Overall energy expenditure consists of
- Basal metabolic rate (60%)
- Thermic effect of food (10%)
- Nonexercise activity (5%)
- Purposeful activity (~25%, but obviously highly variable)
Basal metabolic rate is
- Energy required to sustain chemical reactions of resting body
- = energy liberated from food when...
- At rest, but awake
- Fasted state (not eaten for 12 hours)
- Not exercised for 12 hours
- Supine
- Thermoneutral temperature
- All sources of excitement removed
Determinants of BMR (ABCDEFGHI)
- Age: \(\downarrow\) 2%/decade
- Body mass and composition: \(\uparrow \uparrow\) with \(\uparrow\) lean mass, \(\uparrow\) with obesity
- Climate: BMR \(\uparrow\) 20% in arctic vs tropics
- Disease (fever, burns, trauma, malignancy) \(\to \ \uparrow\)BMR
- Endocrine (all \(\uparrow\) BMR): thyroxine, catecholamines, testosterone
- "Fertility" (Pregnancy) \(\uparrow\) BMR
- Gender - men > women
- "Hibernation" (Sleep) - \(\downarrow\) CNS activity \(\to \ \downarrow\)10%
- Intake - starvation \(\to \ \downarrow\)BMR
Measurement of BMR
- Above conditions satisfied (at rest, fasted etc)
- Direct calorimetry: Subject placed in insulated chamber filled with H2O \(\to \Delta \text{Temp} \propto \text{BMR}\)
- Indirect calorimetry: 1L O2 consumption \(\approx\) 5kCal - but depends on respiratory quotient. Mixed expired gas is collected with known FiO2 and MV; \(F\bar{e}O_2\) and \(F\bar{e}CO_2\) measured. \(\dot{V}_{O_2}\) and \(\dot{V}_{CO_2}\) calculated \(\to\) metabolic rate calculated with Weir equation.