Macronutrients — Protein, Fats and Carbohydrates
Defining Macronutrients
The spec gives a precise definition to reproduce: macronutrients are “a class of chemical compounds which humans consume in the largest quantities.” There are three: protein, fats (lipids), and carbohydrates. These are the nutrients you eat in gram-sized amounts every day — contrasted with micronutrients (vitamins, minerals), which are needed in milligram or microgram quantities. Getting this definition word-for-word correct is a reliable early mark on any principles of nutrition question.
Energy values to memorise: protein provides 4 kcal per gram; carbohydrate provides 4 kcal per gram; fat provides 9 kcal per gram. Fat is more than double the energy density of the other two macronutrients — relevant when planning diets for athletes (high energy needs) or those managing weight.
Protein: Essential and Non-Essential Amino Acids
Proteins are chains of amino acids. There are twenty amino acids in total, but the spec focuses on the split between essential and non-essential. Essential amino acids cannot be made by the body and must come from food. The nine on the spec are: histidine, isoleucine, lysine, leucine, methionine, phenylalanine, threonine, tryptophan and valine. Non-essential amino acids — including alanine, asparagine, aspartic acid and glutamic acid — can be synthesised by the body from other compounds.
Foods that contain all nine essential amino acids in adequate amounts are called high biological-value (HBV) proteins. Animal foods — meat, fish, poultry, eggs, dairy — are HBV. Most plant proteins are low biological-value (LBV) because they lack one or more essential amino acids. Soya and quinoa are notable exceptions — both are complete plant proteins. For a 14-year-old vegan athlete, combining lentil curry with brown rice at dinner provides a complete amino acid profile, because lentils are low in methionine (plentiful in rice) and rice is low in lysine (plentiful in lentils).
The primary functions of protein are: growth and repair of body tissues; production of enzymes, hormones and antibodies; and as a secondary energy source (the body uses protein for energy when carbohydrate stores are depleted). Recommended daily intake for adults is approximately 0.75 g per kg of body weight — higher for adolescents, pregnant women and those recovering from illness or surgery.
Fats, Oils and Lipids
Dietary fats are classified by their chemical structure — specifically by the number of double bonds between carbon atoms in their fatty acid chains. Saturated fats have no double bonds and tend to be solid at room temperature. They are found mainly in animal products (butter, lard, fatty meat, full-fat dairy, palm oil) and are associated with raised LDL (“bad”) cholesterol and an increased risk of cardiovascular disease (CVD) when consumed in excess. Monounsaturated fats have one double bond. Olive oil, rapeseed oil, avocados and most nuts are rich in monounsaturated fats; they are associated with a more favourable cholesterol profile.
Polyunsaturated fats have two or more double bonds. This category includes the essential fatty acids — omega-3 (alpha-linolenic acid) and omega-6 (linoleic acid) — which cannot be synthesised by the body and must be obtained from food. Oily fish (salmon, mackerel, sardines) are the richest source of long-chain omega-3 fatty acids, which support brain function, reduce inflammation, and lower triglyceride levels. Vegetable oils such as sunflower and corn oil are high in omega-6. The current dietary advice is to increase omega-3 intake and moderate omega-6, since Western diets tend to be omega-6 heavy.
The functions of dietary fat extend beyond energy: fat carries and enables absorption of the fat-soluble vitamins A, D, E and K; provides insulation for organs; and supplies essential fatty acids that the body cannot make. A diet very low in fat may therefore lead to deficiencies in fat-soluble vitamins even if those vitamins are consumed — because without fat, they cannot be absorbed in the small intestine.
High intake of saturated fat → raised LDL cholesterol in the blood → LDL deposits on artery walls → formation of plaques (atherosclerosis) → narrowed arteries → increased risk of coronary heart disease (CHD) or stroke. The key vocabulary to use in an exam: saturated fat, LDL cholesterol, atherosclerosis, coronary heart disease, cardiovascular disease. A student who writes “saturated fat causes heart disease” scores 1 mark; a student who writes the full chain using this vocabulary scores much more in an extended response.
Carbohydrates: Monosaccharides, Disaccharides and Polysaccharides
Carbohydrates are the body’s preferred energy source. They are classified by the number of sugar units they contain. Monosaccharides are single sugar units: glucose (found in fruit and honey; the form of sugar the body uses for energy), fructose (fruit sugar) and galactose (found in milk). Disaccharides are two monosaccharides bonded together: sucrose (glucose + fructose — table sugar), lactose (glucose + galactose — milk sugar), and maltose (glucose + glucose — from malt). Polysaccharides are long chains of glucose units. Starch (from plants: potatoes, flour, rice) and glycogen (stored in the liver and muscle) are digestible polysaccharides. Non-starch polysaccharides (NSP) — dietary fibre — are not digested in the small intestine, passing instead to the colon where they feed beneficial bacteria and support bowel health.
Starchy polysaccharides release energy more slowly than simple sugars because they require digestion before glucose enters the bloodstream. This is why a bowl of porridge oats before a 5 km run sustains energy better than a spoonful of sugar — the polysaccharide structure means glucose is absorbed gradually, maintaining a stable blood sugar level rather than a rapid spike and crash. This is particularly important for managing type 2 diabetes, where the glycaemic response to food is a key dietary concern.
Monosaccharides: glucose, fructose, galactose. Disaccharides: sucrose (table sugar), lactose (milk sugar), maltose. Polysaccharides: starch, glycogen, non-starch polysaccharides (dietary fibre). The written paper can ask students to classify a named carbohydrate — knowing which group each falls into is a reliable recall mark.
Malnutrition means ‘wrong nutrition’ in either direction — too much or too little. Protein deficiency causes stunted growth, poor wound healing, muscle wasting and impaired immunity; in severe global poverty, the condition kwashiorkor (characterised by oedema) can develop. Excess protein puts strain on the kidneys, which must excrete the nitrogenous waste. Excess saturated fat raises CVD risk and contributes to obesity; fat deficiency impairs fat-soluble vitamin absorption and disrupts hormone production. Excess refined carbohydrate / free sugars contributes to type 2 diabetes, dental caries and obesity; insufficient carbohydrate causes fatigue and forces the body to break down protein and fat for energy, producing ketones. For the exam, tie each consequence to a named individual (e.g. an obese adult, a vegan with inadequate protein, a child with dental caries) to access AO2 application marks.