Energy needs: sources of energy from food, basal metabolic rate (BMR) and physical activity level (PAL), how requirements vary with age, sex and activity, energy balance, and the proportion of energy from each macronutrient.

What this dot point is asking

Eduqas 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 $1$ kcal is about $4.2$ kJ. The three macronutrients supply this energy:

• Carbohydrate: about $4$ kcal per gram.
• Protein: about $4$ kcal per gram.
• Fat: about $9$ kcal per gram (the most energy-dense).

Alcohol also provides energy (about $7$ 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 $1500$ kcal and a PAL of $1.6$ needs $1500 \times 1.6 = 2400$ 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 $2000$ kcal a day for an average woman and $2500$ 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. $1400 \times 1.5 = 2100$ kcal a day.