Cambridge IGCSE0970

Diet

Biology 0970 Chapter Notes

What this chapter covers

DietDigestive systemPhysical digestionChemical digestionAbsorption
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In this chapter

  1. 1The Seven Components of a Balanced Diet
  2. 2Macronutrients: Fuel and Building Blocks
  3. 3Micronutrients: Vitamins and Minerals
  4. 4Fibre and Water: The Unsung Heroes
  5. 5Energy Balance and Malnutrition
  6. 6Health Risks of a Poor Diet

1. The Seven Components of a Balanced Diet

A balanced diet is essential for health. It means eating a wide variety of foods in the right proportions to supply the body with all the nutrients it needs to function correctly. Organisms require food for three main reasons: to provide energy for life processes (like movement and keeping warm), to provide raw materials for growth and to repair tissues, and to provide substances to keep the body healthy and fight off diseases. A balanced diet contains seven key components: carbohydrates, proteins, fats (lipids), vitamins, minerals, dietary fibre, and water.

Key term

Balanced Diet: A diet that provides all the essential nutrients in the correct amounts and proportions to maintain good health and carry out life processes.

Examiner insight

Examiners expect students to name all seven components of a balanced diet and understand that 'correct proportions' is the key part of the definition, not just the presence of the nutrients.

Worked example 15 marks

State three reasons why humans need to eat food, and list the seven types of nutrients required for a balanced diet.

  1. 1
    1. State the first reason for needing food: To provide energy for metabolic processes, movement, and warmth.
  2. 2
    1. State the second reason: To provide raw materials (like amino acids) for building new cells for growth and repairing damaged tissues.
  3. 3
    1. State the third reason: To provide essential substances like vitamins and minerals that are needed for chemical reactions and to maintain health.
  4. 4
    1. List the seven components of a balanced diet: Carbohydrates, Proteins, Fats (Lipids), Vitamins, Minerals, Dietary Fibre, Water.

Recap

  • A balanced diet provides all necessary nutrients in the correct proportions.
  • Food provides energy, materials for growth and repair, and substances to maintain health.
  • The seven components of a balanced diet are carbohydrates, proteins, fats, vitamins, minerals, fibre, and water.
  • Eating a variety of foods is the best way to achieve a balanced diet.

Quick check

  1. List the seven components of a balanced diet.2 marks

2. Macronutrients: Fuel and Building Blocks

Macronutrients are the nutrients the body needs in large amounts. They are carbohydrates, proteins, and fats.

  • Carbohydrates: These are the body's main source of energy. They are broken down into glucose, which is used in respiration to release energy. Sources include pasta, bread, rice, and potatoes.
  • Proteins: These are essential for growth and repairing damaged tissues. They are made of amino acids. The body uses them to make new cells, enzymes, antibodies, and hormones. Sources include meat, fish, eggs, and beans.
  • Fats (Lipids): These are a rich source of energy and provide more than twice the energy per gram compared to carbohydrates. They also act as an energy store, provide insulation, and protect organs. Sources include butter, oil, and nuts.

Key term

Macronutrient: A type of food, such as carbohydrate, fat, or protein, that is required in large amounts in the diet.

Common pitfall

Confusing the primary role of proteins (growth and repair) with their secondary role as an energy source, which is only used when carbohydrate and fat stores are low.

Fun fact

The human brain uses about 20% of the body's total energy, primarily from carbohydrates, even though it only makes up about 2% of your body weight.

Worked example 13 marks

A bodybuilder is training to increase muscle mass. Explain why they should ensure their diet is rich in protein.

  1. 1
    1. Identify the function of protein: Protein is the key nutrient for growth and the repair of tissues.
  2. 2
    1. Relate this to the context: Bodybuilding involves intense exercise which causes microscopic tears in muscle fibres.
  3. 3
    1. Explain the role of protein in this process: The body uses amino acids from digested protein to repair these tears and build new muscle tissue, leading to an increase in muscle mass.
  4. 4
    1. Conclude: Therefore, a protein-rich diet is essential to provide the necessary building blocks for muscle growth.

Worked example 23 marks

Compare the roles of carbohydrates and fats as energy sources.

  1. 1
    1. State the role of carbohydrates: Carbohydrates are the primary and most readily available source of energy for the body's cells.
  2. 2
    1. State the role of fats: Fats are a more concentrated energy source, providing more than double the energy per gram. They are used for long-term energy storage.
  3. 3
    1. State a key difference: Carbohydrates provide quick-release energy, while fats provide slow-release energy and are stored for later use.

Recap

  • Carbohydrates are the main source of immediate energy.
  • Proteins are essential for growth, repair, and making enzymes.
  • Fats provide a long-term energy store and insulation.
  • Macronutrients are needed in large quantities in the diet.

Quick check

  1. What is the main function of protein in the body?1 mark

3. Micronutrients: Vitamins and Minerals

Micronutrients are substances required in very small amounts to maintain health. They are not used for energy but are vital for the body's chemical reactions.

  • Vitamins: Organic compounds essential for health. For example, Vitamin C (found in citrus fruits) is needed to make healthy skin and gums; a lack of it causes scurvy. Vitamin D (made by the skin in sunlight, also in oily fish) is needed to help the body absorb calcium for strong bones and teeth; a lack of it causes rickets.
  • Minerals: Inorganic elements needed for various functions. For example, Iron (found in red meat and spinach) is needed to make haemoglobin in red blood cells to carry oxygen; a lack of it causes anaemia. Calcium (found in dairy products) is needed for strong bones and teeth.

Key term

Deficiency Disease: A disease caused by the lack of a specific essential nutrient in the diet.

Examiner insight

Marks are often awarded for correctly linking a specific vitamin or mineral to its function and the corresponding deficiency disease. Be precise.

Worked example 13 marks

A child living in a region with little sunlight and a diet low in oily fish develops bones that are soft and deformed. Name the condition and the nutrient deficiency that caused it.

  1. 1
    1. Identify the symptoms: Soft and deformed bones are the classic signs of a specific condition.
  2. 2
    1. Name the condition: The condition is called rickets.
  3. 3
    1. Identify the cause: Rickets is caused by a deficiency of Vitamin D.
  4. 4
    1. Explain the link: Vitamin D is needed for the absorption of calcium, which is essential for hardening bones. Sunlight is a primary source, so a lack of it can cause deficiency.

Worked example 23 marks

A person feels constantly tired and looks pale. A blood test reveals they have anaemia. Which mineral is likely deficient in their diet, and why does this cause tiredness?

  1. 1
    1. Name the deficient mineral: Anaemia is caused by a deficiency of iron.
  2. 2
    1. Explain the role of iron: Iron is a key component of haemoglobin, the protein in red blood cells that binds to and transports oxygen.
  3. 3
    1. Link the deficiency to the symptom: A lack of iron means less haemoglobin is made, so the blood's oxygen-carrying capacity is reduced. Tissues and muscles receive less oxygen for respiration, leading to fatigue and tiredness.

Recap

  • Vitamins and minerals are needed in small amounts to maintain health.
  • A lack of Vitamin C causes scurvy.
  • A lack of Vitamin D can lead to rickets.
  • Iron is needed to make haemoglobin; a deficiency causes anaemia.
  • Calcium is essential for strong bones and teeth.

Quick check

  1. Name the disease caused by a lack of Vitamin C.1 mark

4. Fibre and Water: The Unsung Heroes

Although they provide no energy, fibre and water are crucial for a healthy diet.

  • Dietary Fibre (Roughage): This is the indigestible part of plant-based foods, mainly cellulose from plant cell walls. It is not absorbed by the body. Its main role is to add bulk to the faeces, making them softer and easier to pass. This stimulates the muscles of the intestine wall to contract (peristalsis), pushing food along the gut and preventing constipation. Good sources include wholemeal bread, vegetables, and fruit.
  • Water: Water makes up about 70% of the human body. It is vital for life. It acts as a solvent for chemical reactions, helps transport nutrients and waste products in the blood, and is essential for regulating body temperature through sweating.

Key term

Dietary Fibre: Indigestible plant material, mainly cellulose, that adds bulk to the faeces and stimulates peristalsis.

Common pitfall

Stating that fibre is digested or provides energy for humans. Its key feature is that it is indigestible.

Worked example 13 marks

Explain how eating a diet rich in dietary fibre can help prevent constipation.

  1. 1
    1. Define the role of fibre: Dietary fibre is indigestible material that passes through the digestive system.
  2. 2
    1. Explain its effect on faeces: It absorbs water and adds bulk (mass) to the faeces.
  3. 3
    1. Explain its effect on the intestine: The increased bulk stimulates the muscular walls of the large intestine to contract more strongly and regularly.
  4. 4
    1. Link to the outcome: This process, called peristalsis, moves the faeces along the gut more efficiently, preventing them from becoming hard and difficult to pass, thus preventing constipation.

Recap

  • Dietary fibre is indigestible and adds bulk to faeces.
  • Fibre stimulates peristalsis and helps prevent constipation.
  • Good sources of fibre include whole grains, fruits, and vegetables.
  • Water is essential as a solvent, for transport, and for temperature regulation.
  • Dehydration can impair physical and mental performance.

Quick check

  1. State two reasons why water is essential in the diet.2 marks

5. Energy Balance and Malnutrition

The energy we get from food should be balanced with the energy our body uses. Energy requirements vary depending on factors like age (growing children need more), gender (males generally need more), body size, and activity level (a manual labourer needs more than an office worker). When energy intake equals energy expenditure, body mass remains stable. Malnutrition is a broad term for ill-health caused by a poor diet. It can mean:

  • Undernutrition: Not getting enough nutrients. This can lead to conditions like Kwashiorkor, caused by severe protein deficiency, resulting in a swollen abdomen and stunted growth.
  • Overnutrition: Consuming too many calories, especially from fats and sugars. This leads to the excess energy being stored as fat, causing obesity, which increases the risk of many diseases like type 2 diabetes and heart disease.
  • Imbalanced Diet: Getting the wrong proportions of nutrients, even if the total energy is sufficient.

Energy released from food (J) = Mass of water (g) × 4.2 J/g°C × Temperature change (°C)

Key term

Malnutrition: Ill-health caused by a diet that is unbalanced, insufficient, or excessive.

Examiner insight

Students should be able to clearly explain why energy requirements differ between individuals, citing at least two distinct and well-explained factors like activity level and age.

Worked example 13 marks

Explain why a 45-year-old male construction worker has a higher daily energy requirement than a 45-year-old male office worker.

  1. 1
    1. State the key variable: The primary difference between the two individuals is their activity level or occupation.
  2. 2
    1. Describe the construction worker's activity: A construction worker performs physically demanding manual labour, which requires significant muscle contraction and movement.
  3. 3
    1. Describe the office worker's activity: An office worker is largely sedentary, using far less energy for physical activity.
  4. 4
    1. Link activity to energy demand: Since the construction worker uses more energy for muscle activity throughout the day, their total daily energy expenditure is higher, and therefore they require a greater energy intake from food to maintain their body mass and health.

Worked example 22 marks

Describe the cause and a visible symptom of kwashiorkor.

  1. 1
    1. State the cause: Kwashiorkor is a form of severe malnutrition caused by a deficiency of protein in the diet, even if calorie intake is adequate.
  2. 2
    1. Describe a visible symptom: A key symptom is oedema, which is swelling caused by fluid collecting in tissues, particularly visible as a swollen, distended abdomen.

Recap

  • Energy needs vary with age, gender, and activity level.
  • A stable body weight requires energy intake to balance energy expenditure.
  • Malnutrition refers to any form of poor nutrition, including undernutrition and overnutrition.
  • Obesity is caused by consistently consuming more energy than is used.
  • Kwashiorkor is a protein-deficiency disease.

Quick check

  1. List two factors that affect a person's daily energy requirement.2 marks

6. Health Risks of a Poor Diet

An unbalanced diet is a major risk factor for several non-communicable diseases.

  • Coronary Heart Disease (CHD): A diet high in saturated fats and cholesterol can lead to fatty deposits (plaques) building up inside the coronary arteries. This process, called atherosclerosis, narrows the arteries, restricting blood flow to the heart muscle. A blockage can cause a heart attack.
  • High Blood Pressure (Hypertension): Consuming too much salt (sodium) causes the body to retain water, increasing the volume of blood. This raises blood pressure, putting a strain on the heart and arteries, and increasing the risk of strokes and heart disease.
  • Tooth Decay: Sugary foods and drinks are broken down by bacteria in the mouth, producing acid. This acid dissolves the tooth enamel, leading to cavities (dental caries).
  • Type 2 Diabetes: Obesity, often resulting from a diet high in fat and sugar, is the biggest risk factor for developing type 2 diabetes. In this condition, the body either doesn't produce enough insulin or its cells stop responding to insulin.

Key term

Coronary Heart Disease (CHD): A condition where the coronary arteries that supply blood to the heart muscle become narrowed by a build-up of fatty materials.

Common pitfall

Vaguely stating 'too much fat' causes heart disease. Examiners look for the specific mention of 'saturated fat' and its role in forming plaques in arteries.

Worked example 14 marks

Match the dietary problem with the health condition it is most likely to cause. Dietary Problem:

  1. Too much saturated fat
  2. Too little dietary fibre
  3. Too much salt
  4. Too much sugar

Health Condition: A. High blood pressure B. Constipation C. Tooth decay D. Coronary heart disease

  1. 1
    1. Too much saturated fat is linked to the build-up of cholesterol in arteries, causing Coronary heart disease. Match: 1-D.
  2. 2
    1. Too little dietary fibre leads to difficulty passing faeces. Match: 2-B.
  3. 3
    1. Too much salt increases blood volume and pressure. Match: 3-A.
  4. 4
    1. Too much sugar is converted to acid by mouth bacteria. Match: 4-C.

Recap

  • Excess saturated fat in the diet is a major risk factor for coronary heart disease.
  • A diet high in salt can lead to high blood pressure.
  • Frequent consumption of sugary foods and drinks causes tooth decay.
  • Obesity resulting from a poor diet increases the risk of type 2 diabetes.

Quick check

  1. What health problem is associated with a diet high in saturated fat?1 mark

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. Define the term 'balanced diet'.2 marks
  2. Explain the difference in the main dietary roles of carbohydrates and proteins.4 marks
  3. A diet is found to be severely lacking in Vitamin D and Calcium. Describe two potential health problems that could arise from these deficiencies.4 marks
  4. Explain why a professional long-distance runner and a person with a sedentary office job have very different daily energy requirements.3 marks
  5. Describe the role of dietary fibre in the digestive system and explain one consequence of a diet that is low in fibre.4 marks
  6. What is malnutrition? Describe two different examples of malnutrition, stating the specific dietary cause for each.5 marks
  7. Explain how a diet that is consistently high in salt and saturated fat can lead to long-term health problems.4 marks
  8. In a food energy experiment, burning a 2g sample of peanut heated 50 cm³ of water from 20°C to 60°C. Calculate the energy content of the peanut in joules per gram (J/g). Assume the specific heat capacity of water is 4.2 J/g/°C and the density of water is 1 g/cm³.4 marks
  9. A doctor advises a teenager to reduce their intake of sugary drinks. State two different health reasons that support this advice.2 marks
  10. The table shows the nutrient content per 100g of four foods. Justify which food (A, B, C, or D) would be most suitable for an individual recovering from major surgery and needing to rebuild tissues. | Food | Protein (g) | Fat (g) | Carbohydrate (g) | | :--- | :---: | :---: | :---: | | A | 1 | 0 | 25 | | B | 25 | 15 | 0 | | C | 2 | 30 | 1 | | D | 8 | 1 | 45 |3 marks

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