Components of Energy Expenditure
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Direct answer
Three compartments make up daily energy expenditure. Basal metabolic rate consumes roughly 60-70 per cent — the cost of keeping cells alive. The thermic effect of food burns about 10 per cent of intake, and physical activity, the most variable compartment, claims 15-30 per cent in sedentary to moderately active adults; within it sits non-exercise activity thermogenesis (NEAT), the fidgeting, standing and walking that can differ between individuals by hundreds of kilocalories a day. Measurement rests on indirect calorimetry — oxygen consumption and carbon dioxide production converted by the Weir equation — with doubly labelled water measuring free-living expenditure over one to two weeks; the respiratory quotient, VCO2 divided by VO2, reveals what mixture of fuels is burning (0.7 fat, 1.0 carbohydrate).
What you must remember
- The split to quote: BMR 60-70 per cent, thermic effect of food about 10 per cent, physical activity 15-30 per cent — with physical activity the only freely modifiable compartment.
- Thermic effect by nutrient: protein costs 20-30 per cent of its own calories to process, carbohydrate 5-10 per cent, fat barely 0-3 per cent — the mechanistic case for high-protein weight-loss diets.
- Respiratory quotient: 0.7 for pure fat oxidation, 1.0 for carbohydrate, about 0.85 on a mixed diet, and above 1.0 signals lipogenesis (or overfeeding).
- Weir equation: energy (kcal/day) = (3.9 × VO2 + 1.1 × VCO2) × 1440, with gas volumes in litres per minute — the number the ventilated-patient cart produces.
- Doubly labelled water: drink water labelled with deuterium and oxygen-18; the difference in their washout rates equals carbon dioxide production — the gold standard for free-living expenditure.
- NEAT: the adaptive compartment; experimental overfeeding shows lean gainers burn hundreds of extra kilocalories through spontaneous movement while low-NEAT gainers store them.
- Disease multipliers: major burns can approach double basal needs; sepsis and trauma raise expenditure 20-50 per cent; fever adds roughly 10-13 per cent per degree Celsius (classically quoted).
- Overfeeding's respiratory price: excess carbohydrate drives VCO2 up, and in a CO2-retaining COPD patient, hypercaloric feeding worsens hypercapnia — a genuine bedside consequence of the RQ concept.
An ICU patient, fed by equation
A ventilated patient with chronic obstructive pulmonary disease fails weaning trials while on high-calorie enteral feed. The indirect calorimetry cart reports VO2 280 mL/min and VCO2 240 mL/min: respiratory quotient 0.86, resting energy expenditure about 2400 kcal/day by the Weir equation — yet the prescription was 3000 kcal, mostly carbohydrate. The excess carbon dioxide must be exhaled by lungs that cannot exhale it; the RQ pushed toward and above 1.0 reflects conversion of surplus carbohydrate to fat, a CO2-generating process. Reduce the feed toward the measured expenditure, shift the composition toward more fat (lower RQ per calorie), and the arterial CO2 eases, unloading the ventilator. One measurement prevented an iatrogenic failure-to-wean. For contexts without a metabolic cart, predictive equations (Harris-Benedict and successors, multiplied by stress factors) are the fallback — accurate to within 10-15 per cent at best, which is why guideline language prefers measured expenditure wherever available.
How the exam frames it
The one-mark staples: largest compartment (basal), most variable (physical activity), highest thermic effect (protein), RQ of fat versus carbohydrate (0.7 and 1.0). The applied traps follow. Students quote RQ 0.7 as "anaerobic" — it is fat oxidation; anaerobic metabolism is a different axis entirely. The RQ above 1.0 gets called impossible — it is lipogenesis or overfeeding, and naming it is a distinction mark. Direct versus indirect calorimetry: heat capture versus gas exchange; doubly labelled water extends measurement into free-living life, which neither chamber method achieves. Indian postgraduate vivas tie the compartment model to obesity counselling: prescribing exercise alone rarely outpaces a small excess intake (a 500 kcal daily surplus dwarfs a 30-minute walk), so energy-in modification must accompany energy-out increases — and NEAT, the least glamorous compartment, may be where the largest long-term differences live.
Frequently asked questions
Which component contributes most to daily energy expenditure?
Basal metabolic rate, roughly 60-70 per cent, reflecting liver, brain, heart and kidney work at rest.
Why does protein have the highest thermic effect?
Its processing — deamination, urea synthesis, gluconeogenesis — is metabolically expensive, consuming 20-30 per cent of its own energy as heat.
What does a respiratory quotient of 0.7 versus 1.0 indicate?
0.7 indicates predominantly fat oxidation and 1.0 carbohydrate oxidation; a mixed diet yields about 0.85, and values above 1.0 indicate net lipogenesis.
What is NEAT?
Non-exercise activity thermogenesis — the energy of spontaneous standing, fidgeting and ambulation — which varies between individuals by hundreds of kilocalories per day.
Which technique measures free-living energy expenditure over days?
Doubly labelled water: the differential washout of deuterium and oxygen-18 yields average daily carbon dioxide production and hence energy expenditure.