What is protein, why does the body need it, and what is the difference between high and low biological value protein?
Protein as a macronutrient: its function in the body, amino acids and essential amino acids, high and low biological value protein, protein complementation, food sources, and the effects of too little or too much.
A focused answer to the WJEC Food Preparation and Nutrition principles of nutrition topic on protein, covering its function, amino acids, high and low biological value protein, protein complementation, food sources, and the effects of deficiency and excess.
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What this dot point is asking
You need to know what protein does in the body, that it is made of amino acids, the difference between high and low biological value protein, how protein complementation works, the main food sources, and what happens if you eat too little or too much.
The function of protein
Protein is a macronutrient (needed in large amounts). Its main roles are:
- growth of the body, especially in children, teenagers and pregnancy,
- repair and maintenance of cells and tissues, including healing wounds,
- making enzymes, hormones and antibodies,
- providing energy as a secondary source, when spare protein is left over (about 4 kcal per gram).
Amino acids
There are around twenty amino acids in total. The need for essential amino acids is why the quality of protein matters, not just the amount.
High and low biological value protein
- High biological value (HBV) proteins contain all the essential amino acids. Sources: meat, fish, eggs, dairy and soya.
- Low biological value (LBV) proteins are missing one or more essential amino acids. Sources: most plant foods, such as beans, lentils, nuts, seeds and cereals.
Protein complementation means combining different LBV proteins so the amino acids missing from one are provided by another, giving the full set, for example beans on toast, hummus and pitta, or rice and peas. This is essential planning for vegetarian and vegan diets.
How much protein, and life stages
Protein needs are higher during periods of growth: children, teenagers, pregnancy and breastfeeding, and during recovery from illness or injury. Most people in the UK eat enough or more than enough protein, so protein deficiency is rare here but a serious problem in parts of the world where food is scarce.
A rough guide for adults is about 0.75 g of protein per kilogram of body weight each day, so a 60 kg adult needs around 45 g. The exam will not ask you to calculate exact amounts, but it may ask you to say who needs more protein (growing children and teenagers, pregnant and breastfeeding women, and people recovering from injury) and to suggest protein foods to meet those needs.
The effects of too little or too much
Too little protein (deficiency):
- poor or slow growth (serious in children),
- slow healing of wounds and muscle wasting,
- lowered resistance to infection, as antibodies are made from protein.
Too much protein:
- spare protein is converted and stored as fat, which can lead to weight gain,
- it can put extra strain on the kidneys, which process the waste from protein breakdown.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC style6 marksExplain the difference between high biological value and low biological value protein, and describe how a vegan can still get all the essential amino acids.Show worked answer →
A 6-mark question. Mark it for a clear distinction plus the idea of complementation.
Proteins are made of amino acids. There are essential amino acids that the body cannot make and must get from food. High biological value (HBV) proteins contain all the essential amino acids, and come mainly from animal foods such as meat, fish, eggs and dairy, plus soya. Low biological value (LBV) proteins are missing one or more essential amino acids, and come mainly from plant foods such as beans, lentils, nuts and cereals.
A vegan eats no animal foods, so relies on plant protein. By combining different plant proteins so that the amino acids missing from one are supplied by another (protein complementation), the diet still provides all the essential amino acids, for example beans on toast or rice and peas. Soya and tofu are useful because they are HBV plant proteins.
A top answer defines HBV and LBV with sources, explains essential amino acids, and explains complementation with a correct example.
WJEC style3 marksDescribe the effects on the body of eating too little protein.Show worked answer →
A 3-mark question on protein deficiency.
Protein is needed for growth and repair, so too little protein slows or stops growth, which is especially serious in children. Wounds and damaged tissue heal more slowly, and muscle may waste. The body becomes more prone to infection because antibodies and enzymes are made from protein. In severe cases there is fluid retention and swelling.
Markers reward: slow or poor growth (especially in children), slow healing or muscle wasting, and lowered resistance to infection. Linking each effect to protein's role in growth and repair strengthens the answer.
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Sources & how we know this
- WJEC Eduqas GCSE Food Preparation and Nutrition specification (from 2016) — WJEC Eduqas (2016)