By the end of this lesson
- Relate activity level to energy requirement.
- Explain effects of age and pregnancy.
- Apply energy balance without oversimplifying health.
BIG QUESTION
Why can two people eating the same meal move toward opposite energy balances?
Energy requirement reflects basal metabolism, activity, growth and physiological state—not one universal number.
Energy requirement has several components
basal metabolismThe energy the body needs at complete rest to keep vital processes going, such as breathing, heartbeat and keeping warm. supplies energy for processes such as ion transport, breathing, circulation and temperature regulation at rest. Physical activity adds variable demand.
Digestion, growth, tissue repair and thermoregulation also contribute, so an estimate is not a fixed personal constant.
Activity changes demand quickly
Muscle contraction requires ATP. More frequent, intense or prolonged activity therefore raises respiratory substrate use and total energy expenditure.
Occupation and daily movement matter as well as planned sport.
Age changes growth and body composition
Children and adolescents need energy for growth as well as maintenance and activity. Requirements per kilogram are often higher during rapid growth.
With ageing, average muscle mass and activity may fall, reducing total requirement, but individual variation is large.
Pregnancy requires additional resources
Pregnancy supports growth of fetal and maternal tissues and changes maternal metabolism. Energy and nutrient requirements can increase, particularly later in pregnancy.
“Eating for two” is misleading: quality and appropriate increase matter more than doubling intake.
Energy balance is not a moral judgement
When intake persistently exceeds expenditure, energy stores tend to increase; when it remains below expenditure, stores tend to decrease.
Body mass is also influenced by water, illness, medication, hormones, access to food and other factors. A short observation cannot diagnose a person’s health.
Factors affecting requirements
Use a biological reason rather than simply saying someone “needs more food.”
| Factor | Likely effect | Biological reason |
|---|---|---|
| Greater activity | Higher requirement. | More muscle contraction and respiration. |
| Rapid growth | Higher requirement. | Synthesis of new cells and tissues. |
| Older age on average | May lower requirement. | Often less growth, muscle mass or activity. |
| Pregnancy | Requirement increases. | Growth of fetal/maternal tissues and altered metabolism. |
| Cold environment | May raise requirement. | More energy used in thermoregulation. |
Language that earns precision
- energy requirement
- Energy needed to support metabolism, activity, growth and physiological state.
- basal metabolism
- Minimum energy expenditure supporting essential functions at rest.
- energy balance
- Relationship between energy intake and expenditure over time.
- kilojoule
- A unit of energy equal to 1000 joules.
Common misconceptions
Claim: Everyone of the same age needs the same energy intake.
Correction: Body size, activity, health and physiology differ.
Claim: Older people never need much energy.
Correction: Needs vary greatly and can remain high with activity or illness.
Claim: Pregnancy requires double the energy intake.
Correction: Requirement rises, but not by a simple doubling.
Claim: One high-energy meal immediately causes lasting fat gain.
Correction: Longer-term energy balance and many physiological factors determine body stores.
Match the answer to the command word
Link the named factor to a cellular energy demand.
Keep units consistent and state whether the result is intake, expenditure or balance.
Avoid universal claims from population averages.
Recall, apply, then justify
Explain why a highly active person generally needs more dietary energy.
Hint
Muscle → ATP → respiration.
Suggested marking points
- Muscle contraction requires ATP/energy released by respiration.
- More activity increases respiration and substrate use.
- Greater intake may be needed to maintain energy balance.
Explain two reasons why an adolescent may require more energy than an inactive older adult.
Hint
Use growth and activity/body composition.
Suggested marking points
- Adolescents may be synthesising new tissue during growth.
- Growth requires energy and materials.
- They may have higher activity or metabolically active tissue.
- The comparison is a tendency and individuals vary.
A person consumes 9,200 kJ and expends 10,000 kJ in one day. Calculate the energy balance and interpret it.
Hint
Intake minus expenditure.
Suggested marking points
- 9,200 − 10,000 = −800 kJ.
- There is an 800 kJ deficit for that day; one day alone does not establish a long-term trend.
Critique the use of a single population-average energy value as a personal dietary target.
Planning hint
Identify both usefulness and limits.
What a strong design includes
- It provides a simple reference for planning and comparison.
- Individual body size and composition vary.
- Activity and environmental temperature vary.
- Growth, pregnancy, illness and medication can alter requirements.
- Food-label intake and expenditure estimates have uncertainty.
- A range or personalised longitudinal evidence is more defensible than one exact universal value.