Macronutrients: Carbohydrates, Proteins and Lipids
What this domain tests
Macronutrients supply energy and building material. Carbohydrates provide 4 kcal/g, proteins 4 kcal/g and fats 9 kcal/g. The exam tests functions, food sources, digestion and the effects of deficiency and excess.
Core rules
- Carbohydrates include sugars, starch and fibre. Starch and sugars supply energy; fibre aids bowel function, slows glucose absorption and lowers cholesterol.
- Proteins are made of amino acids. Nine are essential for adults; animal proteins are generally complete, while most plant proteins are limiting in one or more amino acids. Cereal–pulse combinations improve quality.
- Fats supply essential fatty acids and carry fat-soluble vitamins. Saturated fats raise LDL cholesterol; unsaturated fats are preferable, but all fats are energy-dense.
- Digestion begins in the mouth for carbohydrate, in the stomach for protein, and in the small intestine for fat, with bile emulsifying fat.
- Excess energy from any macronutrient is stored as fat.
Worked example
A meal contains 50 g carbohydrate, 10 g protein and 10 g fat. Energy = (50 × 4) + (10 × 4) + (10 × 9) = 200 + 40 + 90 = 330 kcal. Reasoning: use the Atwater factors, not the weight of food. If the same meal had 20 g fat, energy would rise by 90 kcal, showing how fat increases energy density without adding bulk.
Traps that cost marks
- Using 9 kcal/g for protein or 4 kcal/g for fat.
- Saying all plant proteins are complete proteins.
- Treating fibre as a nutrient that supplies 4 kcal/g in the usual sense.
- Forgetting that essential fatty acids must come from the diet.