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Macronutrients (carbohydrate, protein and fat) are needed every day in large amounts. As a rounded rule, a gram of carbohydrate or protein provides about 4 kcal and a gram of fat about 9 kcal. Excess energy is stored by the body as fat.

The nutritional Calorie (kcal) is the heat needed to raise the temperature of 1 kg of water by 1 °C. Labels give energy in kcal and kJ: the NHS quotes 2,000 kcal as 8,400 kJ, so 1 kcal ≈ 4.2 kJ. Energy needs vary with age, size and activity.

Micronutrients: vitamins are organic compounds needed for the body's chemical reactions; minerals are inorganic and cannot be made in the body. Fat-soluble vitamins (A, D, E, K) can build up in the body, while excess water-soluble vitamins (the B vitamins and C) are lost in urine, so overload is rare except with supplements.

Experiment: Testing the 4–4–9 rule on food labels

1. For each product, copy the values per 100 g: energy (kcal and kJ), fat, carbohydrate and protein.
2. Predict the energy: E = 4 × carbohydrate + 4 × protein + 9 × fat (in kcal per 100 g).
3. Calculate the difference from the label value and the percentage difference: (predicted − label) ÷ label × 100.
4. Check the label's kJ ÷ kcal ratio for each product. Is it close to 4.2?
5. Work out the percentage of the product's energy that comes from fat: (9 × fat) ÷ label kcal × 100.
6. Evaluate: label values are rounded and 4–4–9 is an average rule. Which products differ most, and what could explain it?

Product	Label energy (kcal/100 g)	Predicted 4–4–9 energy (kcal)	% difference	% of energy from fat

Questions

1. A cereal has 68 g carbohydrate, 10 g protein and 6 g fat per 100 g. Predict its energy with 4–4–9 and convert it to kJ.

2. The same cereal has 18 g sugars per 100 g. Is it high, medium or low in sugars on a traffic-light label? Explain.

3. Explain why guidelines such as 'free sugars below 10% of energy' are written as percentages, and calculate 5% of 2,000 kcal in grams of sugar.

4. Give two reasons why your 4–4–9 predictions did not match the labels exactly.

Sources for every fact on this sheet: echoesof.science/school/healthy-eating/sources/

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