Biochemical Society Transactions
Daily energy expenditure in human subjects
JOHN HASLAM and MALCOLM BANNER,
School of Life, Basic Medical and Health Sciences,
King's College London, Campden Hill Road, London WE
7AH, U.K.
In teaching
the subject of bio-energetics
biochemists often d o not consider the whole organism
but rather concentrate o n the roles of mitochondria
and chloroplasts and o n the molecular events which
take place in the various electron transport chains,
usually illustrated by experiments using animal or
plant tissue preparations.
On the other hand those
teachers concerned with subjects such as physiology
and human biology, where "whole body" experiments are
common, realise that experiments involving humans can
often capture the interest of students, particularly
at the elementary level. Whole body experiments also
help t o keep in perspective the sub-cellular and
molecular events which are the somewhat narrow focus
of the biochemist.
Ultimately the energy expended by a n organism is
accountable (dissipated) as heat which can be
measured accurately by calorimetry.
However, for
human subjects calorimetry is a costly and difficult
procedure which is generally inappropriate for
teaching.
We use a n alternative procedure,
(described below) for measuring energy expenditure,
which can easily be performed by undergraduates.
The energy cost of human activities may be
estimated indirectly by measuring the amount of
oxygen consumed.
The validity of using oxygen
consumption as a basis for measuring
energy
expenditure has long been established [I]. Oxygen
consumption ('VO,)
is simply calculated as the
difference between the amount of oxygen inspired and
expired, usually expressed at standard temperature
and pressure dry (STPD), per unit time.
It
represents the oxygen used in cellular processes
which can be related t o specific biochemical events.
Of
the
several
techniques
available
for
measurement of oxygen consumption in humans the most
straightforward is the classical Douglas bag method
which involves the collection of expired air over a
measured period of time. In our class experiments
expired air is collected into Douglas bags over short
periods of time (10 to 20 min) from a subject
performing each of several activities; see Table 1.
(These kinds of activities represent those that the
subject is likely t o undergo in a 'normal" 24-hour
period.)
A small sample of expired air of known
volume is withdrawn, dried and its percentages of
oxygen and carbon dioxide are measured using a
Servomex paramagnetic oxygen analyser and HartmannBraun infra-red carbon dioxide analyser connected in
series. The volume of the remaining expired air is
determined, together with its temperature, by
emptying the contents of the bag through a gas meter
into which a thermometer is incorporated.
The
barometric pressure is also recorded. The data is
used to calculate the volume of air expired at STPD.
The percentages of oxygen and carbon dioxide in
dry fresh air, 20.93% and 0.03% respectively, are
always the same. The fractional concentrations of
Flm2) and nitrogen (FIN2) may be
these gases (F
inspired air by assuming that it
calculated in
contains only oxygen, carbon dioxide and nitrogen.
do?;
Percentage m t r a t l m of gas
100
Thus
: Fim
-
0.2093;
Flw2
-
0.0003;
FIN2
-
0.7904
( 1 99 1 ) 19 433s
T o calculate oxygen consumption (VO2) during a
particular activity the volume of oxygen expired is
subtracted from the volume inspired over the same
time period. These two volumes are calculated from
a knowledge of the volumes of inspired air (VI) and
expired air (V ) and their fractional oxygen
concentrations
& FEO2respectively).
(fIo2
Thus:
V02 = (VI X Flo2)
-
(VE X FEo2)
Since the volume of expired air and its fractional
oxygen concentration are measured experimentally the
only unknown is the volume of inspired air. But this
can be determined from the knowledge that the amount
of nitrogen inspired is unchanged in the body and is
therefore equal to the amount of nitrogen expired.
1'
Thus :
VI
Go2
-
-
[VE x
-
(FEW2
FIN2
E'
VE x FEN2
+
FE02)
FEN2
FIN2
x 0.2093) - [V
E x FEo2]
0.7904
The subject keeps a diary detailing the time spent
From
o n each of the activities during 24 hours.
this, and from a knowledge of the energy cost of each
activity, daily energy expenditure is estimated
The latter is determined on the basis of
(Table 1).
the measured oxygen consumption and the caloric
equivalent of a litre of oxygen, assumed t o be
Thus;
5 kcal 1-' (20.93 kJ 1-').
caloric cost
of activity
-
oxygen cost x
of activity
caloric equivalent
per litre oxygen
Table 1. Estimation of Daily Energy Expenditure
Activity
Time
(min)
Lying
Sitting
Standing
Walking
540
600
150
150
TOTALS
1440
Energy Cost
of activity
kcal min-'
( k min
~
-')
Total
Energy
kcal
(MJ)
1.2 (5.02)
648(2.7)
840(3.5)
285(1.2)
480( 2.0)
1.4 (5.86)
1.9 (7.95)
3.2 (13.4)
7.7 (32.23)
2253( 9.4)
Thus, from the collated data students are able t o
obtain a value for the total 24-hour energy
expenditure, in our example 2253 kcal (9.4 MJ).
[ l ] Astrand, P-0. & Rodahl, K., (1986) Textbook of
Work Physiology, 3rd edn., McGraw-Hill International.