Table of Contents
What Is Basal Metabolic Rate?
Basal metabolic rate (BMR) is the amount of energy an organism uses per unit time in a state of complete rest and under precisely defined standard conditions. It reflects the minimum energy needed to keep vital functions going:
- maintenance of ion gradients (e.g. Na\^+/K\^+-ATPase in membranes)
- basic activity of heart and respiratory muscles
- baseline activity of liver, kidneys, brain, endocrine system
- continuous synthesis and breakdown of cell components (turnover)
- minimal thermoregulation
BMR is usually expressed as energy per time, for example:
- in humans: kJ/day or kcal/day
- in comparative physiology: W (watts) or mL O\_2 per minute
Standard Conditions for Measuring Basal Metabolic Rate
To be called basal (not just “resting”), several conditions must be met:
- organism is awake but physically and mentally at rest
- no food intake for several hours (postabsorptive state; in humans usually 12–14 h fasting)
- comfortable thermoneutral temperature (no shivering, no sweating)
- lying position (or comparable resting posture)
- no acute stress (pain, fear, strong emotions, stimulants)
If one or more of these conditions are not met, the measured value is called resting metabolic rate (RMR), which is usually slightly higher than BMR.
Basal Metabolic Rate as “Base Consumption”
BMR can be seen as the “fixed running costs” of the organism. On top of this base, all additional activities increase energy consumption. Graphically:
$$
\text{Total daily energy expenditure} = \text{BMR} + \text{Energy for activity} + \text{Specific dynamic effect of food} + \text{Thermoregulation (extra)}
$$
The “specific dynamic effect of food” (diet-induced thermogenesis) and additional thermoregulation are typically not included in the basal value.
Factors Influencing Basal Metabolic Rate
Although BMR is defined under standard conditions, it still varies between individuals and species. Important influences include:
Body Size and Body Composition
- Body surface area: Larger animals have a higher absolute BMR, but a lower BMR per unit body mass than small animals.
- Lean mass vs. fat mass:
- Lean tissue (muscles, organs) is metabolically active and strongly increases BMR.
- Adipose tissue has relatively low metabolic activity per unit mass.
In humans, BMR is often approximated from body mass, height, age, and sex using empirical equations (e.g. Harris–Benedict, Mifflin–St Jeor; formulas are not essential here, but they reflect the dependence on these factors).
Sex and Age
- On average, males have a higher BMR than females of the same body mass, largely due to higher muscle proportion.
- Age: BMR is high in infants and children (growth), decreases slowly with age, particularly as muscle mass declines.
Hormones
BMR is strongly affected by endocrine status:
- Thyroid hormones (thyroxine T\_4, triiodothyronine T\_3):
- Elevated levels → markedly increase BMR (hyperthyroidism).
- Low levels → reduce BMR (hypothyroidism).
- Catecholamines (adrenaline, noradrenaline): at rest they have a smaller but measurable effect.
- Sex steroids, growth hormone, and insulin also influence BMR via their effects on tissue growth and metabolism.
Environmental and Physiological State
- Ambient temperature:
- In the thermoneutral zone (TNZ), BMR roughly reflects minimal thermoregulatory cost.
- At temperatures below or above TNZ, metabolism rises for active heating or cooling; these extra costs are not counted in BMR by definition.
- Nutritional status:
- Long-term undernutrition or starvation → BMR decreases (energy-sparing adaptations).
- Overnutrition and some endocrine changes can increase BMR.
- Species-specific adaptations:
- Hibernating animals can drastically lower metabolic rate (basal level during torpor is much lower than normal BMR).
Performance Metabolism (Working Metabolic Rate)
While BMR describes energy turnover at minimal activity, performance metabolism (also called working or activity metabolism) covers energy consumption during any form of work or performance:
- voluntary muscle activity: locomotion, posture, speech, manipulation
- involuntary or reflex activities: shivering, increased ventilation during exercise
- specific work of organs above basal level: e.g. high cognitive load, intensive digestion after large meals, immune responses, lactation
Performance metabolism is typically expressed as the increment above basal or resting metabolism.
Components of Daily Energy Expenditure
From the perspective of a full day (24 h), total energy use can be divided into:
- Basal metabolic rate (BMR) – obligatory costs
- Activity-related energy expenditure – all forms of physical activity
- Diet-induced thermogenesis – energy cost of digestion, absorption, and processing of nutrients
- Adaptive thermogenesis – extra heat production under cold or heat stress, beyond basal thermoregulation
In many humans with a typical lifestyle:
- BMR accounts for roughly 50–70% of total daily expenditure
- Activity and performance metabolism account for 20–40% (but can be much higher in athletes or heavy labor)
- Diet-induced thermogenesis is often around 5–15%
Measuring and Comparing Basal and Performance Metabolism
The parent chapter on determining energy conversion covers methods (direct and indirect calorimetry). Here, the focus is on what is specifically measured:
From Oxygen Consumption to Energy Expenditure
Performance metabolism is often assessed via indirect calorimetry:
- At rest → measurement gives BMR (under basal conditions) or RMR.
- During activity → oxygen consumption (and sometimes CO\_2 production) rises in proportion to metabolic rate.
Energy expenditure can be estimated, for example, as:
$$
\text{Energy expenditure} \approx \text{VO}_2 \times \text{energy equivalent of O}_2
$$
The “energy equivalent” depends slightly on the mix of substrates (fats, carbohydrates, proteins) but is often approximated as about 20 kJ per liter O\_2 for mixed diets.
Work Levels and Metabolic Rate
Performance metabolism is sometimes expressed as a multiple of BMR:
- 1 MET (metabolic equivalent) ≈ resting metabolic rate in humans
- Light office work: ~1.5–2 MET
- Walking briskly: ~3–4 MET
- Heavy physical labor or intense sports: 6–10+ MET
Thus:
$$
\text{Work metabolic rate} = \text{BMR} \times \text{MET}
$$
This representation highlights that performance metabolism adds on top of the basal level.
Aerobic Capacity and Maximum Performance Metabolism
During increasing work intensity, metabolic rate rises until a maximum oxygen uptake is reached (VO\_2 max). Beyond this point, further energy has to come mainly from anaerobic processes:
- VO\_2 max represents maximum aerobic performance metabolism.
- It is a key measure of endurance capacity and cardiovascular fitness.
At VO\_2 max:
- total metabolic rate can reach many times the BMR (e.g. 10–15× in trained humans, much more in some athletic animals).
Relationship Between Basal and Performance Metabolism
BMR and performance metabolism are tightly linked but reflect different aspects:
- BMR captures:
- structural and functional “quality” of the organism
- basic organ sizes and activities
- hormonal status and long-term adaptations
- Performance metabolism reflects:
- current level of muscular and organ work
- flexibility of the organism to respond to demands
- cardiovascular and respiratory capacity to deliver O\_2 and remove CO\_2 and heat
From an ecological and evolutionary viewpoint:
- Species with high BMR often show:
- high activity, rapid growth, high turnover (“fast life histories”)
- Species with low BMR often rely on:
- energy-saving strategies, reduced mobility, or intermittent activity (“slow life histories”)
On the individual level, understanding both BMR and performance metabolism is essential for:
- estimating energy requirements (e.g. nutrition, clinical settings)
- assessing fitness and adaptation to workloads
- analyzing how organisms allocate energy between maintenance, growth, reproduction, and activity.