From: sbharris@ix.netcom.com(Steven B. Harris)
Newsgroups: sci.med,sci.med.nutrition
Subject: Re: Oxygen-18 supplies (metabolism with D2 O-18)
Date: 5 May 1998 09:49:07 GMT

In <Pine.GSO.3.95.980504083202.17356A-100000@holyrood.ed.ac.uk> Ian
Stevenson <ians@holyrood.ed.ac.uk> writes:

>We're studying the energetics of egg laying in wild birds. To avoid
disturbance, and interruption of laying, we need to introduce the label
via food. We're using such high purity O18 so that it can be placed in
a sufficiently small piece of food (a moth caterpillar) that the bird
is almost certain to eat the entire dose. We've tried various other
concentrations, but only the high concentration gives good results.<



   Fascinating.  Hope you're going some good controls, using captive
birds fed the same ration and calorimetized the old fashioned way.
Theory is good, but having controls is better.

    For those wondering what the devil is being discussed, the doublely
labeled water technique (using deuterium and O-18) is a way of
measuring average metabolic rate (average energy use over time) in
critters that are hard to catch, or put in a calorimeter, or for some
reason are doing something that you can't have them in a calorimeter
while they're doing.  Like flying about getting caterpillars to get
energy to feed chicks.

    The theory is a little hard to explain in a short synopsis, but
basically the principle is this:  Most oxygen is O-16, and a little is
0-17 and O-18 (these are all non radioactive).  When you put water with
0-18 into an animal, the extra O-18 goes out in basically two ways:
it's excreted as water, and it's eliminated as CO2 (the oxygen in water
equilibrates with oxygen in CO2 via carbonic anhydrase).  If you also
put heavy hydrogen (deuterium, again non-radioactive) into the animal,
you can use its loss to subtract for, or correct for, the loss of O-18
as water.  The rest (the extra loss) is loss as CO2, which tells you
how much CO2 the animal is making over time.  This is directly
proportional to energy use, if you only have some idea of respiratory
quotient for the animal (which is dependent on the fat/carbohydrate
composition of the diet, and doesn't vary that much for each animal, or
even for similar animals eating similar diets).  And that's your
answer.

   Now---- getting the dose of O-18 and D INTO the wild animal can be a
problem.  You can shoot a whale or bear with a dart full of doubley
labeled water (D2O-18).  Then you just need one more blood sample, a
few days to a few weeks later.  But a bird?  A dose of D20-18 in a nice
fat caterpillar....


                                         Steve Harris, M.D.