Energy Expenditure

Chapter 5

Total energy expenditure breakdown

• Resting metabolic rate ~60-75%

• Thermic effect of food ~10%

• Physical activity ~15-30%

Direct measurement of energy expenditure

measures heat generated by metabolic reactions

Indirect measurement of energy expenditure

gas analysis, HR monitor

Heat production _____ with energy production

increases

can be measured in a calorimeter

Direct calorimeter

used for measurement of energy expenditure by an exercising human subject

heat generated by exercising body is transferred to air + walls of chamber — heat produced = measure of metabolic rate

Pros of direct calorimeter

accurate over time, good for resting metabolic measurement

Cons of direct calorimeter

expensive, slow, heat added by equipment, sweat creates measurement errors, not practical, and not accurate for exercise

Indirect calorimetry

estimates energy expenditure using O₂ used and CO₂ produced

measures respiratory gas concentrations

Why is indirect calorimetry only accurate for stead-state exercise?

Once anerobic metabolism kicks in, hard to look at the respiratory gas exchange

VO₂

volume of O₂ consumed per minute

VCO₂

volume of CO₂ produced

Ve

ventilations per minute

Respiratory exchange ratio

ratio between CO₂ production and O₂ usage (VCO₂/VO₂)

RER: O₂ usage depends on type of fuel

• glucose requires less O₂ than lipid metabolism

• start exercise using fat and transition to more carbs

1 glucose = ___ RER

1.0 RER
6 O₂ + C₆H₁₂O₆ —> 6 CO₂ + 6 H₂O + 32 ATP
6 CO₂/6 O₂ = 1.0 RER

1 fatty acid = ____ RER

0.70 RER
23 O₂ + C₆H₁₂O₆ —> 16 CO₂ + 16 H₂O + 129 ATP
16 CO₂/23 O₂ = 0.70 RER

limitations of indirect calorimetry

• CO₂ production may not = CO₂ exhalation

• RER is inaccurate for protein oxidation

• RER near 1.0 may be inaccurate when lactate buildup increases CO₂ exhalation

• Gluconeogenesis produces RER <0.70

Increased psychological stress leads to…

higher RER and increased carb/glucose metabolism

Heart rate monitoring can be used to estimate…

• VO₂

• linear relationship, considerable error rate (electronics)

• used for estimation of submaximal exercise

Limitations of heart rate monitoring

• only aerobic metabolism

• complicated by body temp, upper body exercise, and fitness level

• poor correlation for sedentary/low-intensity activity

Energy expenditure: Walk/Run

• estimate: only during steady-state submaximal aerobic exercise; specific to body mass

• Divide metabolic rate into…

  • resting = 3.5 mL/kg/min = 1 MET

    • + energy cost for horizontal (0.1ml/kg/min)

    • + energy cost for vertical movement (+1.8mL/kg/min)

    • + much more (walking swing, height, etc.)

  • non-resting

Energy Expenditure: Cycle

• looking for Watts

• estimate: resistance/load (kg), distance (wheel size=m/rev), cadence (rev/min or RPM)

• POWER (kg/m/min) = resistance (kg) x flywheel size (m/rev) x cadence (rev/min)

• energy cost also includes external resistance, body weight, and bike weight

metabolic rate

rate of energy use by the body

based on whole-body O₂ consumption

~2000 kcal/day

What is the average RER at rest?

0.80

Basal metabolic rate

energy expenditure at rest — minimum requirement for living

measured in supine position after 8 hr sleep & 12 hr fasting

related to fat-free mass and influenced by size, age, stress, hormones, and temp

resting metabolic rate (RMR)

like BMR (+5%/10%) — more realistic without fast

1200-1400 kcal/day

total daily metabolic activity includes

normal daily activity

1800-3000 kcal/day

Fick equation

tells how much oxygen is being delivered to and removed from the tissues

VO₂=HR x SV (cardiac output) x a-vO₂ difference

Cardiac output

quantity of blood pumped each contraction

cardiac output = stroke volume x heart rate

How many kcals used per liter of oxygen consumed?

5kcal

estimated over time

In submaximal aerobic exercise, metabolic rate ….

increases with exercise intensity

shared with oxygen use

At higher outputs, VO₂ ______ & more type ____ fiber recruitments

continues to increase

more type II muscle fibers recruited (less efficient)

VO₂ drift

upward drift observed even at low power outputs when person reaches a certain point

possibly due to ventilatory or hormone changes

VO_2max

maximal O₂ uptake

point at which O₂ consumption does not increase with greater exercise intensity

more training allows athlete to compete at higher % of VO_2max — adaptation plateaus after 8-12 weeks of chronic training

Best measurement of aerobic fitness?

VO_2max

Economy of movement is very important for…

aerobic athletes (think running form, swimmer’s technique, etc.)

How does gender affect VO_2max?

• testosterone & protein synthesis

  • increased testosterone —> males can synthesize more hemoglobin

• also, males usually have slightly larger lungs and heart

Absolute measurement of VO_2max

Not dependent on bodyweight

L of O₂ / minute

Relative measurement of VO_2max

Helps compare athletes of different weights

greater muscle mass involved = greater VO₂

mL of O₂ x min^-1 x kg^-1 = relative VO_2max

Maximal aerobic exercise

nothing is 100% aerobic or anaerobic

Estimates of anaerobic effort involve…

• Excess of postexercise O₂ consumption (EPOC)

• Lactate threshold (hard to quantify)

Early in exercise O₂ demand is…

greater than O₂ consumed

initial energy comes from anaerobic metabolism as aerobic takes awhile to ramp up

this builds an O₂ deficit

Post-exercise O₂ consumed is…

greater than O₂ demand

EPOC: Excessive Postexercise O₂ consumption

EPOC: Excessive Postexercise O₂ consumption

• replenishes ATP/PCr stores

• converts lactate to glycogen

• replenishes hemomyoglobin and clears CO₂

Do we have lactate in the blood at rest?

Yes- some tissues have no mitochondria; RBCs push out lactate b/c they have no use for it (no aerobic metabolism)

Lactate Threshold (L1)

blood lactate accumulation increases markedly at this point (~2 mmol/L)

at this point, lactate production > lactate clearance (mix of aerobic and anaerobic systems)

usually expressed as a percentage of VO_2max

Lactate threshold is a good indicator of potential for ______ exercise

endurance exercise

Economy of Effort

as athletes train, they use less energy at a similar pace

this is independent of VO_2max

body learns this with practice

Economy increases with distance — multifactorial phenomenon

form: practice lends better economy of movement

varies with type of exercise (running, swimming)

Successful endurance athletes have…

• high VO_2max

• high lactate threshold

• high economy of effort

• high % of Type I muscle fibers