Caloric and Isotopic methods Flashcards
What is the energy balance equation?
Energy intake = Energy expenditure ± ∆ stores
What has to be measured when we want to involve the energy balance equation?
energy intake and expenditure
and body composition
what are the major difficulties in measuring energy intake?
- qualitative/quantitative description of intake
- reliability of reference data
- deliberate and unconscious bias
- indirect recording for infants/toddlers
What are the fundamental energetics of any biological system?
Energy in = Energy out ± Δ Energy content
Total work done is INDEPENDENT of pathways
What are the possible types of energy in the body?
- chemical
- urinal/faecal
- mechanical
- electrical
- thermal
As an outcome of metabolism to result in ‘work’… what is produced?
heat
What did Boyle investigate regarding calorimetry methods?
fire and life are combustion processes
What did Lavoiser and Laplace investigate regarding calorimetry methods?
CO2 and heat production
What did Magnus investigate regarding calorimetry methods?
arteries have increased O2 and decreased CO2
What did Helmholtz investigate regarding calorimetry methods?
heat produced by isolated muscle
What did Lavoisier (on his own) investigate regarding calorimetry methods?
digestion, PA and cold increase VO2
What did Regnault and Reiset investigate regarding calorimetry methods?
Ratio of CO2 and O2 in gas exchange related to dietary composition
What also defines Joule?
watts per second
what is the SI unit of Joules?
kg/m2/s2
What defines calorie?
the heat required to raise 1g water by 1^C
How much is 1 kcal in J?
4.184
What are the current 4 modern approaches to total energy expenditure?
- Douglas bag - fast, but unrepresentative
- Indirect calorimetry - accurate, fast, but constrained
- Direct calorimetry - accurate, fast, but constrained
- Doubly labelled water - accurate, free-living but slow response
What is SMR?
- sleeping
- replaces the RMR for infants
What does the classical indirect calorimetry involve?
- standardised conditions
- assumes steady state
- BMR, RMR, SMR
- studies of physiology
What are the output results from indirect calorimetry method?
- oxygen uptake
- carbon dioxide production
- resting energy expenditure
- fuel utilisation (RQ)
What is the EE formula for indirect calorimetry?
EE = 3.941 VO2 + 1.106 VCO2 - 2.17 Un
Un = urea nitrogen production
How much % is the effect of neglecting nitrogen in EE?
1%
Give the general understanding of doubly labelled water:
- uses isotopes 2^H and 18^H
- traces the kinetics of H2O and CO2
- estimates CO2 elimination rate
What is the key principle in doubly labelled water?
O2 in respiratory CO2 is in isotopic equilibrium with O2 in body water
What is collected in doubly labelled water method, to analyse?
blood, saliva, urine samples
What is the formula for the expression of isotopic enrichment, the mass spectrometer analysis?
δ = [R(sample) - R(reference) x 1000] DIVIDED BY R(reference)
What is the fundamental calculation for rCO2, in DLW?
rCO2 = N [k(o) - k(d)]/2
N = dilution space k = rate constant
What is the fundamental calculation for VO2, in DLW?
VO2 = VCO2/RQ
What is the fundamental calculation for TEE, in DLW?
TEE = k(1)VO2 + K(2)VCO2
What are the assumptions of DLW?
- TBW is constant
- isotopes only equilibrate with TBW
- Isotopes only leave as water or CO2
- rates of influx and efflux constant during measurement
- no isotopic fractionation
- no tracer re-enters TBW once lost
- natural isotopic levels unchanged
What are the two approaches in calculating dilution spaces (N) in DLW?
- plateau method
- back extrapolation method
What is the plateau result when isotope turnover is slow?
plateau is relatively stable
What is the plateau result when isotope turnover is fast?
- plateau occurs at specific time
- tracee intake during equilibration period is high
Which named approach reduces error in the rCO2 calculation in DLW?
Bayesian approach
Is there fractionation in DLW?
only possible in reaction not going to completion
what is the calculation for fractionation factor (f)?
(f) = [Ratio of heavy : light isotopes in product] DIVIDE BY [Ratio of heavy : light isotopes in reactant]
Explain why isotopic intake is not constant
- 2^H and 18^O occur at natural abundance levels
- levels vary by diet, geographic location
- Pre-dose sample provides this information
What is the random variability in DLW?
50% internal imprecision
what is the systemic variability in DLW?
- weaning in infants
- expedition in adults
- marked dietary interventions
How do we address the precision of EE internally, in DLW?
- calculate in relation to slopes and intercepts
- address sample times and mass spec
How do we address the precision of EE externally, in DLW?
address consistency of behaviour
What is the formula for lean mass, in regards to body composition from isotopes?
Lean mass = Total body water/lean tissue hydration
What is the formula for fat mass, in regards to body composition from isotopes?
= weight - lean mass
What is the measuring metabolisable energy intake (MEi) formula?
MEi = Total energy expenditure + energy stored
