Edexcel IGCSE Biology · Spec 2.24-2.26
A Balanced Diet
The seven components of a balanced diet, their sources and functions, and how energy needs vary from person to person.
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A Balanced Diet
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Explained
Seven components, and what each one is for
A balanced diet contains the right proportions of seven components. Learn each as a source and a function, because questions ask for both and mark them separately.
The seven
- Carbohydrates, from bread, rice, pasta and potatoes. The body's main source of energy, released by respiration.
- Proteins, from meat, fish, eggs and beans. Needed for growth and repair of tissues, and to make enzymes and antibodies.
- Lipids, meaning fats and oils, from butter, oils and nuts. An energy store, insulation, and part of every cell membrane.
- Vitamins, needed in small amounts. Vitamin A for vision in dim light and healthy epithelial tissue, vitamin C for connective tissue, vitamin D for absorbing calcium.
- Minerals, also needed in small amounts. Calcium for bones and teeth, iron for making haemoglobin.
- Water, which most of the body is made of. A solvent for reactions, the medium for transport in blood, and part of temperature control through sweating.
- Dietary fibre, which is not digested at all. It adds bulk so the muscles of the gut can grip, which helps peristalsis move material along and prevents constipation.
Fibre and water are the two most often left off a list of seven, and both are worth a mark.
Deficiency diseases
Too little vitamin C causes scurvy. Too little vitamin D or calcium causes rickets in children, where the bones are too soft. Too little iron causes anaemia, because there is not enough haemoglobin to carry oxygen, so the person is tired and pale.
Each of those follows from the function. Iron makes haemoglobin, so a shortage means less oxygen carried, so less aerobic respiration and less energy released. Giving that chain is worth more than naming the disease.
Energy needs vary
Age: children and teenagers need more energy per kilogram because they are growing.
Activity: someone doing physical work or exercise respires more, so needs more energy.
Sex: males typically need more than females of the same age and mass, because of differences in body composition.
Pregnancy and breastfeeding raise requirements, because the mother is supplying a growing fetus or producing milk.
What the mark scheme accepts and rejects
An Edexcel International GCSE Biology mark scheme asks why someone might choose a drink containing less sugar. It credits that it has fewer calories or kilojoules and less energy, that this means less mass gain or a wish to lose weight or to prevent obesity and heart disease, and that it means less tooth decay or a lower risk of diabetes.
It also allows an answer built round the idea that some people are diabetic and cannot release enough insulin to control their blood glucose.
Notice how specific each of those is. Healthier is not on the list, and neither is better for you. The marks are for naming a particular consequence, and there are several to choose from.
On a related question the same mark scheme credits a chain rather than a fact: the drink with the most glucose supplies more for respiration, which releases more energy or ATP, which supports muscle contraction, so the athlete can run faster or for longer. Its note adds that a candidate who picked the wrong drink can still score the respiration, energy and muscle marks.
So even if the first choice is wrong, the reasoning is credited separately, which is a good reason to explain your answer fully rather than stopping at the choice.
Malnutrition means unbalanced, not just too little
Malnutrition covers any diet with the wrong proportions, so eating too much of something counts.
Taking in more energy than is used leads to the excess being stored as fat, which raises the risk of obesity, type 2 diabetes and heart disease. Taking in too little energy or protein leads to weight loss and poor growth. Both are malnutrition, and questions sometimes test whether you have assumed it only means starvation.
Spec 2.24-2.26
What you need to know
- List the parts of a balanced diet
- Give the source and function of each
- Explain how energy needs vary
Active recall
Quick check
Answer each question before opening the answer.
Name the components of a balanced diet.
Carbohydrates, proteins, lipids, vitamins, minerals, water and dietary fibre.
State a deficiency disease caused by lack of vitamin C, vitamin D and iron.
Vitamin C — scurvy; vitamin D — rickets; iron — anaemia.
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