Explain energy balance, calculate the energy provided by food from its macronutrients, and describe the factors that affect a person's energy needs

What this dot point is asking

SEAB wants you to explain energy balance, to calculate the energy in a food from its protein, fat and carbohydrate content, and to describe the factors that change how much energy a person needs. The big idea is a simple comparison: energy in (from food) against energy out (used by the body). When they match, weight is steady; when they do not, weight goes up or down. The calculation and the factors are common, quick marks.

The answer

Where energy comes from and its units

The body gets energy from three macronutrients. Energy is measured in kilojoules (kJ) or kilocalories (kcal):

• Protein: about $17\ \text{kJ}$ ($4\ \text{kcal}$) per gram.
• Carbohydrate: about $16\ \text{kJ}$ ($4\ \text{kcal}$) per gram.
• Fat: about $37\ \text{kJ}$ ($9\ \text{kcal}$) per gram.

Fat is the most energy-dense, giving more than twice the energy of protein or carbohydrate, which is why fatty foods are easy to over-consume.

Calculating the energy in a food

To find the energy in a food, multiply the grams of each macronutrient by its energy value and add the results. This is exactly how the energy figure on a nutrition label is worked out.

What energy balance means

Energy balance is when the energy taken in from food equals the energy used by the body for movement and its internal processes. In balance, body weight stays steady. There are two ways to be out of balance:

• Energy in greater than energy out: the extra is stored as fat, so weight increases. Over a long time this leads to overweight or obesity.
• Energy in less than energy out: the body uses its stored fat (and then muscle) for energy, so weight decreases.

Factors that affect energy needs

Different people need different amounts of energy. The main factors are:

• Age: children and teenagers need plenty for growth; needs fall in older age.
• Gender: males generally need more than females of the same age.
• Body size: a larger body uses more energy.
• Activity level and occupation: an active person or a manual worker needs more than someone who is mostly seated.
• Special states: pregnancy, breastfeeding and recovery from illness raise needs.

Examples in context

Example 1. The energy in a local breakfast. Two kaya toast sets with half-boiled eggs and kopi can add up to a substantial amount of energy, much of it from the butter and sugar (fat and carbohydrate). Working out the energy from the macronutrients shows why a seemingly small breakfast can be high in energy, helping a person keep their intake in balance.

Example 2. Energy needs of an active student. A teenager on the school football team uses far more energy than a classmate who is mostly seated, because of higher activity and the energy of growth. The footballer therefore needs larger or more frequent meals to stay in energy balance, showing how activity level changes daily energy needs.

Try this

Q1. State the energy value in kcal of 1 g of protein, 1 g of fat and 1 g of carbohydrate. [3 marks]

• Cue. Protein 4 kcal, fat 9 kcal, carbohydrate 4 kcal per gram.

Q2. A food contains 5 g protein, 8 g fat and 15 g carbohydrate. Calculate its energy in kilocalories. [3 marks]

• Cue. $(5 \times 4) + (8 \times 9) + (15 \times 4) = 20 + 72 + 60 = 152$ kcal.

Q3. State three factors that affect how much energy a person needs each day. [3 marks]

• Cue. Any three of: age, gender, body size, activity level or occupation, and special states such as pregnancy.