How much energy does the body need, and how do we calculate it from BMR and activity?
Energy needs: the sources of energy from food, basal metabolic rate (BMR) and physical activity level (PAL), how energy requirements vary with age, sex and activity, energy balance, and the proportion of energy that should come from each macronutrient.
A focused answer on energy needs for OCR GCSE Food Preparation and Nutrition (J309), covering energy from macronutrients, basal metabolic rate (BMR), physical activity level (PAL), how needs vary with age, sex and activity, energy balance, and calculating total energy requirements.
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What this dot point is asking
OCR wants you to understand where the body gets energy, how much it needs, and how to estimate the requirement using basal metabolic rate (BMR) and physical activity level (PAL). You also need energy balance and how the recommended proportions of energy from each macronutrient fit in.
Energy from food
Energy is measured in kilocalories (kcal) or kilojoules (kJ), where kcal is about kJ. The three macronutrients supply this energy:
- Carbohydrate: about kcal per gram.
- Protein: about kcal per gram.
- Fat: about kcal per gram (the most energy-dense).
Alcohol also provides energy (about kcal per gram) but is not a nutrient the body needs.
Basal metabolic rate (BMR)
BMR is higher in people with more muscle (muscle uses more energy than fat), in larger and taller people, in men compared with women on average, and in younger people; it falls slowly with age. This is one reason men and growing teenagers usually need more energy than older or smaller people.
Physical activity level (PAL)
Total daily energy requirement is found by multiplying BMR by PAL:
So an adult with a BMR of kcal and a PAL of needs kcal a day.
How energy needs vary
Energy needs change with age (babies and growing teenagers need a lot for their size; needs fall in older age as BMR drops and activity often falls), sex (men usually need more than women because of larger size and more muscle), body size and composition (larger and more muscular bodies need more), activity (active jobs and exercise raise needs), and special states such as pregnancy and breastfeeding (extra energy). Government reference intakes are about kcal a day for an average woman and kcal for an average man, but these are guides, not targets for every individual.
Energy balance
- If energy in is consistently greater than energy out, the surplus is stored as fat, so the person gains weight and may become overweight or obese, raising the risk of type 2 diabetes, heart disease and high blood pressure.
- If energy in is consistently less than energy out, the body uses fat and then muscle for energy, so the person loses weight; if extreme this causes being underweight, tiredness, a weak immune system and, in children, poor growth.
The macronutrient split
Government guidance suggests roughly half of energy from carbohydrate (mostly starch, not free sugar), no more than about 35% from fat (with a low share of saturated fat), and the remainder from protein. You may be asked to check whether a meal fits this split, which means working out the energy from each macronutrient and finding it as a percentage of the total.
Try this
Q1. State the equation linking total energy requirement, BMR and PAL. [1 mark]
- Cue. Total energy equals BMR multiplied by PAL.
Q2. A person has a BMR of 1400 kcal and a PAL of 1.5. Calculate their daily energy need. [2 marks]
- Cue. kcal a day.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20195 marksAn adult has a basal metabolic rate (BMR) of 1500 kcal and a physical activity level (PAL) of 1.6. Calculate their total daily energy requirement and explain what BMR and PAL mean.Show worked answer →
A 5-mark question combining a calculation with definitions.
Total energy = BMR multiplied by PAL: kcal per day.
BMR (basal metabolic rate) is the amount of energy the body uses at complete rest just to stay alive: to keep the heart beating, the lungs breathing, the body warm and the organs working. PAL (physical activity level) is a number that represents how active a person is; it multiplies the BMR to allow for the energy used in movement and exercise.
Markers award method marks for multiplying BMR by PAL and the correct answer of 2400 kcal, plus clear definitions of BMR (resting energy) and PAL (activity multiplier).
OCR 20216 marksExplain what is meant by energy balance, and describe what happens to the body when energy intake is consistently greater than, or less than, energy used.Show worked answer →
A 6-mark free-response question.
Energy balance is when the energy taken in from food and drink equals the energy used by the body (in BMR and physical activity), so body weight stays stable.
If energy intake is consistently greater than energy used, the surplus energy is stored as fat, so the person gains weight and over time may become overweight or obese, raising the risk of type 2 diabetes, heart disease and high blood pressure.
If energy intake is consistently less than energy used, the body uses its fat and then muscle stores for energy, so the person loses weight; if extreme or prolonged this leads to being underweight, tiredness, a weak immune system and, in children, poor growth.
Top-band answers (5 to 6 marks) define energy balance and explain both directions clearly, naming the health consequences of each.
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