Protein, fats and carbohydrates: their composition, functions, sources, biological and complementary value of protein, saturated and unsaturated fats, simple and complex carbohydrates, and the effects of excess or deficiency.

What this dot point is asking

Eduqas wants you to know the composition (what they are made from), the main functions, the food sources and the effects of having too much or too little of each macronutrient: protein, fats and carbohydrates. You also need biological value of protein, protein complementation, the difference between saturated and unsaturated fat and between simple and complex carbohydrate.

Protein

Protein is needed for growth, repair and maintenance of body tissues, the production of enzymes and some hormones, and as a secondary source of energy.

Protein complementation means combining two LBV proteins, such as beans on toast or rice and peas, so that together they supply all the essential amino acids. This matters for vegetarians and vegans, who rely largely on plant proteins.

Too little protein leads to slow growth in children, poor wound healing, a weak immune system and oedema (fluid retention). Too much protein can strain the kidneys, and the excess is converted to fat and stored.

Fats

Fat (a lipid that is solid at room temperature) and oil (liquid at room temperature) are both made of fatty acids and glycerol. They provide a concentrated source of energy, insulation to keep the body warm, protection of organs, and they carry the fat-soluble vitamins A, D, E and K.

• Saturated fats have no double bonds in their fatty acids, are usually solid at room temperature and come from animal sources such as butter, lard, suet and fatty meat, plus coconut and palm oil. Eating too much raises blood cholesterol and the risk of coronary heart disease.
• Unsaturated fats have one (mono-) or more (poly-) double bonds, are usually liquid at room temperature and come from plant and fish sources such as olive oil, rapeseed oil, sunflower oil, nuts and oily fish. These are the healthier fats.

Too much fat causes weight gain and obesity and raises heart-disease risk; too little leads to lack of energy, feeling cold and deficiency of the fat-soluble vitamins.

Carbohydrates

Carbohydrate is the body's main source of energy. It is grouped as:

• Simple carbohydrates (sugars) such as glucose, sucrose and lactose, which release energy quickly. These include the free sugars added to foods or found in honey and fruit juice.
• Complex carbohydrates (starch) such as bread, rice, pasta and potatoes, which release energy more slowly and steadily.

Dietary fibre is also a carbohydrate, treated separately (see water and fibre). Too much carbohydrate, especially free sugar, leads to obesity and tooth decay; too little causes tiredness and the body breaking down fat and then protein for energy.

Energy from macronutrients

The three macronutrients also supply the body's energy, but not equally. Protein and carbohydrate each provide about 4 kcal per gram (about $17$ kJ), while fat provides about 9 kcal per gram (about $37$ kJ), more than twice as much. This is why fat is described as energy-dense and why high-fat foods are easy to over-consume. You can use these values to calculate the energy in a food from its protein, fat and carbohydrate content, which is the basis of the figures on a nutrition label.

Try this

Q1. Explain the difference between HBV and LBV protein and give one example of each. [3 marks]

• Cue. HBV has all essential amino acids (e.g. eggs); LBV is missing one or more (e.g. lentils); state that animal sources and soya/quinoa are HBV.

Q2. Give two functions of fat in the diet. [2 marks]

• Cue. Any two of energy, insulation, organ protection, carrying fat-soluble vitamins.