What is protein made of, what does it do in the body, and how much do we need?
Protein as a macronutrient: amino acids, the distinction between essential and non-essential amino acids, high and low biological value protein, protein complementation, the functions and food sources of protein, and the consequences of deficiency and excess.
A CCEA A-Level Nutrition and Food Science answer on protein: its amino acid structure, essential and non-essential amino acids, high and low biological value, protein complementation, the functions and sources of protein, and the effects of deficiency and excess.
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What this dot point is asking
CCEA wants you to describe protein as a macronutrient: its building blocks (amino acids), the difference between essential and non-essential amino acids, what high and low biological value mean, how protein complementation works, the functions and food sources of protein, and what happens when intake is too low or too high.
Amino acids and biological value
There are about twenty amino acids found in food proteins. Around eight are essential for adults (with histidine also essential for infants), meaning the diet must supply them. The body builds its own proteins by arranging amino acids in a specific sequence, so the supply of every essential amino acid matters.
The limiting amino acid is the essential amino acid present in the smallest amount relative to need; it limits how much of the protein the body can use. Cereals are typically limited by lysine, while pulses are typically limited by methionine.
Protein complementation
Protein complementation (the complementary action of proteins) means eating two or more LBV foods whose limiting amino acids differ, so that the amino acid lacking in one is supplied by the other. Together the foods provide all the essential amino acids even though no single food is HBV. Classic examples are beans on toast (pulse plus cereal), lentil dahl with rice, hummus with pitta bread, and rice with peas. This is the key strategy that lets vegetarians and vegans obtain a complete amino acid supply from plant foods, and CCEA expects you to be able to explain and exemplify it.
Functions, sources and balance
Requirements are proportionally highest during periods of rapid growth, so children, adolescents, pregnant and breastfeeding women need relatively more. The reference intake for an average adult is roughly 0.75 grams of protein per kilogram of body mass per day, about 45 to 55 grams.
Examples in context
Example 1. A vegan day's meals. Breakfast of porridge oats with peanut butter on toast, a lunch of lentil and rice salad, and a dinner of chickpea curry with flatbread each combine a cereal with a pulse or nut. Across the day the differing limiting amino acids of the two food groups cancel out, so the vegan receives every essential amino acid without any animal protein. This is protein complementation working in a realistic diet, and it shows why CCEA links protein quality to food choice rather than to a single food.
Example 2. Protein needs in pregnancy. During pregnancy the mother is building the tissues of a growing fetus as well as her own expanded blood volume and uterus, so protein requirement rises. A diet including HBV sources such as eggs, dairy and lean meat (or soya and complemented plant proteins for a vegetarian) supports fetal growth and the formation of new maternal tissue. This connects protein directly to the life-stage content of the module.
Try this
Q1. Explain the difference between an essential and a non-essential amino acid. [2 marks]
- Cue. Essential cannot be made by the body and must come from food; non-essential can be synthesised in the body.
Q2. Give two foods that could be combined to provide complementary proteins, and name the food groups. [2 marks]
- Cue. A cereal plus a pulse, for example beans (pulse) on toast (cereal); their limiting amino acids differ.
Q3. State the energy value of 1 gram of protein. [1 mark]
- Cue. About 17 kJ (4 kcal) per gram.
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 AS 20186 marksExplain the term biological value and discuss how protein complementation can be used to provide a vegan with a supply of all the essential amino acids.Show worked answer →
A 6-mark answer needs a clear definition of biological value and a worked explanation of complementation.
Biological value (BV) is a measure of how well a protein supplies the essential amino acids the body needs. A high biological value (HBV) protein contains all the essential amino acids in roughly the proportions the body requires; most come from animal sources such as meat, fish, eggs, milk and cheese, with soya being the main plant exception. A low biological value (LBV) protein is missing or short of one or more essential amino acids; most plant proteins such as cereals, pulses, nuts and seeds are LBV.
Protein complementation means combining two or more LBV foods whose limiting amino acids differ, so that together they supply all the essential amino acids. Cereals are typically low in lysine but adequate in methionine, while pulses are typically low in methionine but rich in lysine. A vegan eating beans on toast, lentil dahl with rice, or hummus (chickpeas) with pitta therefore receives a complete amino acid supply from the meal even though no single food is HBV.
Markers reward a correct definition of biological value, the HBV/LBV distinction with examples, a clear statement that LBV foods have differing limiting amino acids, and a valid complementation example relevant to a vegan.
CCEA AS 20204 marksDescribe the functions of protein in the body.Show worked answer →
A 4-mark describe answer needs four distinct, correctly stated functions.
Protein is needed for growth, building new cells and tissues, which is why requirements are proportionally high in childhood, adolescence and pregnancy. It is needed for the repair and maintenance of body tissues, replacing cells that are constantly worn out, such as skin and the lining of the gut.
Protein is used to make enzymes, hormones (such as insulin) and antibodies, so it supports digestion, regulation and the immune system. Protein can also be used as a secondary source of energy, providing about 17 kilojoules (4 kilocalories) per gram when carbohydrate and fat intake is insufficient.
Markers reward four separate functions: growth, repair and maintenance, the synthesis of enzymes, hormones and antibodies, and use as an energy source. Listing "growth and repair" as one point counts once.
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Sources & how we know this
- CCEA GCE Nutrition and Food Science specification — CCEA (2016)
- Government Dietary Recommendations (PHE) — Public Health England (2016)