Describe the functions, food sources and effects of deficiency or excess of protein, and explain high and low biological value protein and protein complementation

What this dot point is asking

SEAB wants you to describe what protein does in the body, name the foods that supply it, and explain what happens when a person eats too little or too much. You also need to explain the difference between high biological value and low biological value protein, and how protein complementation lets someone who avoids meat still get all the amino acids they need. The big idea is that protein is the body's building material: it is the nutrient used to grow and to repair.

The answer

What protein is made of

Protein is made of small units called amino acids joined in long chains. There are about 20 different amino acids. Some are essential amino acids, which means the body cannot make them and they must come from food. The rest are non-essential because the body can make them itself.

Functions of protein

Protein has three main jobs in the body:

• Growth of new body tissues, which is why children, teenagers and pregnant women need more.
• Repair and maintenance of worn-out or damaged tissues, such as healing a cut or replacing dead skin.
• A secondary source of energy when there is not enough carbohydrate or fat. Protein gives about $17\ \text{kJ}$ (around $4\ \text{kcal}$) per gram.

Food sources of protein

• High biological value (HBV) proteins contain all the essential amino acids. They come mainly from animals: meat, fish, eggs, milk and cheese. The plant foods soya (tofu, soya milk) and quinoa are also HBV.
• Low biological value (LBV) proteins are missing one or more essential amino acids. They come from most plant foods: beans, lentils, peas, nuts and cereals such as rice and wheat.

Protein complementation

A person who does not eat animal foods can still get all the essential amino acids by protein complementation: combining two different LBV foods so that the amino acid missing from one is supplied by the other. Familiar Singapore examples are rice and dhal, baked beans on toast, or roti with lentil curry.

Too little or too much protein

Too little protein (deficiency) causes slow or stunted growth in children, poor wound healing, weak muscles, a weakened immune system, and in severe cases a swollen tummy (oedema). Too much protein is stored as fat and can put extra strain on the kidneys, so eating far more than you need is wasteful, not helpful.

Examples in context

Example 1. A growing teenager's breakfast. A secondary school student in Singapore who eats two half-boiled eggs with wholemeal toast gets HBV protein from the eggs for growth and repair during a period of fast growth, plus carbohydrate from the bread for energy to focus in class. This shows protein supporting growth at a life stage when needs are high.

Example 2. A vegetarian hawker meal. A plate of thosai with dhal and coconut chutney combines a rice-and-lentil flour pancake (cereal, LBV) with lentil curry (pulse, LBV). The two LBV foods complement each other, so the meal supplies all the essential amino acids without any meat, a common everyday example of complementation in Singapore.

Try this

Q1. State two functions of protein in the body. [2 marks]

• Cue. Any two of: growth of new tissue, repair and maintenance of tissue, and a secondary source of energy.

Q2. Explain the difference between HBV and LBV protein and give one example of each. [3 marks]

• Cue. HBV has all the essential amino acids (e.g. egg or soya); LBV is missing one or more (e.g. lentils or rice).

Q3. Suggest how a person who does not eat meat or fish can still get all the essential amino acids, using one example. [2 marks]

• Cue. Eat plant HBV foods such as soya, or use complementation, combining two LBV foods such as rice and dhal so together they give all the essential amino acids.