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Topic 2 of 6 · Form 5

Nutrition, energy and metabolism

Energy values: carbohydrate 17 kJ (4 kcal) per gram; protein 17 kJ (4 kcal); fat 37 kJ (9 kcal); alcohol 29 kJ (7 kcal).

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  1. PDFNutrition, energy and metabolism – Food Science and Nutrition notes (ZIMSEC A Level)47.6 KB

Macronutrients and energy

Energy values: carbohydrate 17 kJ (4 kcal) per gram; protein 17 kJ (4 kcal); fat 37 kJ (9 kcal); alcohol 29 kJ (7 kcal).

Worked example: A snack contains 30 g carbohydrate, 5 g protein and 10 g fat. Find its energy value in kcal.

30 × 4 + 5 × 4 + 10 × 9 = 120 + 20 + 90 = 230 kcal

Energy requirements

Basal metabolic rate (BMR) × physical activity level (PAL) = estimated average requirement. Factors: age, sex, body size, activity, pregnancy and lactation, climate.

Micronutrients

Fat-soluble vitamins (A, D, E, K) and water-soluble vitamins (B group, C); minerals (iron – haem and non-haem; calcium with vitamin D; iodine; zinc). Bioavailability: vitamin C improves non-haem iron absorption; phytates and tannins reduce it.

Protein quality

Biological value, limiting amino acids (lysine in maize, methionine in legumes); protein complementation (sadza with beans).

Dietary reference values

Recommended intakes for population groups; nutrition labelling; food-based dietary guidelines (variety, fruits and vegetables, legumes, limiting salt, sugar and fat).

Assessing nutritional status

Anthropometry (BMI = mass in kg ÷ height in m²; mid-upper arm circumference for children), biochemical tests, clinical signs and dietary surveys.

Worked example: Find the BMI of an adult of 70 kg and 1.75 m.

70 ÷ 1.75² = 70 ÷ 3.0625 = 22.9 (normal range 18.5–24.9)

Exam tip: Always state units – kJ or kcal – and convert carefully (1 kcal = 4.184 kJ).

Check yourself

  1. Why are maize and beans eaten together a good protein combination?
  2. Name a factor that increases non-haem iron absorption.
  3. Find the energy in 20 g of fat in kJ.
Show answers
  1. Maize lacks lysine and beans lack methionine; together they complement each other.
  2. Vitamin C (ascorbic acid).
  3. 20 × 37 = 740 kJ
Next topicDiet, health and food security

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