School › Food and nutrients › Ages 15–16
Nutrition by numbers: energy, labels and guidelines
Macronutrients and energy values, kcal and kJ, vitamins and minerals, international guidelines given as percentages of energy, and reading food labels quantitatively. With a label investigation that tests the 4–4–9 rule and its uncertainty.
Read
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.[6]
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.[7][1][8]
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.[7]
WHO guidelines are written as proportions of energy: free sugars below 10% of total energy intake (with further benefits below 5%), total fat 30% or less, saturated fat no more than 10% and trans fat no more than 1%. Adults should keep salt below 5 g a day, and everyone over 10 should eat at least 400 g of fruit and vegetables a day.[5]
Labels compare foods per 100 g. In UK traffic-light labelling a food is high in sugars above 22.5 g per 100 g and low at 5 g or less; the high mark is 17.5 g for fat, 5 g for saturates and 1.5 g for salt.[8][3]
Fibre: the UK guideline is 30 g a day for adults and about 25 g for ages 11 to 16, but adults average only about 20 g. Eating plenty of fibre is linked to a lower risk of heart disease, stroke, type 2 diabetes and bowel cancer: an association from studies, not a single cause.[4]
Try it: Testing the 4–4–9 rule on food labels
You need
- 5–6 empty, clean food packets with a full nutrition table (carbohydrate, protein, fat, fibre)
- Calculator or spreadsheet
- Record sheet
Safety
Use empty, clean packets; do not open or taste food (allergies). Analyse products, not people: no personal food diaries or calorie counting of pupils.
What to do
- For each product, copy the values per 100 g: energy (kcal and kJ), fat, carbohydrate and protein.
- Predict the energy: E = 4 × carbohydrate + 4 × protein + 9 × fat (in kcal per 100 g).
- Calculate the difference from the label value and the percentage difference: (predicted − label) ÷ label × 100.
- Check the label's kJ ÷ kcal ratio for each product. Is it close to 4.2?
- Work out the percentage of the product's energy that comes from fat: (9 × fat) ÷ label kcal × 100.
- Evaluate: label values are rounded and 4–4–9 is an average rule. Which products differ most, and what could explain it?
What happens, and why?
The 4–4–9 values are rounded averages, so predictions usually come close to, but not exactly to, the printed energy. Labels give energy in both kcal and kJ, with about 4.2 kJ per kcal. Working out the share of energy from fat or sugars lets you compare a product with guidelines that are written as percentages of energy.[6][1][5]
Play: Carbohydrate, protein or fat?
Each statement describes one macronutrient. Which one?
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Quiz
Other ages: Ages 6–7 · Ages 10–11