Food, nutrition and digestion: nutrients and a balanced diet

Learning objectives

By the end of this lesson, you should be able to:

• distinguish food, nutrients, nutrition and digestion
• identify the major classes of food and their functions
• describe the components of a balanced diet
• explain malnutrition and common deficiency diseases
• apply nutrition concepts to practical situations in Sierra Leone

Food, nutrients and nutrition

Food is any substance eaten or drunk that supplies materials and energy needed by the body. Nutrients are the useful chemical substances in food. Nutrition is the process by which organisms obtain and use food. Digestion is the breakdown of large, insoluble food molecules into smaller, soluble molecules that can be absorbed.

The main classes of nutrients are carbohydrates, proteins, fats and oils, vitamins, mineral salts and water. Roughage, also called dietary fibre, is not digested by human enzymes but is important for normal movement of food through the gut.

Carbohydrates

Carbohydrates are a major energy source. Starch is a storage carbohydrate in plants and is found in foods such as rice, cassava, yam, maize and bread. Sugars, including glucose, are simpler carbohydrates. During respiration, glucose is broken down in cells to release usable energy.

A positive iodine test for starch changes the iodine solution from yellow-brown to blue-black. This test identifies starch, not all carbohydrates. Benedict's solution is used to test for reducing sugars: after heating with the sample, a colour change from blue through green, yellow and orange to brick-red indicates increasing amounts of reducing sugar.

Proteins

Proteins are needed for growth and repair of tissues and for making enzymes, some hormones and antibodies. They are built from amino acids. Sources include fish, eggs, milk, meat, beans, groundnuts and other legumes. A prolonged shortage of protein, especially in children, can cause kwashiorkor. Signs may include oedema (swelling), poor growth, muscle wasting and changes to hair or skin. Protein-energy malnutrition can also occur when both energy and protein intake are inadequate.

Fats and oils

Fats provide concentrated energy, store energy, insulate the body and protect delicate organs. They also help the body absorb vitamins A, D, E and K. Sources include palm oil, groundnuts, seeds, avocado and animal fats. Excessive intake of energy-dense foods over time can contribute to overweight and increased risk of some non-communicable diseases. Fat should not be eliminated from the diet because it has essential functions.

Vitamins, minerals and water

Vitamins are required in small amounts to regulate body processes. Vitamin A supports normal vision and healthy epithelial tissues; deficiency can cause night blindness. Vitamin C is needed for healthy connective tissue and wound healing; severe deficiency causes scurvy. Vitamin D helps the body use calcium and phosphate for strong bones; deficiency in children can cause rickets.

Minerals also have specific roles. Iron is needed to make haemoglobin; iron deficiency can cause anaemia, tiredness and reduced ability to concentrate. Calcium is important for bones, teeth, muscle contraction and blood clotting. Iodine is required for thyroid hormone production; deficiency can cause goitre.

Water acts as a solvent, transports substances, supports chemical reactions and helps regulate body temperature. The body loses water in urine, sweat, faeces and exhaled air, so losses must be replaced.

A balanced diet

A balanced diet supplies all essential nutrients in suitable proportions for a person's age, sex, activity level and health. It should provide sufficient energy without excessive energy intake. A practical meal can combine a starchy staple such as rice, cassava or potato with vegetables and a protein source such as fish, beans or egg. Fruit and safe drinking water should also be included. No single food provides every nutrient in the required amounts.

Dietary needs vary. A growing child needs materials for growth; a physically active person generally needs more energy than a sedentary person; pregnancy increases the need for several nutrients, including iron and folate. Food availability, income, illness and cultural practices can affect diet quality.

Malnutrition

Malnutrition includes undernutrition, nutrient deficiencies and overnutrition. It can result from inadequate food intake, an unbalanced diet, disease that interferes with absorption, or increased nutrient needs. A person may consume enough calories but still lack vitamins or minerals. This is why quantity alone does not guarantee a healthy diet.

WASSCE examination focus

When asked to state the functions of nutrients, link each nutrient to a specific role and give a food source. Do not confuse a food with a nutrient: rice is a food, while starch is a nutrient present in rice. In deficiency questions, name the nutrient, the disease or condition, and a characteristic effect.

Quick check

1. State two differences between digestion and nutrition.
2. Name the nutrient needed to make haemoglobin and one deficiency effect.
3. Why is dietary fibre useful even though human digestive enzymes do not digest it?
4. Give a balanced meal using foods commonly available in Sierra Leone.

Answers: 1. Nutrition is the overall process of obtaining and using food; digestion is the breakdown of large food molecules into small soluble molecules. 2. Iron; deficiency can cause anaemia and tiredness. 3. It adds bulk to intestinal contents and helps normal peristalsis and bowel movement. 4. For example, rice with fish and leafy vegetables, plus fruit and safe water; other combinations can also be balanced.