Energy balance is the relationship between the energy taken into the body through food and drink and the energy consumed through metabolism and physical activity. Surplus energy is stored, whereas an energy deficit requires the body to use stored energy.
The relationship can be represented as:
Energy intake is the chemical energy obtained mainly from carbohydrates, fats, and proteins. This energy is commonly measured in kilojoules or kilocalories.
Energy consumption, more precisely called energy expenditure, has three principal components:
- Basal metabolic rate (BMR): energy required to maintain essential functions at rest, such as breathing and circulation.
- Thermic effect of food: energy used to digest, absorb, and process nutrients.
- Physical activity: energy used during exercise and everyday movement. This is usually the most variable component.
The body's response depends on the relationship between intake and expenditure:
| Energy state | Consequence |
|---|---|
| Intake equals expenditure | Neutral energy balance; body energy stores and body mass are generally maintained. |
| Intake exceeds expenditure | Positive energy balance; surplus energy is stored, potentially increasing body mass. |
| Intake is below expenditure | Negative energy balance; stored energy is mobilized, potentially decreasing body mass. |
Short-term energy is stored mainly as glycogen in the liver and skeletal muscles. Larger long-term surpluses are stored primarily as triglycerides in adipose tissue. During a deficit, glycogen and fat stores provide energy; prolonged or severe deficits may also increase protein breakdown.
A common misconception is that physical activity alone determines energy balance. In reality, energy balance depends on both intake and total expenditure, while factors such as age, sex, body composition, and activity level affect expenditure.
Exam technique: Define energy balance, identify intake and all three components of expenditure, and explain what happens during positive, neutral, and negative balance. Link sustained imbalance to changes in body composition and health, as required in subtopic A.3.2.
