Lecture 7 Flashcards
(18 cards)
Measuring and evaluating energy expenditure
All metabolic processes ultimately result in heat production
- substrate metabolism efficiency
- 40% of substrate energy —> ATP
- 60% of substrate energy —> heat
Heat production rate from cells, tissues, and whole body operationally defines metabolic rate
- heat production increases with energy production
- calorie (kcal) represents basic unit of heat measurement
- calorimetry refers to measurement of heat transfer
The kilocalorie (kcal) and food energy
- Unit of energy
- Amount of heat energy needed to raise the temperature of 1 kg of water by 1 degree Celsius
- 1 kcal = 4.184 kilojoules
- Measured with bomb calorimeter
Burning of 148 g of potato heats 1 kg of water by 110 degree Celsius
*often written as “calories” but “kcal” is the correct unit
- *can calculate any food
- A weighed sample of food is sealed in a thick-walled reaction chamber called the “bomb” that is filled with pure oxygen (O2), and the bomb is submerged in water in an insulated container
- An electric signal ignites the food sample, which burns explosively in the pure O2. The amount of heat released is proportional to the amount of available chemical energy in the final sample
- The surrounding water absorbs the heat from the combustion
- The amount of heat released is calculated by measuring how much the water warms, using a high-accuracy thermometer
Quantifying human energy expenditure
All metabolic processes in the body ultimately…
… depend on oxygen use
Indirect calorimetry
- assesses human energy metabolism by measuring oxygen consumption and carbon dioxide production
- oxygen consumption
… result in heat production
Direct calorimetry
- assesses human energy metabolism by measuring heat production (similar to method for determining energy value of food by bomb calorimetry)
Measuring energy expenditure: direct calorimetry
Human calorimeter - direct measure of heat production and thus kcal expended
Measuring energy expenditure: indirect calorimetry
- measuring a person’s rate of oxygen uptake (VO2) provides an indirect but accurate estimate of energy expenditure
- oxygen consumption
Closed-circuit spirometry
- rebreathing 100% O2 in a closed system
- rate of volume reduction = VO2 in L/min
Open-circuit spirometry
- measure inspired and expired O2 volumes
- magnitude of difference divided by time = VO2 in L/min
- Douglas bag
- computerized spirometry
- portable spirometry
Indirect calorimetry
- oxidative metabolism of glucose and fat uses O2 and produces CO2 and water
- the rate of O2 and CO2 exchange in the lungs normally equals the rate of their usage and release by the tissue
- thus, energy expenditure can be estimated by measuring these variables at the mouth
- difference in volume divided by time = rate
- fraction of O2 inspired > fraction of O2 expired
- fraction of CO2 inspired < fraction of CO2 expired
- difference in volume divided by time = rate
Rate of oxygen consumed
- VO2 = VO2 inspired - VO2 expired
Rate of which CO2 is produced
- VCO2 = VCO2 expired - VCO2 inspired
Equations you are familiar with…
Oxygen uptake (L/min)
Inspired - Expired
VO2 = (Ve x ((1 - FeO2 - FeCO2)/.7904) x .2093) - (Ve x FeO2)
VO2 = VO2 divided T collections
L. Divided. Min
L/min
Carbon Dioxide Production (L/min)
VCO2 = Ve x FeCO2
VCO2 = VCO2/T collection
*where all Ve are first corrected to STPD (standardized to STPD)
Automated VO2
VO2 = 1.67 L/min
Douglas Bag VO2
VO2 = (Ve x ((1 - FeO2 - FeCO2)/ .7904) x .2093) - (Ve x FeO2)
Areas is total volume of O2 taken up (VO2)
- Ve = 74 L (STPD)
- F2O2 = 16.0%
- FeCO2 = 4.40%
- T collection is 120-240 or 2 min
VO2 = (74 x ((1 - 0.16 - 0.044)/ .7904) x .2093) - (74 x 0.16)
VO2 = 3.344 L
VO2= VO2/T collection
VO2 = 3.344/2
VO2 = 1.67 L/min
Measuring energy expenditure
- to estimate amount of energy used by the body it is necessary to know the type of food substrate(combination of carbohydrate, fat and protein) being oxidized
- mount of O2 used during metabolism depends on type of fuel being oxidized
- we can use VCO2 and VO2 from indirect calorimetry to determine substrate utilization and quantify energy expenditure
- in general, amount of O2 needed to oxidize 1 molecule of CHO or FAT is proportional to amount of carbon in the fuel:
C6H12O6 + 6O2 —> 6CO2 + 32 ATP + 6H2O
C16H32O2 + 23 O2 —> 16 CO2 + 106 ATP + 16 H2O
Respiratory exchange ratio (RER)
Ratio of CO2 production to O2 uptake:
RER = VCO2/VO2
- RER for 1 molecule of glucose = 1.0
6 O2 + C6H12O6 —> 6 CO2 + 6 H2O
RER = VCO2/VO2 = 6 CO2/ 6 O2 = 1.00
- ReR for 1 molecule palmitique acid = 0.70
23 O2 + C16H32O2 —> 16 CO2 + 16H20
RER = VCO2/VO2 = 16 CO2/ 23O2 = 0.70
- predicts substrate use, kilocalories, and O2 efficiency
RER, substrate utilization and energy yield
Respiratory exchange ratio (RER) as a function of energy derived from various fuel mixtures
RER 0.71 - Carbohydrates 0 and fats 100, energy 4.69
RER 0.75 - Carbohydrates 16 and fats 84, energy 4.74
RER 0.80 - Carbohydrates 33 and fats 67, energy 4.80
RER 0.85 - Carbohydrates 51 and fats 49 (about even), energy 4.86
RER 0.90 - Carbohydrates 68 and fats 32, energy 4.92
RER 0.95 - Carbohydrates 84 and fats 16, energy 4.99
RER 1.00 - Carbohydrates 100 and fats 0, energy 5.05
Fat goes down and carbohydrates increase, when RER increase
Kcal per LO2 CHO > kcal per LO2 FAT (by ~8%) - at the same power output, more O2 molecules need to be consumed when fat is oxidized
CHO - less kcal dense (756 kca/mol); 32 ATP per 6 O2 = ATP/O of 32/12 = 2.7 mol ATP/mol of O2
FAT - more kcal dense (7826 kcal/mol); 109 ATP per 23 O2 = ATP/O of 109/46 = 2.4 mol ATP/mol of O2
Respiratory quotient (RQ)
- RQ and RER are the same measurement but are obtained differently
- RQ = cell respiration
- RER = exhaled air from lung - under certain circumstances the values can differ…
Maximal range of RQ is from 0.70 to 1.00
Maximal range of RER may vary from <0.70 to >1.20
When might RER (not equal to) RQ??
- Hyperventilation - increases CO2 output in excess of metabolically produced CO2; increase VCO2 (RER > RQ)
- Metabolic acidosis - buffering of H+ ions via bicarbonate yields additional, “non-metabolic” CO2 which is breathed out at the mouth; increase VCO2 (RER > 1.0)
- Non-steady-state exercise - VCO2 takes longer to attain steady-state compared to VO2 at exercise onset; temporal distortion (RER < RQ)
- Prolonged exercise - if CHO nutrition is poor and glycogen stores low, longer exercise duration could result in amino acid oxidation (RER doesn’t equal RQ)
Fat vs Carbohydrate Use During Exercise
- exercise at 100 W after 5 days high fat/low carb (white) vs 5 days low fat/high carb diet (black)
- if the power output is the same (i.e. equal ATP resynthesis demand), why were VO2 different???
RER = 0.75 (high fat)
RER = 0.99 (high carbohydrate)
Review so far
Application of energy expenditure measurement…
- different forms of activity require (demand) different levels of energy which is supplied in the form of ATP
- metabolic rate the rate at which the body uses energy or the rate at which ATP must be resynthesized to satisfy bodily demands
- gold standard estimate of metabolic rate is whole body VO2
- VO2 ~ metabolic rate
Measuring energy expenditure at rest
Resting (basal) metabolic rate: minimal amount of energy required to carry out essential physiological functions
- example: during supine rest at 8 am after 12 h fast:
- VO2 = 0.25 L/min; RER = 0.80 (4.80 kcal/LO2)
Kcal/day = 0.25 L/min x 1444 min/day (how many minutes that are in a day) x 4.80 kcal/L
= 1728 kcal/day
Energy intake - energy output = energy balance
(Calories consumed) - (calories expended) =
Positive = increase body mass
Negative = decrease body mass
1 lbs of fat ~ 3500 kcal
Organ - oxygen uptake - percentage of resting metabolism
Liver - 67 - 27
Brain - 47 - 19
Heart - 17 - 7
Kidneys - 26 - 10
Skeletal muscle - 45 - 18
Remainder - 48 - 19
Total - 250 - 100
Energy expenditure during exercise
30 min cycling @ 100 W
VO2 = 1670 mL/min; RER = 0.80 (4.80 kcal/LO2)
Kcal = 1.67 L/min x 3 min x 4.80 kcal/L = 240 kcal
30 min cycling @ 200 W
VO2 = 2860 mL/min; RER = 099 (5.03 kcal/LO2)
Kcal = 2.86 L/min x 30 min x 5.03 kcal/L = 432 kcal
Increase power output —> increase motor units —> increase energy demand —> increase energy supply
