What does protein do in the body, where do we get it, and what happens if we eat too much or too little?
The function and dietary sources of protein, the difference between proteins of high and low biological value, and the effects on health of eating too much or too little protein.
An SQA National 5 Health and Food Technology answer on protein, covering its functions in the body, dietary sources, the difference between high and low biological value protein, and the effects on health of too much or too little protein.
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What this dot point is asking
The SQA wants you to state what protein does in the body, name foods that supply it, explain the difference between high and low biological value protein, and describe what happens to health when someone eats too much or too little protein.
What protein is made of
Protein is built from small units called amino acids joined in chains. There are around twenty amino acids. The body can make some of them for itself, but others, called essential amino acids, must come from food because the body cannot make them.
The functions of protein
Protein has three functions you must be able to state for the exam:
Growth and repair are the main jobs. Energy is a back-up role: the body would rather use carbohydrate and fat for energy, and only uses protein for energy when there is a surplus.
Sources of protein
Protein-rich foods come from both animals and plants:
- Animal sources. Meat, poultry, fish, eggs, milk, cheese and yoghurt.
- Plant sources. Beans, peas, lentils, nuts, seeds, soya products such as tofu, and cereals such as bread and rice (cereals supply smaller amounts).
High and low biological value
National 5 makes a clear distinction between two kinds of protein based on which amino acids they contain:
- HBV protein comes mainly from animal foods (meat, fish, eggs, milk, cheese) and from soya, which is the main plant exception.
- LBV protein comes from most plant foods (beans, lentils, nuts, cereals).
The practical point is that someone who eats little or no animal food can still get every essential amino acid by combining LBV proteins in one meal, so that the amino acids missing from one food are supplied by another. Beans on toast, lentil dahl with rice, and hummus with pitta are classic examples.
Too much and too little protein
A diet very short of protein over a long time can cause serious deficiency, but in Scotland the more common problem is eating more protein (and energy) than the body needs.
Examples in context
Example 1. A teenager during the growth spurt. A 14-year-old is growing quickly and is also very active in sport. They need plenty of protein for the rapid growth of new tissue, and HBV sources such as milk, eggs and lean meat make this easy to supply. A snack of a glass of milk and a boiled egg provides growth-supporting protein.
Example 2. Recovering after an operation. A patient healing after surgery needs extra protein for the repair of damaged tissue and to make antibodies that resist infection. Fish, chicken and eggs are easy-to-digest HBV sources that support recovery.
Try this
Q1. Name the two main functions of protein in the body. [1 mark]
- Cue. Growth (of new cells and tissues) and repair (of worn-out or damaged cells and tissues).
Q2. Name one plant food that is a source of high biological value protein. [1 mark]
- Cue. Soya (for example tofu).
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 N5 style4 marksDescribe two functions of protein in the body, and for each give one food source that supplies it.Show worked answer →
A 4-mark describe-and-source answer needs two functions, each with a matching source, so plan two function marks and two source marks.
Function 1. Protein is needed for the growth of new body cells and tissues, which matters most during childhood, the teenage growth spurt and pregnancy. A food source is lean meat such as chicken.
Function 2. Protein is needed for the repair of worn-out or damaged body cells and tissues, for example healing a wound or replacing skin cells. A food source is fish.
A third function that would also score is providing energy: if more protein is eaten than the body needs for growth and repair, the surplus is used to release energy. Eggs, milk, cheese, beans, lentils and tofu are all acceptable sources.
Markers reward each correct function (1 mark) and each correct source (1 mark). Naming a source that supplies little protein, such as butter or sugar, scores nothing.
SQA N5 style3 marksExplain the difference between protein of high biological value and protein of low biological value, and give one example of each.Show worked answer →
This question tests the key National 5 idea that not all protein is equal, so each statement must be clear and an example given.
Protein of high biological value (HBV) contains all of the essential amino acids the body cannot make for itself. It comes mainly from animal foods such as meat, fish, eggs, milk and cheese, and also from soya. An example is eggs.
Protein of low biological value (LBV) is missing one or more of the essential amino acids. It comes mainly from plant foods such as beans, peas, lentils, nuts and cereals. An example is baked beans.
A useful extra point that scores is that a vegetarian or vegan can still get all the essential amino acids by combining different LBV proteins in a meal, for example beans on wholemeal toast, so that one food supplies what the other lacks.
Markers reward the correct meaning of HBV, the correct meaning of LBV, and a valid example. Saying only "animal protein is better" without the amino-acid reason does not earn full marks.
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Sources & how we know this
- SQA National 5 Health and Food Technology Course Specification — SQA (2017)
- British Nutrition Foundation - Protein — British Nutrition Foundation (2023)