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This lesson covers energy balance, basal metabolic rate (BMR), and physical activity level (PAL), as required by AQA GCSE Food Preparation and Nutrition specification 8585, section 3.2.3. You must understand how the body uses energy, what determines how much energy a person needs, and the consequences of energy imbalance.
In nutrition, energy is the fuel the body needs to carry out all its functions. Energy comes from the macronutrients in food: carbohydrates, fats, proteins, and (to a lesser extent) alcohol. Energy is measured in kilocalories (kcal) or kilojoules (kJ).
| Macronutrient | Energy per gram |
|---|---|
| Carbohydrate | 3.75 kcal (16 kJ) |
| Protein | 4 kcal (17 kJ) |
| Fat | 9 kcal (37 kJ) |
| Alcohol | 7 kcal (29 kJ) |
Exam Tip: Fat provides more than twice the energy per gram compared to carbohydrate or protein. This is why high-fat foods are energy-dense and why excessive fat consumption can easily lead to energy surplus and weight gain.
Basal metabolic rate (BMR) is the amount of energy the body needs at complete rest to carry out essential life processes. These include:
BMR accounts for approximately 60–75% of a person's total daily energy expenditure. Even if you lay in bed all day and did nothing, your body would still use this much energy just to stay alive.
| Factor | Effect on BMR |
|---|---|
| Age | BMR decreases with age because muscle mass declines and is replaced by fat (which is less metabolically active). |
| Sex | Males generally have a higher BMR than females because they tend to have more muscle mass and larger body size. |
| Body size | Larger bodies have a higher BMR because there is more tissue to maintain. Taller and heavier people use more energy at rest. |
| Body composition | More muscle mass = higher BMR. Muscle tissue is more metabolically active than fat tissue, so it uses more energy at rest. |
| Genetics | Some people inherit a naturally faster or slower metabolism. |
| Climate | People living in cold climates may have a slightly higher BMR because the body uses energy to maintain its temperature. |
| Health | Illness, fever, and certain medical conditions (e.g. hyperthyroidism) can increase BMR. |
Exam Tip: If asked to explain why two people of the same age need different amounts of energy, refer to the factors above — particularly sex, body size, body composition, and activity level. Always explain why the factor matters (e.g. "Males typically have more muscle mass, which is more metabolically active, so they need more energy at rest").
Physical activity level (PAL) is a number that represents how active a person is. It is used alongside BMR to calculate a person's total daily energy requirement.
| Activity Level | PAL Value | Examples |
|---|---|---|
| Sedentary (little or no exercise) | 1.4 | Office worker, student who does not exercise |
| Moderately active | 1.6–1.7 | Person who walks regularly, does light exercise |
| Active | 1.8–1.9 | Person who exercises most days, manual worker |
| Very active | 2.0–2.4 | Athlete, heavy manual labourer, professional sports player |
The formula is:
Total daily energy requirement = BMR × PAL
For example:
Energy balance describes the relationship between the energy a person takes in from food and drink (energy in) and the energy the body uses through BMR, physical activity, and the thermic effect of food (energy out).
graph LR
A["Energy In<br/>(food and drink)"] -->|"compared to"| B["Energy Out<br/>(BMR + activity + thermic effect)"]
B --> C{"Balance?"}
C -->|"Energy In = Energy Out"| D["Energy Balance<br/>Weight stays the same"]
C -->|"Energy In > Energy Out"| E["Energy Surplus<br/>Weight gain"]
C -->|"Energy In < Energy Out"| F["Energy Deficit<br/>Weight loss"]
style D fill:#27ae60,color:#fff
style E fill:#e74c3c,color:#fff
style F fill:#f39c12,color:#fff
| State | What It Means | Consequence |
|---|---|---|
| Energy balance | Energy in = energy out | Body weight remains stable. This is the ideal state for most adults. |
| Positive energy balance (surplus) | Energy in > energy out | Excess energy is stored as body fat. Over time, this leads to weight gain and potentially obesity. |
| Negative energy balance (deficit) | Energy in < energy out | The body uses its energy reserves (glycogen, then fat, then muscle). This leads to weight loss. Prolonged deficit can cause malnutrition. |
| Component | Proportion of Total | Description |
|---|---|---|
| BMR | ~60–75% | Energy used at rest for essential body functions |
| Physical activity | ~15–30% | Energy used during movement and exercise |
| Thermic effect of food (TEF) | ~10% | Energy used to digest, absorb, and process food |
Understanding energy balance is essential for preventing diet-related health conditions:
| Life Stage | Average Daily Energy Needs | Notes |
|---|---|---|
| Children (1–3 years) | ~1,200 kcal | High energy needs per kg body weight for growth |
| Children (4–6 years) | ~1,500 kcal | Active and growing |
| Boys (11–14 years) | ~2,200 kcal | Growth spurts, increasing muscle mass |
| Girls (11–14 years) | ~1,800 kcal | Growth and development |
| Boys (15–18 years) | ~2,800 kcal | Peak energy requirements |
| Girls (15–18 years) | ~2,100 kcal | High but lower than boys |
| Adult men (19–64) | ~2,500 kcal | Maintenance, matched to activity |
| Adult women (19–64) | ~2,000 kcal | Maintenance, matched to activity |
| Elderly (65+) | ~2,000 kcal (men), ~1,800 kcal (women) | Reduced BMR and activity |
| Pregnant women (3rd trimester) | Extra ~200 kcal/day | Supporting foetal growth |
| Breastfeeding women | Extra ~500 kcal/day | Producing breast milk |
Exam Tip: These are approximate values. The exact energy requirement depends on the individual's BMR, PAL, body size, and body composition. In the exam, use these as a guide and always explain the factors that cause variation.
A meal contains 50 g carbohydrate, 20 g protein, and 15 g fat. Calculate the total energy in kcal.
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