What are proteins, fats and carbohydrates, and what do they do for health?
The functions, food sources and health implications of the macronutrients - protein (including biological value), fats and oils (saturated and unsaturated), and carbohydrates (sugars, starch and non-starch polysaccharide) - and the role of food in supplying energy.
An SQA Higher Health and Food Technology answer on the macronutrients, covering the functions, sources and health effects of protein (and biological value), fats and oils, and carbohydrates, plus how food supplies the body with energy.
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What this dot point is asking
The SQA wants you to state the functions, main food sources and health implications of the three macronutrients - protein, fats and oils, and carbohydrates - and to explain how food supplies the body with energy. Higher expects precise terms such as biological value, essential amino acids, saturated and unsaturated, and free sugars.
Protein
The main functions of protein are the growth and repair of body tissue (muscle, skin, organs), making enzymes, hormones and antibodies, and acting as a secondary source of energy when carbohydrate and fat are short. Children, teenagers, pregnant women and people recovering from illness or surgery need more protein because they are building new tissue.
Biological value measures protein quality:
Both too little and too much protein cause problems. Severe deficiency in children causes the wasting condition kwashiorkor; a long-term large excess places strain on the kidneys and liver.
Fats and oils
The functions of fat are to provide a concentrated store of energy, to insulate the body and protect organs, to carry the fat-soluble vitamins A, D, E and K, and to supply the essential fatty acids the body cannot make. Fat also gives food flavour, a moist or creamy texture, and a feeling of fullness.
Fats differ in their effect on health:
- Saturated fat (in butter, lard, fatty meat, cheese, palm oil) raises blood LDL cholesterol, increasing the risk of coronary heart disease. Current advice is to reduce saturated fat.
- Unsaturated fats - monounsaturated (olive oil, rapeseed oil) and polyunsaturated (oily fish, sunflower oil, nuts) - are healthier and can help lower LDL cholesterol. Omega-3 fatty acids from oily fish support heart health.
- Trans fats, formed when oils are hydrogenated, are the most harmful and should be minimised.
Too much fat of any kind is energy-dense and contributes to obesity; too little risks deficiency of essential fatty acids and the fat-soluble vitamins.
Carbohydrates
- Starch is a complex carbohydrate in bread, pasta, rice, potatoes and cereals. It is digested slowly to release energy steadily and is the carbohydrate dietary advice encourages, especially wholegrain forms.
- Sugars are simple carbohydrates. Free sugars - those added to food and drink plus sugars in honey, syrups and fruit juice - are the ones to cut. They give energy with few other nutrients (empty calories), promote dental caries, and contribute to obesity and type 2 diabetes when eaten in excess.
- NSP (fibre) is the indigestible part of plant foods (wholegrains, fruit, vegetables, pulses). It adds bulk to the diet, aids the movement of food through the gut, helps prevent constipation and bowel disorders, and gives a feeling of fullness that helps control weight.
Food as a source of energy
The body needs energy for basal metabolism (keeping the body alive at rest), for physical activity and, in children, for growth. Energy comes mainly from carbohydrate and fat, with protein as a back-up. Energy needs vary with age, sex, body size and how active a person is.
Examples in context
Example 1. A vegan athlete. A vegan eats no animal protein, so every source is LBV. To get all the essential amino acids the athlete combines foods - lentil dahl with rice, hummus with pitta, tofu with noodles. This is protein complementation, showing why protein quality matters as much as quantity.
Example 2. Reformulating a ready meal. To make a lasagne healthier, a manufacturer swaps saturated fat (cream, cheese) for unsaturated oils, cuts added sugar to reduce free sugars, and adds wholewheat pasta and vegetables for starch and NSP - each change tracking current dietary advice.
Try this
Q1. State two functions of protein in the body. [2 marks]
- Cue. Growth and repair of tissue; making enzymes, hormones or antibodies (energy as a back-up also accepted).
Q2. Which macronutrient is the most energy-dense, and how much energy does it supply per gram? [2 marks]
- Cue. Fat, about 37 kJ (9 kcal) per gram.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA Higher (specimen)4 marksExplain the term biological value and describe how two foods can be combined to improve the protein quality of a vegetarian meal.Show worked answer →
A 4-mark explain-and-describe answer needs the definition and a worked example of complementation.
Biological value (BV) measures how closely the balance of amino acids in a protein matches what the body needs to make its own proteins. A high-BV protein supplies all the essential amino acids in roughly the right proportions, so most of it can be used.
Animal proteins (meat, fish, eggs, dairy) are high BV (HBV) because they contain all the essential amino acids. Most single plant proteins are low BV (LBV) because each is short of one or more essential amino acid - the limiting amino acid.
Two LBV foods can be combined so that the amino acid missing from one is supplied by the other; this is protein complementation. For example, beans on wholemeal toast: pulses are low in the amino acid methionine but cereals supply it, while cereals are low in lysine but pulses supply it. Eaten together they give a protein of higher overall value.
Markers reward (1) BV defined as amino-acid match to need, (2) the idea of a limiting/essential amino acid, (3) complementation explained, and (4) a correct paired example such as beans and toast or rice and lentils.
SQA Higher (past paper style)6 marksEvaluate the health implications of a diet that is high in saturated fat and free sugars but low in starchy carbohydrate.Show worked answer →
A 6-mark evaluate answer should link each nutrient feature to a specific health consequence, with both negative and any positive points.
High saturated fat: raises blood LDL cholesterol, which builds fatty deposits (atheroma) in artery walls, narrowing them and increasing the risk of coronary heart disease and stroke. Fat is also very energy-dense (37 kJ per gram), so a high-fat diet supplies excess energy that is stored as body fat, contributing to obesity.
High free sugars: free sugars give energy with few other nutrients (empty calories), again risking weight gain, and they are the main cause of dental caries because mouth bacteria ferment sugar to acid that dissolves enamel. A high-sugar load is also linked to a raised risk of type 2 diabetes.
Low starchy carbohydrate: starchy foods (especially wholegrain) are the main source of energy and of NSP/fibre. A diet low in them tends to be low in fibre, which is linked to constipation and bowel disorders, and may leave the person reliant on fat and sugar for energy.
A balanced evaluation notes that some fat is essential (fat-soluble vitamins, essential fatty acids) and some sugar is harmless within an overall balanced diet, so the problem is the imbalance, not the nutrients themselves.
Markers reward correct nutrient-to-disease links (CHD, obesity, dental caries, diabetes, bowel health), use of the term energy-dense, and a genuine evaluative judgement rather than a list.
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