What are the macronutrients and what do they do in the body?
Protein, fat and carbohydrate: their chemical structure, functions, sources, biological and complementary value, and what happens with excess or deficiency.
A focused answer on the three macronutrients for AQA GCSE Food Preparation and Nutrition, covering the structure, functions, sources and deficiency or excess of protein, fat and carbohydrate.
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What this dot point is asking
AQA wants you to know the chemical make-up, main functions, food sources and the effects of having too much or too little of each macronutrient: protein, fat and carbohydrate. You also need to understand biological value of protein and protein complementation.
Protein
Protein is needed for growth, repair of body tissues and maintenance, and gives a secondary source of energy.
Protein complementation means combining two LBV proteins, such as beans on toast or rice and peas, so that together they supply all the essential amino acids. This matters for vegetarians and vegans.
Too little protein leads to slow growth in children, poor wound healing, weak immunity and oedema (fluid retention). Too much protein can strain the kidneys and excess is converted to fat.
Fat
Fat (lipid) is made of fatty acids and glycerol. It provides a concentrated source of energy, insulation to keep the body warm, protection of organs, and carries the fat-soluble vitamins A, D, E and K.
- Saturated fats have no double bonds, are usually solid at room temperature and come from animal sources such as butter, lard and fatty meat. Eating too much raises blood cholesterol and the risk of heart disease.
- Unsaturated fats have one or more double bonds, are usually liquid at room temperature and come from plant and fish sources such as olive oil, sunflower oil and oily fish.
Too much fat causes weight gain and obesity; too little leads to lack of energy, feeling cold and vitamin deficiency.
Carbohydrate
Carbohydrate is the body's main source of energy. It is grouped as:
- Simple carbohydrates (sugars) such as glucose, sucrose and lactose, which release energy quickly.
- Complex carbohydrates (starch) such as bread, rice, pasta and potatoes, which release energy slowly.
- Dietary fibre (NSP) such as wholegrains, fruit and vegetables, which aids digestion and prevents constipation.
Too much carbohydrate, especially free sugar, leads to obesity and tooth decay; too little causes tiredness and the body breaking down fat and then protein for energy.
Energy from macronutrients
The three macronutrients also supply the body's energy, but not equally. Protein and carbohydrate each provide about 4 kcal per gram, while fat provides about 9 kcal per gram, more than twice as much. This is why fat is described as energy-dense and why high-fat foods are easy to over-consume. You can use these values to calculate the energy in a food from its protein, fat and carbohydrate content, which is the basis of the figures on a nutrition label. Government guidance suggests roughly half of energy should come from carbohydrate (mostly starch, not free sugar), no more than about 35% from fat, and the remainder from protein.
Try this
Q1. Explain the difference between HBV and LBV protein and give one example of each. [3 marks]
- Cue. HBV has all essential amino acids (e.g. eggs); LBV is missing one or more (e.g. lentils).
Q2. Give two functions of fat in the diet. [2 marks]
- Cue. Any two of energy, insulation, organ protection, carrying fat-soluble vitamins.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20186 marksExplain the difference between high biological value and low biological value protein, and discuss how a vegan can still meet their protein needs. (Paper 1, Section B)Show worked answer →
High biological value (HBV) protein contains all the essential amino acids and comes mainly from animal sources such as meat, fish, eggs and dairy, plus the plant foods soya and quinoa. Low biological value (LBV) protein is missing one or more essential amino acids and comes from most plant foods such as beans, lentils, nuts and cereals.
A vegan can meet protein needs by protein complementation, combining two LBV proteins (such as beans on toast or rice and peas) so that together they supply all the essential amino acids, and by including soya and quinoa, which are plant HBV proteins.
Top-band answers (5 to 6 marks) define HBV and LBV with examples and clearly explain complementation as the solution.
AQA 20214 marksA 60 g portion of food contains 8 g of protein, 5 g of fat and 30 g of carbohydrate. Calculate the energy it provides in kilocalories. (Paper 1, Section A)Show worked answer →
Use the energy values: protein and carbohydrate give about 4 kcal per gram, and fat gives about 9 kcal per gram.
Protein: kcal. Carbohydrate: kcal. Fat: kcal.
Total energy is kcal. Markers award method marks for the correct energy value per gram of each macronutrient and the final total, and note that fat is the most energy-dense at more than double protein or carbohydrate.
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