Hello joebus
I found a useful summary of the basic principles in a web page from
the Medical School of the University of Tasmania, which is now only
available from the Google cache:
http://216.239.39.100/search?q=cache:x9vys31xZogC:www.healthsci.utas.edu.au/medicine/teaching/weller/%2Bdocs/metab1.htm+%22metabolic+rate%22+humans+calorimetry&hl=en&ie=UTF-8
The most accurate measurement is by means of direct calorimetry. From
the cached web page: "Except for that portion of the energy output
that can be measured in the form of mechanical work, the end point of
all metabolic processes in the body is heat. So by measuring the heat
produced by the body, a direct estimate of metabolic rate can be made.
Such measurements are performed in a device known as a human
calorimeter or whole-body calorimeter. The subject is sealed in the
calorimeter chamber and the heat given up to the circulating air and
to water flowing through the chamber walls is measured once a steady
state has been achieved. Human calorimeters are costly and difficult
to operate and maintain, so direct calorimetry is not routinely
performed."
The Thermonetics Corporation is one producer of human calorimeters.
You can view some, including one that provides a whole room with space
for various activities, with descriptions on their web site at
http://www.electriciti.com/~thermo/sec_hum.html
Because of the drawbacks of direct calorimetry, most researchers use
indirect calorimetry. Because steady-state metabolism is aerobic, the
rate of oxygen uptake by the body can be used to estimate metabolic
rate. However, it is also necessary to take into account that
different amounts of oxygen are needed to process different food
components. To convert one gram into energy requires 1.96 liters of
oxygen for fats, 0.94 liters for protein and 0.81 liters for
carbohydrate. The amount of carbon dioxide released as a result also
varies, as does the amount of energy released (the cached page cited
above has a table with these values).
If you divide the amount of CO2 produced (VCO2) by the amount of
oxygen uptake (VO2), you get a value called the respiratory quotient
or RQ. The RQ for the metabolism of carbohydrates is 1.00, for fats
0.71 and for protein 0.80. "Knowing the RQ allows us to assign an
energy equivalent to each litre of O2 consumed. If it is not possible
to measure the VCO2, and hence calculate the RQ, a value of RQ=0.82
may be adopted for a subject on a normally balanced mixed diet."
"the value for VO2 may be given as volume per unit time, e.g as ml
O2/min. If the RQ is known (or alternatively can be safely assumed),
then the energy equivalent of the VO2 can be found, in which case
metabolic rate is usually given in kJ (the joule has largely replaced
the calorie as a unit of energy). To standardise for individuals of
differing sizes, it is useful to express metabolism per unit of body
surface area (BSA), i.e in kJ/m2. Lastly, if we wish to express
metabolic rate in units of heat, so that mechanical work and
metabolism can be considered in the same units, we can express the
value in watts (W), remembering that 1 W = 1J/sec."
Techniques of indirect calorimetry based on finding the RQ:
Again, a whole room system can be used, but there are few of these
because of the expense involved.
"an airtight environmental room utilized to assess 24-hour energy
expenditure. This room provides facilities for daily living and
bridges the difference between laboratory and free-living
environments. Oxygen consumption (VO2) and carbon dioxide production
(VCO2) are calculated by measuring the changes of oxygen and carbon
dioxide contents of the air inside the room calorimeter and by the
flow rate of the purged air multiplied by its concentration of gases."
http://www.mc.vanderbilt.edu/gcrc/r_nutrition_support.html (Vanderbilt
Medical Center)
Douglas Bag: Two leaflets from suppliers:
http://www.cgs.com.br/H/F047.pdf
http://www.harvardbioscience.com/pdffiles/B2K_F046.pdf
The Douglas bag is viewed as the gold standard against which to
validate other systems. However, at least one person disagrees with
this: http://www.zest.net/vacumed/l2-2460.html (nb this is on the web
site of a supplier of one system for metabolic studies, so the
information could be biased)
Various ergospirometry systems, fixed and mobile (also called
metabolic carts): eg three systems from one supplier can be seen at
http://www.jaeger-toennies.com/english/products/cardio-respiratory/cardiopulmonary-ex-testing/e-oxyco.html
(click on pictures to get descriptions - nb this equipment is used for
other purposes as well) And another portable system:
http://www.cosmed.it/pdf/k4b2_en.pdf
A hand-held device marketed for medical use:
http://www.healthetech.com/corp/medical/medgem.jsp;sid=aZBH9m1Dls-b
and domestic use: http://www.healthetech.com/corp/consumer/bodygem.jsp;sid=aZBH9m1Dls-b
Ventilated hood: a picture of a young subject being studied at Dundee
University: http://www.dundee.ac.uk/biocentre/SLSBDIV6pww.htm
Doubly-labeled water technique: "A recently developed technique is the
use of doubly labelled water (2H218O or D218O) for measuring energy
expenditure in ambulatory subjects. The heavy oxygen (18O) can be
incorporated into carbon dioxide C18O2 during metabolism or excreted
as water (H218O). Deuterium, on the other hand, can only be excreted
as water (D2O). Thus the rate at which 18O declines relative to the
decline in deuterium is a measurement of metabolic rate." (from Diet
and Health: Implications for Reducing Chronic Disease Risk
(1989)Commission on Life Sciences
http://www.nap.edu/books/0309039940/html/141.html)
"The technique involves sampling a sample of body water (maybe from
urine or water vapour in exhaled breath) at some point after dosing.
During the intervening period the animal can be released into its
natural surroundings and might even not have to be captured again (for
example urine can be collected after the animal has moved on). One
major problem with the technique is that it is based on CO2 production
which, as detailed above, can vary widely depending on fuel source.
Thus some information on the types of food an animal has been eating
is also required." http://helios.bto.ed.ac.uk/bto/biology/compneut/Notes/Requirements/OptGrowth/FactorsInflMNR/Energy/Techniques/techniques.htm
- this page is about measurements in animals, but the technique is
used in humans too
Another technique might offer advantages over direct and indirect
calorimetry:
"Due to inherent problems with both the direct and indirect
calorimetry methods, the development of a simple and direct method of
assessing heat loss would be of immense value. A new and viable method
may be infrared thermography which uses a lightweight, portable, and
direct imaging radiometer. One of our faculty members has shown that
mean skin temperature and, ultimately, total heat loss can be
determined when infrared thermography is used in conjunction with heat
loss equations and specialized computer software. This technique has
been validated for use in adults and infants against indirect
calorimetry... infrared thermography is a valid, sensitive, and
reliable method for measuring energy expenditure... it is
noninvasive, nonconfining, poses no risk to subjects, and can
potentially deliver immediate results.
http://www.ag.uiuc.edu/~vista/html_pubs/9294rep/nutsci.html (Division
of Nutritional Sciences, University of Illinois College of
Agricultural, Consumer and Environmental Sciences)
Finally, an article from the NASA Glenn Research Center looks at ways
in which new technologies could be used for this purpose, especially
for studies in space
http://microgravity.grc.nasa.gov/grcbio/documents/fitness.pdf |
