Short answer
Metabolic rate is the speed at which chemical reactions in your cells release energy from food. Your basal metabolic rate (BMR) is the energy you need at rest just to keep vital organs working. Metabolic rate is higher in people with more muscle, in younger individuals, during exercise, and when the hormone thyroxine is produced in greater quantities.
At a glance
- Key stage
- Key Stage 3
- Subject
- Biology
- Type
- Guide
- For
- Students
- Read time
- 7 min
- Last updated
- 8 October 2026
Where this fits
- Key Stage 3Years 7–9This article
- GCSEYears 10–11
What is metabolic rate?
Metabolism refers to all the chemical reactions happening in the cells of the body — reactions that break down molecules (releasing energy), build new molecules, and carry out the work of living. The metabolic rate is the overall speed of these reactions — how quickly energy is being used by the body at any given moment.
Metabolic rate is usually measured in kilojoules per hour (kJ/h) or sometimes kilocalories per day. You can also think of it as the rate of oxygen consumption, since oxygen is used in aerobic respiration (the main energy-releasing reaction in cells).
What is basal metabolic rate (BMR)?
Basal metabolic rate (BMR) is the minimum rate of energy release needed to keep a person alive and all vital organs functioning, measured at rest (lying down, not having eaten for 12 hours, at a comfortable temperature). BMR accounts for:
- Keeping the heart beating
- Breathing
- Maintaining body temperature at 37 °C
- Brain function
- Kidney filtration and other organ functions
For an average adult, BMR is roughly 6,000–7,500 kJ/day for females and 7,000–9,000 kJ/day for males, though it varies considerably between individuals. BMR represents about 60–70% of total daily energy expenditure in sedentary people.
What factors affect metabolic rate?
| Factor | Effect on metabolic rate | Reason |
|---|---|---|
| Muscle mass | More muscle → higher metabolic rate | Muscle cells are metabolically active even at rest, using energy to maintain muscle proteins |
| Body size | Larger body → higher absolute metabolic rate | More cells means more chemical reactions occurring |
| Age | Younger → higher metabolic rate per kg | Children and teenagers are growing (building new tissue) and have proportionally more muscle; metabolic rate declines from early adulthood |
| Sex | Males tend to have slightly higher metabolic rate | Males typically have a higher proportion of muscle mass than females of the same body weight |
| Exercise | Exercise increases metabolic rate during activity — and for hours afterwards | Active muscles use more oxygen for respiration; the afterburn effect means cells continue repairing and replenishing energy stores |
| Temperature | Cold environment → body increases heat generation | More energy released by respiration to maintain 37 °C core temperature |
| Thyroxine | More thyroxine → higher metabolic rate | Thyroxine (produced by the thyroid gland) increases the rate of respiration in most body cells |
| Illness/fever | Raises metabolic rate | Each degree increase in body temperature speeds up all enzyme-catalysed reactions |
What is the role of thyroxine in controlling metabolic rate?
Thyroxine is a hormone produced by the thyroid gland, a butterfly-shaped gland in the neck. It is one of the main long-term regulators of metabolic rate, acting on almost every cell in the body to increase the rate of respiration and therefore the rate of energy production.
- Too much thyroxine (hyperthyroidism): metabolic rate is too high — the person feels hot, loses weight (even when eating well), has a fast heart rate, feels anxious, and may have trembling hands.
- Too little thyroxine (hypothyroidism): metabolic rate is too low — the person feels cold, gains weight, feels tired, and may feel depressed.
Thyroxine release is controlled by a negative feedback loop involving the hypothalamus and the pituitary gland. When thyroxine levels fall too low, the pituitary releases TSH (thyroid-stimulating hormone), which tells the thyroid to produce more thyroxine — restoring the level.
How does exercise affect metabolic rate?
During exercise, active muscles require much more energy. Metabolic rate can increase by a factor of 10–20 compared to rest during vigorous activity. After exercise stops, metabolic rate remains elevated for minutes to hours — this is the excess post-exercise oxygen consumption (EPOC) or "afterburn". The body continues using energy at an elevated rate to:
- Replenish ATP stores
- Remove lactic acid from anaerobic respiration
- Repair micro-damage to muscle fibres
- Restore normal body temperature
Regular exercise also increases muscle mass over time, which raises resting metabolic rate permanently — this is one of the long-term health benefits of physical activity.
Why does metabolic rate matter for health?
Energy balance compares the energy taken in from food (chemical energy in nutrients) with the energy used by the body (metabolic rate × time). If a person consumes more energy than they use, the surplus is stored as fat (an energy-dense molecule), leading to weight gain. If they consume less than they use, stored fat is broken down — weight is lost.
Understanding metabolic rate helps explain why:
- Two people can eat identical diets but maintain different body weights (due to differences in muscle mass, activity levels, and thyroid function).
- Crash diets that cause rapid weight loss also reduce muscle mass, lowering BMR and making it harder to maintain the lower weight.
- Physical activity is important not just for burning energy during exercise but for building and maintaining muscle.
Frequently asked questions
Does having more muscle really increase metabolic rate at rest?
Yes. Muscle tissue is metabolically active at rest — it uses energy continuously to maintain protein structures, pump ions across membranes, and sustain electrical potential. Fat tissue is much less metabolically active. A person with greater muscle mass therefore uses more energy even while sitting still or sleeping, compared to a person of the same total body weight but less muscle. This is why resistance exercise (building muscle) raises resting metabolic rate, and why simply reducing body fat without maintaining muscle mass does less to improve long-term energy balance.
Why do children seem to have higher metabolic rates than adults?
Children's bodies are actively growing — synthesising new proteins, building bone, and producing new cells — all of which are energy-demanding processes. Children also have a higher surface area to body volume ratio than adults, which means they lose heat more rapidly and must generate more heat to maintain body temperature. In addition, children tend to be more physically active on average than adults. When metabolic rate is expressed per kilogram of body weight, children's values are significantly higher than those of adults, and the difference is most pronounced in infancy and early childhood.
What happens to metabolic rate during a period of starvation?
During prolonged energy restriction (starvation), the body responds by lowering metabolic rate — a form of energy conservation. The brain signals the thyroid to reduce thyroxine output, slowing cellular respiration. Muscle protein may be broken down for fuel (reducing muscle mass and further lowering BMR). This adaptation means that metabolic rate can fall by 15–20% during severe starvation. This is one reason why very low calorie diets produce rapid initial weight loss that slows over time — the body partly compensates. It is also why weight that is lost gradually with a combination of diet and exercise is more likely to be maintained.
How is metabolic rate measured in practice?
The standard method is indirect calorimetry — measuring the rate of oxygen consumption and CO₂ production, from which the rate of energy release can be calculated (since the amount of oxygen consumed per kilojoule released is known for the major fuel molecules — carbohydrate, fat, and protein). At KS3, you will not need to know the detailed method, but you should understand the principle: measuring how much oxygen a person uses per unit time gives a measure of the speed of aerobic respiration and therefore metabolic rate. Simpler estimates use equations based on age, sex, height, and weight (such as the Harris-Benedict equation).
Professor Darwin at aitutors.me can take you through how the body's systems connect to regulate metabolism, quiz you on the factors that affect metabolic rate, and help you build complete exam answers.
Key terms
- Metabolism
- metabolic rate
- kilojoules per hour (kJ/h)
- Basal metabolic rate (BMR)
- Muscle mass
- Body size
- Age
- Sex