From: fgz@kodak.pa.dec.com (Federico Genoese-Zerbi)
Newsgroups: rec.scuba,sci.med
Subject: Re: Fat not a contributing factor to the onset of DCS?
Date: 8 Oct 92 23:04:55 GMT
Article-I.D.: PA.1992Oct8.230455.1798
Reply-To: fgz@kodak.pa.dec.com (Federico Genoese-Zerbi)
Organization: Digital Equipment Corporation

In article <BvtMLL.4L0@scd.hp.com>, dave@hpdstma.ptp.hp.com (Dave
Waller) writes:
> |> Yet this fact
> |> alone should not make for an added risk of DCS.  If the tissue's half
> |> time is appropriately modeled by the tables or computer being used to
> |> plan dive profiles, then it should be no more a risk factor than any
> |> other slow (or fast) comparment.
> 
> Here is one of the basic misconceptions that leads to the analysis you
> have provided, yet because of the faulty basis, your conclusions are
> not useful.

Even if you throw all my theoretical thoughts, you still have to deal with
the experimental/statistical data (see below...)


> Haldanean based computers model the entire body based on subdividing the
> theoretical continuous range of tissue types by quantizing them into a
> set of "compartments" with specific halftimes. However, there is no
> correspondence to actual tissues, and as such, the approximation of
> quantizing the infinite number of possible halftimes into a discrete
> set of compartments cannot perfectly model the body.

I agree, but since fat is considered to be the "extreme", you would expect
this one to be perfectly modeled (unless there is something grossly wrong
with the exponential ongassing/offgassing model -- which there could be...)

> Similarly, if a body is composed of a disproportionate amount of slow
> tissues, the model no longer represents this diver very well, and the
> individual can wind up bent when the computer says they're fine. Statistics
> seem to confirm this hypothesis.

Right on the first thesis, wrong on the second one.  What statistics do you
have that back up this point?  I have seen statistics that back up a
correlation
between obesity and DCS, but none that suggest a causal relationship between
elevated body fat percentage and DCS. 


> |> And, in the end, I find the experimental evidence that obesity is a
> |> contributive factor DCS to be lacking, which is the ultimate test..
> 
> So you don't believe the statistical evidence? We are talking about a
> statistically significant number here.

Of course I do.  What do you think experimental evidence is?
Statistical evidence == experimental evidence.  We're all participating in
an experiment every time we dive, an experiment for which DAN keeps
pretty good records. 

> |> While it is true that DAN statistics do show correlation between being
> |> overweight and being hit, other numbers contradict this. 
> 
> What other numbers?

 "Percent Bodyfat and Human DCS", Undersea Biomedical Research,
circa 1989 (I'll look up when I go home if I remember to).  (Percentage
of body fat in fit, non-obese divers shows no link to propensity
to get bent.) (summary mine)


> |> First of all, the increased likelyhood is not geometric with
> |> percentage of body fat. [rest of explanation elided for brevity]
> 
> One would not expect a first order relationship such as this. Remember that
> there are alot of other tissues in the body, which must be considered as
> well (DCS does not occur exclusively in fat tissues, thereby skewing the
> statistics).

But again, one would expect *at least* a relationship such as this.


> |> The other forgotten factor is fitness.
> 
> It is not forgotten at all, rather, it is cited as a potential factor
for increased risk of DCS, although there are no theories (to my knowlege)
> that explain a correlation here, nor am I aware of any statistical
> confirmation that there is indeed a connection.

I don't really care about theories....My comment is that it's forgotten in
the discussion of fat, not in the discussion of DCS.  My point is that nobody
(that I know of) has tried to do a study to decouple the direct causative
effects of fat and DCS incidence.  So people claim fat increases chances of
getting hit, but it's never clear if it's the fat or the fitness or
other factors.
Saying fat correlates with getting hit, means nothing by itself.  Even in
the presence of a working hypothesis.


> |> It is possible to be
> |> extrememly fit yet obese.
> 
> Well, I know quite a few Physicians that would argue against this
point vigorously. 
.
.
.
> Further, high blood pressure may be an added risk factor for DCS as well.

Not all obese people have high blood pressure or any of the other 
characteristics you cite.  True, most do but that's not the point. 
What do any of these things have to do with fat besides being common 
in obese individuals?  If you find the few obese persons (200 lbs persons who
should weigh a little less than 170 lbs) who run 10 miles daily, have great
strength, stamina, aerobic endurance and low blood pressure, and you find
that they have elevated chance of DCS, then you can claim that fat
has something to do with it (until you discover that maybe there's
another confound), but otherwhise it seems you're just making
bad conclusions.  There's something to controlling factors when
conducting an experiment or analyzing statistical data.


> |> But the majority of obese individuals are
> |> not very fit.  And since the studies I've seen suggesting a link
> |> between obesity and DCS do not eliminate fitness (which could've been
> |> quantified rather than briefly mentionned as a "potential player") as
> |> a confounding factor, they would seem exceedingly flawed.
> 
> In order to make this assertion, you must first define what "fit" means,
> and then offer at least a hypothesis for how it can affect DCS
> susceptability. Just such a hypothesis does exist for increased body
> fat percentage, and the statistics seem to confirm this hypothesis as
> valid.

All you need in ways of hypothesis to conduct a proper experiment is
something along the lines of "Fitness (defined as XXXX) is causally
related with low incidences of DCS.  Let's prove or disprove this...."
you don't need a mechanism -- good grief a lot of medicine would have
to be tossed if mechanisms had to be hypothesized before any
kind of conclusion from experimental tests (or statistics, FWIW) could
be drawn.  There are lots of things that are but
we don't know why.  But once you do have a hypothesis, 
proving it in the presence of known confounds and not eliminating the
confounds proves nothing and shows bad research method.

> What does "fit" mean, and how can it influence susceptability to DCS?

Fit can be drawing an arbitrary line with a treadmill and monitoring
metabolic efficiency.  Or whatever.  Pick your metric, and then eliminate it as
an experimental confound from your studies of fat.  We do have reason to
suspect that it's a factor, so not quantifying it and (at least trying to)
eliminating it from studies of fat seems intellectually fraudulent.


But back to real world.....

Thanks for the response, Dave.  I don't doubt that fat and DCS are
correlated, I just have trouble concluding that the fat is the culprit
rather than all sorts of other conditions associated with fat (or obese).
The fact that I've yet to see a study or controlled statistic that
eliminates confounds bothers me.  This should not be impossible,
why's nobody doing it?  Or has it been done and I've not seen it?  It's
possible, I'd love to be wrong.

You don't need a hypothesis to realize that other factors have effects
on DCS propensity.  Cavemen knew that lightning could ignite
tree trunks long before they had any idea why.

BTW, DAN stats on women and diving show a slightly lower propensity
for DCS than men (about 22% of DCS and AGE cases in '88 involved women,
and the general diving population is comprised of about 40% women)
But DAN statistics like that are irrelevant unless you compare apples
with apples -- that is start comparing diving frequencies, profiles,
etc.  More relevant are retrospective studies and other controlled
studies which go both ways (leading me to think that it really
does not matter....)

Cheers....

F.

P.S. I'm 5% above my "ideal weight" (defined as the time I was in
college and was an athlete)
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