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Measurement of Energy Expenditure

Measurement of Energy Expenditure. Direct Calorimetry. All metabolic reactions end up as heat given off by the body Energy expenditure EE can be measured using direct calorimetry – this is measures the heat given off by an organism

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Measurement of Energy Expenditure

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  1. Measurement of Energy Expenditure

  2. Direct Calorimetry • All metabolic reactions end up as heat given off by the body • Energy expenditure EE can be measured using direct calorimetry – this is measures the heat given off by an organism • Heat production is measured by an airtight chamber called a direct calorimeter

  3. Temperature changes of water in pipes at the top of the chamber are measured The chamber is heavily insulated to reduce heat loss Procedure is expensive and static Direct Calorimetry Temperature changes are due to heat produced and radiated by individual being tested

  4. Indirect Calorimetry • Since 95% of energy production in the body depends on the process of oxygen, it follows that the measurement of the oxygen consumed by the body over a period of time gives an indirect measure of energy expenditure. This is known as indirect calorimetry.

  5. Portable respirometers allow energy expenditure during occupational and sporting activities

  6. Subject breathes in air of known composition – 20.93% oxygen, 0,03% carbon dioxide and 79.04% nitrogen. • Exhaled air is collected in a bag or analysed directly and compared to inhaled air • Measurement of oxygen consumption requires knowledge of :- • Volume of air breathed out over a given time • The composition of the exhaled air • It is known that approximately 20kJ of energy are released when 1 litre of oxygen is consumed. This can be used to work out EE

  7. Example Volume of air expired in 10 minutes = 100litres % O2in inspired air is 21% of 100l = 21l % O2 in expired air is 18% of 100l = 18l % O2 used in 10 minutes = ( 21-18) = 3l % O2 used / minute = 3/10l = 0.3l/minute Knowing that 20kJ of energy is released when 1 litre of oxygen is consumed allows us to calculate EE For the example above EE = 20 × 0.3 = 6kJ/min

  8. Portable respirometers measure energy expenditure of individuals by measuring total volume of expired air and by keeping small sample of gas for analysis

  9. Heart rate recording • In any individual there is a relationship between heart rate, oxygen consumption and EE during any activity above resting. • The greater the oxygen consumption, the higher the heart rate • ‘Sports testers’ take continuous measurements of HR over 24 hour period and results used to calculate EE

  10. Less fit individual Fit individual

  11. Activity Diaries • An activity diary can be used to give an estimate of EE by recording accurate times for all activities throughout a day. • All 1,440 minutes of each day should be kept over a four to seven day period to give a pattern for an individual each week • The energy expended can be calculated from the total time for each activity X the energy cost of each activity from published results

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