispose tissue which can lead to overweight when is too much.

iii. Cellulose which is a carbohydrate contains fiber which when added to the diet provides bulk to the diet and to the faeces and bowel movement.

  1. Carbohydrate adds variety to the diet.
  2. Carbohydrate has protein sparing action.

vi.Carbohydrate helps in the complete metabolism of fats and oil in the body.

Sources of carbohydrates.

The food sources of carbohydrates include: maize, rice, millet, cassava, beans,banana,cereals,roots and tubers, etc.




Protein molecules contain nitrogen atoms in addition on carbon, oxygen and hydrogen. Protein often contains sulphur and sometimes phosphorus. The basic unit of protein is amino acid. A minimum of about twenty amino acids have been identified out of which eight (8) are referred to as essential amino acid for adults and two additional  essential amino acids  for infants.

The essential amino acids are lysine, methionine, leucine, phenylalanine, tryptophan, alanine, valine, and threonine and the additional two for infants are histidine and arginine.

The essential amino acids are so called because the body cannot synthesize them hence, they need to be supplied by the diet or in the food we eat.




Amino acids are classification into two; namely: essential amino acids and non- essentials amino acids.

  1. Essential amino acids

These are amino acids that the body cannot make at all or cannot make sufficient quantity to meet the body’s needs. They are also referred to as indispensable amino acids. These are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. These amino acids must be supplied from the food we eat.

  1. Non-essential amino acids

These are the amino acids that the body can synthesize. They are eleven in number and are referred to as referred to as dispensable amino acids. They are alanione,arginine, asparagines, aspartic acid, cysteine, glutamic acid, glutamine, glycerine, praline, serine and tyrosine.



Classification of protein

Proteins are classified according to their amino acid content and their food sources.

Proteins that are rich in all the essential amino acids are referred to as 1st class or complete proteins and are gotten from animal sources. Those that lack one or more essential amino acids are called 2nd class protein or incomplete protein.

Functions of the protein

  1. Proteins promote growth particularly in children
  2. They help in repair of body cells, organs and tissues.
  • In the absence of carbohydrates and fats, they serve as a source of energy.
  1. They are the main dish of a meal and other foods are planned around them.
  2. They are important in the production of hormone antibodies that assist the body.
  3. Proteins act as enzymes and thereby help to catalyze chemical reactions from the body.
  • Protein transport substances such as liquids, vitamins, minerals and oxygen around the body.
  • Protein helps to maintain the acid base balance of the body fluid by acting buffers.

Food sources of protein

The major food sources of protein are meat, poultry, fish, eggs, milk, cheese, termites, legumes, and nuts, melon seeds, etc.


Fats and oils

Fats and oils like carbohydrates are substances made up of the elementscarbon, hydrogen and oxygen. It’s the most concentrated sources of energy that provide the body with heat energy. The term saturated and unsaturated fatty acids refer to the absence or presence of double bonds in the fatty acid chain. In saturated fatty acid, each carbon atom in the hydrogen tail has four single covalent bonds. If it is a single double bond in the fatty acid chain, it is known as monounsaturated fatty acids. If there are two or more double bond, it is known as polyunsaturated fatty acids.



Classification of fatty acids


Fatty acids can be classified into essential fatty acids and non-essential fatty acids. The essential fatty acids are those that the body cannot synthesize and must be present in the diet.

The non –essential fatty acids are those that can be synthesized in the body hence not needed in the diet.


Functions of fats and oils.

  1. Fats and oil supply more heat energy than any other food
  2. They spare protein.
  • They act as cushion to delicate organs e.g. lungs, heart.
  1. They are needed to provide the essential fatty acids which cannot be manufactured in the body. E.g. linoleic acid.
  2. They add flavor to the diet
  3. As they take longer time to digest, thereby more satisfying
  • They serve as lubricants in the intestines
  • They facilitate the absorption of fat soluble vitamins (ADEK).
  1. They promote healthy skin and hair.
  2. They aid good vision due to the presence of vitamin A in fats.

Sources of fats and oil

Fats and oil sources include:

Milk and butter, cashew nuts, groundnuts, olive oil, oil palm produce, coconuts, margarine, fatty portion of meat, oily fish, soya bean oil, bread fruit oil, sun flower oil, etc.






Vitamins are organic substances produced in plants and animals, which must be present in small quantities in our meals or diet. Vitamins cannot be produced by the human body hence; they must be included in our diets.

Classes of vitamins

Vitamins are classified into:

  1. Water soluble vitamins are those that dissolve in watere.g. B complex vitamins and vitamin C (ascorbic acid).
  2. Fat soluble vitamins are those that dissolve in fats, oil. They include vitamins A, D, E, and K.

A summary of vitamins, their functions and sources are shown in the table below.



Vitamins B1 (thiamine) It promotes growth and appetite.

It helps in the release of energy from carbohydrate.

It prevents beri beri (a nervous disease).



Cereal, wheat germ, offal, nuts, leafy vegetables,pork,yeast.
Vitamin B2 (riboflavin) It helps in the release of energy from foods.

Promotes growth.

It is involved in the formation of blood cells

Offal, liver,milk, eggs, legumes and leafy vegetable yeast, cheese, etc
Vitamin  B3 (Niacin Nicotinic acid) It promotes the release of energy from carbohydrate food.

It promotes and growth.

It aids the functioning of nerves and skin

Legumes, milk,eggs,meat, fish, whole cereal, offal,pulses,cheese.
Vitamin B5 (panthotenic acid)


It helps in the metabolism of carbohydrate and protein


Meat, eggs, fish,cereals,nuts, and legumes and fats.



Vitamin B6 (Pyridoxine)

In the metabolism of fat and protein (meat), leafy vegetables, cereals.  

Legumes, yeast, eggs, milk, co-enzyme.

Vitamin B 12 (cyanocobala-min) It is involved in the formation of red blood cells. Liver, kidney, fish,milk and meat.
Ascorbic acid

Vitamin C

It promotes healing of wounds.

It helps in the formation of collagen and connective tissues, prevents scurvy, infection, and prevents cold.


Fruits such as oranges, grape fruits,pawpaw, guava, pineapple, cabbage, black currants, tomatoes, carrot.
Folic acid It helps in the production of red blood cell.


Prevent anaemia.

Eggs, cereals, beef, green leafy vegetables.
Niacin Involved in the metabolism of fats and carbohydrate.

It acts as a co-enzyme.

Liver, kidney, milk, egg yolk, banana.






Vitamin A (Retinols) It helps the eye adjust to dim light better.

Provides resistance to infection.

Helps to keep the lining of the mouth, nose, throat and digestive tract healthy.

Helps in normal growth in children

Cold liver oil, liver, palm oil, carrot, eggs, carrot, yellow vegetables, okra.
Vitamin D (cholecalciferol sunshine vitamin) Helps the body to build strong bones and teeth.

Helps in the absorption of cod liver oil,palm oil, salt calcium, and phosphorus in the body.

Sunlight, milk and milk products, eggs yolk, fish, butter and margarine.
Vitamin E (tocopherols) It acts as an anti-sterility factor.

It aids absorption of iron in the body.

Cereals, groundnut,green vegetables, meat, milk, eggs, liver, margarine
Vitamin K( phyloquinone) It promotes normal clotting of blood. Green leafy vegetable, milk, cabbage, liver,eggs.



Minerals are the inorganic elements required by living organisms. They cannot be destroyed by heat, air, acid or mixing. They perform a variety of functions such as the formation of body structures and the maintenance of health.

Classification of minerals

Minerals are classified into two;

  1. Macro minerals: these are required by the body in relative larger quantity. Examples of macro elements are: calcium, sodium, sulphur, phosphorus, magnesium, potassium, chlorine.
  2. Micro minerals or trace minerals: these are required by the body in trace or little amounts. Examples are: iron,iodine,fluorine, manganese, cobalt, copper


Water is made up of elements hydrogen and oxygen. In its pure form, it is colourless, tasteless and odourless.


Functions of water

  1. It aids in the regulation of normal body temperature.
  2. It maintains blood volume
  • It participates in metabolic reaction
  1. It carries nutrients and waste products throughout the body.
  2. They serve as solvents for minerals, vitamins, amino acids, glucose and many other smaller molecules so that they can participate in metabolic activities.
  3. It acts as a lubricant and cushions around joints and inside the eyes, spinal cord and in pregnancy.,

Sources of water to the body

About 20% of water intake comes in food, while the rest comes from drinking water, and assorted beverages.

  1. Liquid: the largest fluid intake is from liquids. An average person is supposed to consume a total of one and half liters of liquid each day.
  2. Solid foods: the water content of foods varies widely, but most foods contain more than 70%.
  • Metabolism of energy nutrients: water is formed during the metabolism of carbohydrates, proteins and fats.



  1. State all the classes of the food nutrients.
  2. What are food nutrients?
  3. List four functions of carbohydrate?


General objectives

Objective test:

  1. Which of the following is a nutrient?
    1. nitrogen
    2. oxygen
    3. fat
    4. hydrogen
  1. Which of the following nutrients does a growing child needs most?
    1. carbohydrate
    2. .vitamins
    3. proteins
    4. fats and oils
  2. Carbohydrates are made up of the following …………………………………..
  3. The amino acids the body produce are called …………………
  4. The mineral element involved in the blood is ………..
    1. vitamin A
    2. vitamin B
    3. Vitamin K
    4. vitamin E
  5. Rancidity is a type of spoilage associated with
    1. frozen beef
    2. butter
    3. frozen chicken
    4. milk

Weekend assignment:

Read Evans food and nutrition for senior secondary school 1 by F.A. Bakare et el; (pages15-40)

Pre- reading assignment:

Read about the functions, sources of mineral element and scientific study of foods.

Reference texts:

  1. Evans food and nutrition for SSS book 1 by F.A et al; Evans brothers Nigeria limited.
  2. Exam focus foods and nutrition for WASSCE and SSCE by J.O Olusanya et al. University press.