
No one else seems to know, so I'll post this.

This topic came up on sci.physics.fusion shortly after the cold-fusion
flap started.  As I recall, its been done to some experimental mice.
They showed various ill effects and eventually died.  The reason is
that deuterium does not have exactly the same reaction rates as
hydrogen due to its extra mass (which causes lower velocity, Boltzman
constant, mumble).  This throws various bits of body biochemistry out
of kilter, and you get sick and die.

I've never heard of anyone being poisened this way, in or out of real
life.  The process takes quite a while.  If anyone wants to write this
book, I would imagine you would have to:

1: Replace a significant fraction of the water in the body with heavy
   water.

2: Wait while normal breakdown and repair processes cause other
   molecules in the body to be synthesised using the deuterium.

During this process the victim would gradually deteriorate and
eventually die, but I imagine it would take weeks during which the
poisoner would have to ensure that a significant proportion of the
water the victim ingested was heavy.

You would get such a mess of symptoms that the doctors would be both
alarmed and confused.  Why should every organ in the body suddenly
begin to deteriorate?  If you can figure out how the poisoner gets the
heavy water into the victim in a hospital then you could have a real
story here.

Come to think of it, <2> would continue even after the heavy water was
no longer being ingested, so hospitalisation might be too late.

The most detectable effect would be that the victim's body fluids
would literally be "heavy".  Water has a molecular weight of 18 and
heavy water has a MW of 20.  Thus the victim's weight will increase by
about 1% for every 10% of body water replaced by heavy water.  Maybe
the detection occurs because some pathologist in the lab notices that
the victim's urine is strangely dense.  Is there any medical test
involving the specific gravity of a body fluid?

Paul.