Energy Balance and BMR

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.

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What Is Energy?

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.

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What Is Basal Metabolic Rate (BMR)?

Basal metabolic rate (BMR) is the amount of energy the body needs at complete rest to carry out essential life processes. These include:

• Breathing
• Circulating blood (heartbeat)
• Maintaining body temperature
• Cell growth and repair
• Brain function
• Organ function (liver, kidneys, etc.)

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.

Factors Affecting BMR

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").

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Physical Activity Level (PAL)

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

Calculating Total Energy Requirement

The formula is:

Total daily energy requirement = BMR × PAL

For example:

• A woman with a BMR of 1,400 kcal and a PAL of 1.6 (moderately active):
  • Total energy = 1,400 × 1.6 = 2,240 kcal per day
• A man with a BMR of 1,800 kcal and a PAL of 2.0 (very active):
  • Total energy = 1,800 × 2.0 = 3,600 kcal per day

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Energy Balance

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

Three States of Energy Balance