Macronutrients — Protein, Fats and Carbohydrates
What Are Macronutrients?
Macronutrients are the nutrients your body needs in large quantities every day. They supply energy and build or repair tissues. All three — protein, fats and carbohydrates — appear on every nutrition label and form the backbone of every meal you cook. Compare them with micronutrients (vitamins and minerals), which are needed in tiny amounts and do not supply energy. Getting the distinction clear is the foundation of this entire unit.
Protein — Functions, Sources and Quality
Protein builds and repairs muscle, makes enzymes and hormones, and supports immune function. The body breaks protein down into amino acids. Of the 20 amino acids in food, nine are essential — we cannot make them ourselves and must eat them. This is where high biological value (HBV) protein comes in. Meat, fish, eggs, dairy, and soya beans all supply HBV protein. In contrast, most plant proteins (beans, lentils, nuts, cereals) are low biological value (LBV) — they’re missing at least one essential amino acid.
HBV proteins (meat, fish, eggs, dairy, soya) contain all nine essential amino acids. LBV proteins (beans, lentils, most cereals) are missing at least one. Vegetarians and vegans can meet all amino acid needs by combining LBV sources.
Protein Complementation
Protein complementation means combining two or more LBV protein sources in the same meal so that the amino acids missing from one are supplied by the other. The classic examples are beans on toast (haricot beans + wheat), rice and peas (rice + kidney beans), and hummus and pitta (chickpeas + wheat). For a vegan teenager eating no animal products, this technique is essential to avoid protein deficiency.
Protein Alternatives
The spec names four main protein alternatives for vegetarian and vegan diets. TVP (textured vegetable protein) is made from defatted soya flour; it absorbs flavour well and can replace mince in a bolognese. Soya is the only plant food that is naturally HBV. Mycoprotein (sold as Quorn) is made from a fungus and is high in fibre as well as protein. Tofu is set soya curd; it has a neutral flavour and takes on marinades readily when diced for a stir-fry.
Students often write that plant proteins are “bad” or “incomplete” and leave it there. This misses the point. LBV proteins are perfectly healthy — they are simply missing one or more essential amino acids on their own. By using complementation (beans on toast, rice and peas, lentil soup with a bread roll), a vegan or vegetarian can get every essential amino acid they need without eating any animal products. Soya is actually HBV and is nutritionally equivalent to meat protein in that respect.
Deficiency and Excess
Protein deficiency is rare in the UK but causes kwashiorkor in severe cases (muscle wasting, swollen abdomen, poor immunity). Mild deficiency leads to slow growth in children and poor wound healing. Excess protein is not stored — the nitrogen is excreted as urea, putting extra strain on the kidneys. Most UK adults consume more protein than the Reference Nutrient Intake (RNI) of around 0.75 g per kg body weight per day.
Fats — Types, Functions and Effects
Fats supply more than twice the energy of protein or carbohydrate per gram (9 kcal/g versus 4 kcal/g). They insulate the body, protect organs, carry fat-soluble vitamins A, D, E and K, and are essential for hormone production. There are two main types.
Saturated fats are solid at room temperature. Think butter, lard, the white fat on a lamb chop, the cream in a dessert. High intake raises LDL (“bad”) cholesterol, which is a risk factor for coronary heart disease (CHD). Current guidelines recommend that saturated fats make up no more than 11% of daily energy intake.
Unsaturated fats are liquid at room temperature and come in two sub-types: monounsaturated (olive oil, avocado, rapeseed oil) and polyunsaturated (sunflower oil, oily fish, walnuts). Replacing saturated fats with unsaturated fats can reduce LDL cholesterol. Oily fish (salmon, mackerel, sardines) are particularly rich in omega-3 polyunsaturated fatty acids, which support heart health.
A common exam error is to write “fat is bad” and then list butter as the only source. Remember: unsaturated fats are essential. Without them, you cannot absorb vitamins A, D, E and K, and you risk deficiency of those nutrients too. The exam expects you to distinguish between saturated and unsaturated, name sources of each, and link saturated fat excess to CHD via raised LDL cholesterol. Fat deficiency — though rare — causes poor absorption of fat-soluble vitamins, dry skin, hormonal disruption and lack of energy.
Carbohydrates — Structure, Functions and Fibre
Carbohydrates are the body’s preferred energy source, supplying 4 kcal per gram. They come in three structural forms:
- Monosaccharides (glucose, fructose, galactose) — the simplest sugars; absorbed directly into the blood.
- Disaccharides (sucrose = glucose + fructose; lactose = glucose + galactose; maltose = glucose + glucose) — two monosaccharides joined together.
- Polysaccharides (starch, glycogen, dietary fibre (NSP)) — long chains of monosaccharides.
Starch is found in bread, pasta, rice, potatoes, cereals and oats. It is the slow-release fuel that should form the majority of a GCSE student’s energy intake. Free sugars (those added to food or naturally present in honey, syrups and fruit juices) should make up no more than 5% of total energy intake. Dietary fibre keeps the gut healthy, prevents constipation and reduces the risk of bowel cancer and type 2 diabetes.
Carbohydrates: monosaccharides (glucose), disaccharides (sucrose), polysaccharides (starch, dietary fibre). Excess free sugars cause tooth decay and contribute to obesity and type 2 diabetes. The recommended maximum for free sugars is 5% of total energy intake.
Deficiency of carbohydrates means the body switches to fat then protein for fuel, producing ketone bodies, which can cause fatigue, brain fog and, in extreme cases, a dangerous condition called ketoacidosis. Deficiency of dietary fibre causes constipation and increases the long-term risk of bowel cancer. Excess carbohydrate (especially refined sugars) is stored as body fat once glycogen stores are full, contributing to obesity.