What makes a balanced diet, and how do we test food for the main nutrients?
The components of a balanced diet and their sources and functions, the consequences of an unbalanced diet, and the chemical food tests for starch, reducing sugar, protein and fat.
A focused CCEA GCSE Double Award Science (Biology Unit B1) answer on nutrition, covering the components of a balanced diet and their functions, the effects of an unbalanced diet, and the food tests for starch, reducing sugar, protein and fat with their colour changes.
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What this dot point is asking
CCEA Double Award wants the components of a balanced diet, what each one does, what happens when the diet is unbalanced, and the four standard chemical food tests with their colour changes. The food tests are a guaranteed source of marks because the colours are fixed and easy to learn.
A balanced diet
The amount of energy a person needs depends on their age, sex and activity level. An active teenager needs more energy than a sedentary adult, so "balanced" means matched to the individual, not a fixed menu.
Consequences of an unbalanced diet
If the diet has too much energy (especially fat and sugar) and too little exercise, the excess is stored as fat, leading to obesity and a higher risk of type 2 diabetes and heart disease. If the diet is short of a nutrient, a deficiency disease results:
- Too little iron causes anaemia, because iron is needed for haemoglobin.
- Too little vitamin C causes scurvy.
- Too little calcium or vitamin D weakens bones and teeth.
- Too little protein in children can stop normal growth.
The four food tests
The reducing-sugar test must be heated, and a fuller positive result moves blue to green to yellow to orange to brick-red as the sugar concentration rises. The fat test needs ethanol first because fat does not dissolve in water.
Examples in context
Example 1. Designing a diet for an athlete. A long-distance runner needs plenty of carbohydrate for energy and enough protein to repair muscle, plus iron to keep their blood carrying oxygen. Balancing the diet to their high energy demand prevents tiredness and supports recovery, showing that "balanced" depends on activity level.
Example 2. Testing a breakfast cereal. A student tests a cereal and finds iodine turns blue-black and heated Benedict's turns brick-red. The cereal therefore contains both starch and reducing sugar. This combination is common in processed breakfast foods and explains the energy they provide.
Try this
Q1. State the function of protein in the diet. [1 mark]
- Cue. Growth and repair of tissues.
Q2. What reagent and colour change show that fat is present? [2 marks]
- Cue. Ethanol then water; a cloudy white emulsion forms.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA-style4 marksDescribe the food test for starch and the food test for protein, giving the reagent and the positive colour change for each.Show worked answer →
Two tests, each worth a reagent mark and a colour-change mark.
Starch: add iodine solution. A positive result changes the orange-brown iodine to a blue-black colour.
Protein: add Biuret solution (or sodium hydroxide then copper sulfate). A positive result changes the blue solution to a purple or lilac colour.
Markers want the correct reagent named and the correct colour change, including the starting colour where it shows the change clearly.
CCEA-style3 marksExplain two health problems that can result from an unbalanced diet.Show worked answer →
Two problems with a cause for three marks.
Too much fat and sugar with too little exercise can lead to obesity, which raises the risk of type 2 diabetes and heart disease.
Too little iron in the diet can cause anaemia, because iron is needed to make haemoglobin, so the blood carries less oxygen.
Other valid answers include too little vitamin C causing scurvy, or too little calcium and vitamin D weakening bones. Markers reward a named problem linked to the missing or excess nutrient.
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Sources & how we know this
- CCEA GCSE Science Double Award specification — CCEA (2017)