Energy Expenditure

Basic unit of heat is the Calorie.

This expresses the quantity of heat necessary to raise the temperature of 1kg (1 L) of water by 1°C.

Direct Calorimetry

Measurement of heat actually produced by the body which is confined in a sealed chamber or calorimeter.

Disadvantages Expensive Slow to generate results Cannot follow rapid

changes in energy expenditure

Motor driven ergometers give off heat as well

Not all heat is liberated from the body

Sweat affects body temperature and mass

Advantages Measures heat directly

Indirect Calorimetry

To date it is easier and less expensive to measure energy expenditure by assessing the exchange of oxygen and carbon dioxide that occurs during oxidative phosphorylation.

Hence, the method estimating total body energy expenditure (indirect calorimetry) is more appropriate.

Closed circuit and open circuit spirometry represent the two methods of indirect calorimetry.

Closed Circuit Spirometry

This is currently used in hospitals and research laboratories to estimate resting energy expenditure.

With this system the subject breathes 100% oxygen from a prefilled container spirometer.

The spirometer acts as the closed system as the individual rebreathes only the gas in the spirometer, and no outside air enters the system.

A canister with soda lime (potassium hydroxide) placed in the breathing circuit absorbs the persons exhaled carbon dioxide.

A drum attached to the spirometer revolves at a known speed and records the difference between the initial and final volumes of oxygen in the calibrated spirometer.

This system is not suitable for use during exercise where the subject movement is required and large volumes of air are exchanged.

Open Circuit Spirometry

Here a subject inhales ambient air with a constant composition.

Changes in oxygen and carbon dioxide percentages in expired air compared with inspired ambient air indirectly reflects the ongoing process of energy metabolism.

The difference between inspired and expired air dictates how much O2 is being taken up and how much CO2 is being produced.

The body’s limited O2 storage, allows for an assumption that the amount of O2 taken up at the lungs accurately reflects the body’s use of O2.

This technique is limited to steady state activities lasting for approx. 1 min or longer, as energy production must be almost completely oxidative.

Anaerobic energy will not allow for respiratory gas measurements to reflect all metabolic processes.

Three common open circuit, indirect calorimetric procedures measure oxygen uptake during physical activity: Bag Technique Portable Spirometry Computerized Instrumentation

The simplest and oldest methods of indirect calorimetry are probably the most accurate methods.

Calculating Oxygen Consumption and Carbon Dioxide Production

Equipment used for indirect calorimetry utilizes the volume of oxygen consumed (VO2) and the volume of carbon dioxide produced (VCO2).

Generally the values are represented as oxygen consumed per minute and carbon dioxide produced per minute.

VO2 = volume of O2 inspired – volume of O2 expired.

Volume of O2 inspired = Volume of air inspired X fraction of that air that is composed of O2.

Volume of O2 expired = Volume of air expired X fraction of the air that is composed of O2.

Same for CO2.

Calculation of VO2 and VCO2 requires: Volume of air inspired (VI) Volume of air expired (VE) Fraction of O2 in the inspired air (FIO2) Fraction of CO2 in the inspired air (FICO2) Fraction of the O2 in the expired air (FEO2) Fraction of the CO2 in the expired air (FECO2)

Equation for VO2

VO2 = (VI X FIO2) – (VE X FEO2)

Equation for VCO2 production VCO2 = (VE X FECO2) – (VI X FICO2)

Haldane Transformation

For many years scientists have attempted to simplify the actual calculation of O2 consumption and CO2 production.

Several of the measurements needed in the equation are known and do not change.

The gas concentration of the three gases that make up inspired air are known: 20.93% O2 0.04% CO2 79.03% N (plus small quantity of inert gases)

Respiratory Exchange Ratio

To estimate the amount of energy used by the body, it is necessary to know the type of food substrate being oxidized.

The carbon and oxygen contents in these substrates vary greatly, hence the amount of oxygen used during metabolism depends on the type of fuel being oxidized.

The ratio between the amount of O2 consumed and CO2 released is termed respiratory exchange ratio (RER).

RER = VCO2 / VO2

Generally, the amount of oxygen needed to completely oxidize a molecule of carbohydrate or fat is proportional to the amount of carbon in that fuel.

For example: 6 O2 + C6H12O6 6CO2 + 6H2O + 38 ATP By evaluating how much CO2 released

compared with the amount of O2 consumed.. RER = 1.0

In contrast to metabolizing free fatty acids where there is considerably more carbon and hydrogen but less oxygen than glucose.

For eg oxidation of palmitic acid: 23 O2 + C16H32O2 16CO2 + 16H2O + 129 ATP

Combustion of fat molecule requires significantly more oxygen than combustion of carbohydrate molecule.

During carbohydrate oxidation, approx. 6.3 molecules of ATP are produced for each molecule of O2 used (38 ATP per 6 O2).

Compared with 5.6 molecules of ATP per molecule of O2 during palmitic acid metabolism (129 ATP per 23 O2)

Although fat provides more energy than carbohydrate, more oxygen is needed to oxidize fat than carbohydrate.

This means that the RER for fat is lower; for palmitic acid RER = VCO2/ VO2 = 16 / 23 = 0.70

Caloric Equivalence of the Respiratory Exchange Ratio (RER) and % kcal from Carbohydrate and

Fats

Energy % kcal

RER Kcal/L O2 Carbohydrates Fats

0.71 4.69 0 100

0.75 4.74 16 84

0.80 4.80 33 67

0.85 4.86 51 49

0.90 4.92 68 32

0.95 4.99 84 16

1.00 5.05 100 0

Once the RER value is determined from the calculated respiratory gas volume. The value can be compared with the table to determine the food mixture being oxidized.

Limitations of Indirect Calorimetry

Assumption that the body’s O2 content remains constant and that CO2 exchange in the lung is proportional to its release from the cells.

CO2 exchange is less constant, and the amount released in the lungs may not represent that being produced in the tissues.

So calculations of carbohydrate and fat used based on gas measurements appear to be valid only at rest and during steady state exercise.

Use of RER can also lead to inaccuracies. Nil calculations of the body’s protein use from

the RER. Recent evidence suggests that exercises lasting

for several hours, protein may contribute up to 5% of the total energy expended under certain circumstance.

Calorimetry

Indirect

Carbon and

Nitrogen Balance

O2

Consumption

Open Circuit

Closed Circuit

Direct

Heat Production

Isotopic Measurements of Energy Metabolism

The use of isotopes has expanded the ability to investigate energy metabolism.

Isotopes are elements with an atypical atomic weight.

They are either radioactive or nonradioactive.

These isotopes are used as tracers, selectively followed in the body.

Tracer techniques involve infusing isotopes into an individual and then following their distribution and movement.

Isotope turnover is relatively slow, energy metabolism must be measured over weeks.

Thus, this method is not well suited for measurement of acute exercise metabolism.

However, its accuracy (>98%) and low risk make it well suited for determining day to day energy expenditure.

The Maximal Oxygen Uptake (VO2max)

The VO2max represents the greatest amount of oxygen a person can use to produce ATP aerobically on a per minute basis.

This usually occurs with high intensity, endurance type exercise.

Athletes who compete in endurance sports records the highest VO2max.

However, this does not mean that only VO2max determines endurance exercise capacity.

VO2max represents a fundamental measure in exercise physiology and serves as a standard to compare performance estimates of aerobic capacity and endurance fitness.

Tests for VO2max use exercise tasks that activate large muscle groups with sufficient intensity and duration to engage maximal aerobic energy transfers.

Research has been directed towards: Developments and standardization of tests for VO2max

Establishments of norms related to age, gender, state of training, and body composition.

Tests of Aerobic Power

There are different standardized tests to measure VO2max.

Such tests remains independent of: Muscle strength Speed Body size Skill

These tests may require a continuous 3 – 5 minute “supermaximal” effort, but it usually consists of increments in exercise intensity (graded exercise test / GXT) until the subject stops.

Two types of VO2max tests are typically used: Continuous test : no rest among exercise

increments. Discontinuous test: several minutes of rest

between exercise increments.

Commonly used treadmill protocols: Naughton Test Astrand Test Bruce Test Balke Test Ellestad Test Harbor Test

Features common to each test include manipulation of exercise duration and treadmill speed and grade.

Factors Affecting Maximal Oxygen Uptake

Exercise Mode

Hereditary

Training State

Gender

Body Composition

Age

Maximal Update Predictions

Heart Rate Predictions of VO2max Tests make use of the essentially linear

relationship between heart rate and oxygen uptake for various intensities of light to moderately heavy exercise.

The slope of the line reflects the individuals aerobic power.

VO2max is estimated by drawing a best fit straight line through several submaximum points that relate to heart rate and oxygen uptake (or exercise intensity) and then extrapolating to an assumed maximum heart rate for the person’s age.

Caution

All predictions involves error. The error is referred to as the standard error of

estimate (SEE) and is computed from the original equation that generated the prediction.