Every cell burns fuel and releases heat. This page explains how energy is spent and stored, how energy needs change across life, how the body keeps its temperature near 37°C, and how to prevent and treat heat illness during exercise.
10Sections
5Figures
4Tables
10Questions
Part 7 · Digestion and Metabolism · Chapter 33 of 47
Learning objectives
By the end of this page you should be able to:
Describe the components of daily energy expenditure.
Explain how energy needs change across the life course.
Describe the main fuel stores and how they are used.
Explain heat production and the routes of heat loss.
Distinguish fever from hyperthermia.
Recognise and manage exertional heat illness.
Why it matters
Exercise prescription changes energy balance, and patients often ask how exercise affects weight. Physiotherapists also supervise exercise in hot environments, where heat illness can be fatal if not recognised and treated quickly.
Older adults, people with spinal cord injury and patients on certain drugs regulate temperature less well, which shapes how we exercise them.
Energy expenditure
Figure 1 · Where daily energy goes
Activity is the part you can change most. In very active people it can exceed half of daily expenditure. Approximate shares.
Basal metabolic rate is the energy needed to keep the body alive at rest. It depends mainly on fat-free mass: muscle and organs. Activity energy expenditure includes both exercise and everyday movement, and varies most between people.
Pontzer and colleagues pooled doubly labelled water measurements from people aged 8 days to 95 years [1]. After allowing for body size, energy expenditure peaked in infancy, settled to adult levels by about 20, stayed stable from 20 to 60, including during pregnancy, and fell after 60 [1].
Figure 2 · Energy use across life
Adult metabolism does not slow down in middle age. The decline begins after about 60 [1]. Schematic of the reported pattern.
This challenges the idea that metabolism slows in middle age. Weight gain in midlife is more about intake and activity than a falling metabolic rate. After 60, falling expenditure partly reflects loss of muscle, which resistance training can slow.
Fuel stores
Store
Approximate energy
Use
Blood glucose
Very small
Moment-to-moment fuel for the brain
Liver glycogen
About 400 kcal
Maintains blood glucose (Chapter 32)
Muscle glycogen
About 1,500–2,000 kcal
Fuel for that muscle in moderate to hard exercise
Fat
Tens of thousands of kcal
Main fuel at rest and in long, steady exercise
Protein
Large, but structural
Used for fuel mainly in starvation or severe illness
At rest and in light exercise, fat supplies most energy. As intensity rises, carbohydrate supplies a growing share. In severe illness, muscle protein is broken down for fuel and repair, which is why patients lose muscle so quickly in intensive care.
Watch out
Unplanned weight loss, low appetite and muscle wasting in older or ill patients signal malnutrition. Exercise without enough energy and protein can worsen it. Screen and refer early.
Temperature regulation
Core temperature is held near 37°C by a control centre in the hypothalamus. It compares signals from skin and core sensors with a set point and adjusts heat production and loss.
Figure 3 · Heat balance
In hot, humid conditions, evaporation fails. When air temperature exceeds skin temperature, sweat evaporation is the only route left, and humidity blocks it.
When too hot
When too cold
Skin vessels dilate, carrying heat to the surface
Skin vessels constrict to keep heat in
Sweating; evaporation removes heat
Shivering produces heat
Behaviour: shade, less clothing, slow down
Behaviour: shelter, clothing, movement
In exercise, muscle heat production can rise more than tenfold. Skin and muscle then compete for blood flow (Chapter 15), and fluid lost as sweat reduces blood volume. Heat acclimatisation over about two weeks increases sweating, plasma volume and comfort in the heat.
Figure 4 · Fever is not the same as heat illness
The difference decides treatment. Fever settles when the cause is treated; exertional hyperthermia needs rapid cooling.
Exertional heat illness
Condition
Features
Action
Heat cramps
Painful muscle cramps during or after exercise
Rest, stretch, fluids and salt
Heat syncope
Fainting on stopping or standing
Lie down, legs raised, cool, fluids
Heat exhaustion
Fatigue, dizziness, nausea, core under 40°C, normal mental state
Stop, cool, fluids; monitor
Exertional heat stroke
Core above about 40.5°C with confusion, collapse or seizures
Emergency: cool first, transport second [2]
The athletic trainers’ position statement stresses that exertional heat stroke remains a leading cause of sudden death in sport, and that rapid on-site cooling is the key to survival [2]. Rectal temperature is the reliable measure; ear and forehead readings can be misleading [2].
Figure 5 · Exertional heat stroke: cool first, transport second
Speed of cooling decides survival. Based on the athletic trainers’ position statement [2].
Practical skill · exercising in heat
Check the conditions, schedule sessions in cooler times, build up gradually over 10–14 days, and plan fluids [2]. Anyone who becomes confused or collapses during hot exercise is treated as heat stroke until proven otherwise.
Clinical relevance for the physiotherapist
Weight management. Adult metabolism is stable until about 60; activity and intake drive change [1].
Older adults. Expenditure and muscle decline after 60; resistance training helps preserve muscle [1].
Heat. Acclimatise, schedule and monitor; cool first in heat stroke [2].
Spinal cord injury. Impaired sweating and vasomotor control below the lesion increase heat and cold risk.
Medications. Diuretics, beta-blockers and some psychiatric drugs impair heat tolerance.
Transporting a heat stroke patient before cooling [2].
Using ear or forehead temperature to rule out heat stroke [2].
Telling adults their weight gain is due to a slowing metabolism [1].
Treating fever and hyperthermia the same way.
Ignoring humidity when judging heat risk.
Exam preparation
Long answer
Describe the regulation of body temperature. Add a note on heat stroke.
Heat production and loss (Figure 3).
Hypothalamic control and effector responses (table).
Fever versus hyperthermia (Figure 4).
Acclimatisation.
Exertional heat stroke recognition and treatment [2] (Figure 5).
Short notes
Basal metabolic rate.
Energy expenditure across life [1].
Fever.
Heat exhaustion versus heat stroke [2].
Viva and practical
“What is the largest part of daily energy use?” — basal metabolic rate.
“Does metabolism slow at 40?” — not when adjusted for body size [1].
“First action in heat stroke?” — cool on site, then transport [2].
OSCE: plan an outdoor session on a hot day.
Current evidence
Exam and traditional teaching
Metabolism slows steadily through adult life; heat stroke victims should be rushed to hospital.
What the evidence adds
Adjusted energy expenditure is stable from 20 to 60 and falls after [1].
Rapid on-site cooling before transport saves lives in exertional heat stroke [2].
What the student should understand
Energy balance is shaped by activity, and heat balance can fail quickly in exercise. Prepare for heat and act fast when it goes wrong.
Check yourself
10 questions on this page. Tap one to see the answer and the reasoning.
Q1. Total daily energy expenditure in the life-course study was measured by:
(A) Food diaries
(B) Doubly labelled water
(C) Heart rate monitors
(D) Questionnaires
Answer: (B) Expenditure was measured by the doubly labelled water method [1].
Q2. Adjusted for fat-free mass, energy expenditure is highest relative to adults at about:
(A) Birth
(B) 1 year of age
(C) 40 years
(D) 80 years
Answer: (B) It peaks about 50% above adult values at about one year [1].
Q3. Between about 20 and 60 years, adjusted energy expenditure:
(A) Falls steadily
(B) Remains stable, even in pregnancy
(C) Doubles
(D) Is unmeasurable
Answer: (B) It remains stable in adulthood, including pregnancy [1].
Q4. Adjusted energy expenditure begins to decline after about:
(A) 20 years
(B) 40 years
(C) 60 years
(D) 90 years
Answer: (C) Decline begins in older adults, after about 60 [1].
Q5. Exertional heat stroke is identified by central nervous system dysfunction plus a core temperature above about:
(A) 37.5°C
(B) 38.5°C
(C) 40.5°C
(D) 43.5°C
Answer: (C) The position statement uses a core temperature above about 40.5°C [2].
Q6. The recommended first treatment of exertional heat stroke is:
(A) Immediate transport without cooling
(B) Rapid whole-body cooling such as cold-water immersion
(C) Paracetamol
(D) Rest in the shade only
Answer: (B) Cool first, transport second, using cold-water immersion [2].
Q7. The most accurate way to measure core temperature in suspected heat stroke is:
(A) Ear thermometer
(B) Rectal temperature
(C) Forehead strip
(D) Oral thermometer
Answer: (B) Rectal temperature is recommended for diagnosis [2].
Q8. Heat acclimatisation is typically achieved over about:
(A) 1 day
(B) 10–14 days
(C) 6 months
(D) Never
Answer: (B) Progressive exposure over about 10–14 days is recommended [2].
Q9. Which statement about adult metabolism is supported by the life-course data?
(A) Metabolism slows markedly in middle age
(B) Metabolism adjusted for body size is stable from 20 to 60
(C) Pregnancy doubles adjusted expenditure
(D) Teenagers have adult values
Answer: (B) Adult values are stable from about 20 to 60 [1].
Q10. A key reason heat illness happens in humid conditions is that:
(A) Shivering increases
(B) Sweat cannot evaporate effectively
(C) The set point falls
(D) Blood volume rises
Answer: (B) Evaporative cooling is limited, which the position statement recognises in its risk guidance [2].
Quick review
Key takeaways
Daily energy: BMR about 65%, activity about 25%, food about 10%.
Adjusted expenditure stable from 20 to 60, falls after 60 [1].
Fat fuels rest and steady exercise; carbohydrate fuels hard exercise.
Heat lost by evaporation, radiation, convection, conduction.
Fever: set point raised; hyperthermia: heat gain exceeds loss.
Heat stroke: core above 40.5°C with confusion; cool first [2].
Further reading
Source
Why it earns its place
Pontzer H et al. — Science, 2021
Energy expenditure across life [1]
Casa DJ et al. — J Athl Train, 2015
Exertional heat illness [2]
McArdle WD et al. — Exercise Physiology
Energy and thermoregulation in exercise
References
Pontzer H, Yamada Y, Sagayama H, et al. Daily energy expenditure through the human life course.Science. 2021;373(6556):808–812.
doi:10.1126/science.abe5017PMID 34385400Pooled cohort analysis
Casa DJ, DeMartini JK, Bergeron MF, et al. National Athletic Trainers’ Association position statement: exertional heat illnesses.Journal of Athletic Training. 2015;50(9):986–1000.
doi:10.4085/1062-6050-50.9.07PMID 26381473Position statement
