Human nutrition

Just like all living organisms, humans need food to survive. One important difference between plant nutrition and human nutrition (animal nutrition) is that green plants can manufacture their own food by photosynthesis but animals have can’t manufacture their own food. They have to eat food made by plants.

Both plants and animals use the food as follows:

  • For growth – food provides the materials needed for building new cells and tissues.
  • As a source of energy – Energy is essential for all the chemical reactions that occur in living organisms for their survival.
  • Replacement of worn out tissues – Some components of food are used by the body to replace worn out tissues such as the skin that is
    worn away and the damage caused by wounds.

Balanced diet

A balanced diet is a diet that contains all the required food groups in their correct proportions. The important components of a balanced diet are:

  • carbohydrates
  • fats
  • protein
  • vitamins
  • mineral salts
  • plant fibre
  • water


Energy is obtained chiefly from carbohydrates and fats, and are thus called energy-giving foods. Many countries have carbohydrates as the main part of their staple foods because carbohydrates are cheapest source of energy. Starch and sugar are important carbohydrates in most staple foods. Starch found abundantly in maize, rice and other cereals. Sugar is found naturally in many fruits such as mangoes.

Cellulose is another carbohydrate found in the cell walls of all plant tissue. Though our bodies can’t digest it enough to utilise it as a source of energy, it is important in diet as fibre to help with bowel movement.

Carbohydrates are compounds of carbon, hydrogen and oxygen (eg glucose is C6H12O6).

Fats and proteins also provide energy but they are expensive as a source of energy.

In a balanced diet, the total energy provided by the energy-giving foods must be sufficient:

  • to maintain internal body processes
  • to produce enough energy to maintain the body temperature
  • to meet the energy requirements of our daily work and activities.

However, energy requirements for the body differ from person to person. For example, a person doing hard manual work or partaking sport activities will require more energy than a person who sitting in the office.

If we eat more than the body immediately needs, the surplus is converted to glycogen in the liver or stored as fat under the skin, especially in abdominal areas.

Fats and Oils

Meat, milk, cheese, butter and egg yolk are good sources of animal fats whilst fruits and seeds (such as sunflower seeds) are good sources of plant fats.

Fats are used by the body for respiration (energy source, they contain twice as much energy as carbohydrates) and also to form cell parts such as cell membrane. Fats are also stored in the body as a long-term energy source and also to cushion delicate internal organs.

Fats contain only carbon, hydrogen and oxygen, but in different proportions to carbohydrates.


All plants contain varying quantities of protein, but groundnuts and beans are the best sources. Meat, fish, eggs, milk and cheese are excellent sources of animal protein.

Proteins are important to the body because they are digested into amino acids which are needed for building body tissue during growth and also to replace worn out body tissue.

Excess amino acids cannot be stored, so they are either converted to glycogen or broken down to urea and excreted as urine. Glycogen is stored in the liver.

Besides carbon, hydrogen and oxygen, proteins also contain nitrogen and sulphur.


Vitamins, are a group of organic substances chemically unrelated to each other, though they have certain similarities:

  • They are not digested for energy.
  • They are not built into the body tissue.
  • They are required by the body in small quantities for cellular chemical reactions.

Deficiency of vitamins in the diet leads to a vitamin-deficiency disease.


Calcium is another important dietary requirement. It is used by the body as calcium phosphate to harden bones and teeth. It also helps in blood clotting.

Excellent sources of calcium are milk and cheese.


The red blood cells contains a red pigment called haemoglobin which contains iron. Haemoglobin plays a crucial part in the transportation of oxygen around the body.

Red meat, especially liver and kidney are excellent sources of iron.

Sodium and potassium

Salts of sodium and potassium are need by the body to regulate blood and tissue osmotic pressure.

Dietary fibre (roughage)

The cell walls of vegetables and plant material we eat consist mainly of cellulose which the body cannot digest. This is called fibre or roughage. The roughage add bulk to the waste products of digestion help them to retain water. This softens the faeces and help to prevent constipation. Most vegetables and whole cereal grains are good sources of fibre.


An average of about 70% of body tissue consists of water. The body fluids, blood, lymph and tissue fluid are all composed mainly of water. Water also carries digested food, salts and vitamins in solution as a component of the blood. Excretory products such as urea are removed from the body in solution by the kidneys.

In all body cells, there are a large number of chemical reactions in which water plays an important part as either a reactant or a solvent.

Special dietary needs


Pregnant women

A pregnant woman needs more energy-giving foods, protein, calcium, iron and vitamin D to meet the needs of the baby growing inside her. The baby uses protein for building its tissues, calcium and vitamin D to properly develop the bones, and iron to make haemoglobin, a component of blood.

Lactating women

Lactation is a stage where a woman produces breast milk for feeding the baby. The body of a lactating woman requires large quantities of proteins and minerals since milk is rich in these components. She will have to increase her intake of proteins, vitamins and calcium in order to produce adequate quantities of good quality milk.

Growing children

Children up to the age of about 12 years generally require less food than adults, but the quantity is more in relation to their body weight. Extra protein is needed for making new body tissue as the child grows. They also need more calcium and vitamin D for growth of bones, iron for manufacturing haemoglobin, and vitamin A for disease resistance and good eyesight.