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> Susan - maybe you could address the osmolality factor - the concentration
of
> salts in the stomach and gut, vs concentration of salts in >the blood
stream.

That sounds good to me. :-))

Okay, the background concept regarding osmolality (very similar to
osmolarity, for those painfully remembering their inorganic chemistry) is
that it's a measurement of how concentrated a solution is.  As it relates to
this discussion, how salty is the liquid slopping around in a horse's
stomach and how does that affect it's absorption into the bloodstream (where
it's used for metabolic chores, or if in excess, gets filtered out by the
kidneys and excreted in the urine).

The important concept here is that if the contents of the stomach are 1)
high fat, 2) high fiber, 3) high protein or 4) high osmolarity...then it'll
either leave the stomach fairly slowly (or several hours), or sit there
until the body has done something to make it more conducive to leaving and
moving on into the small intestine.

When you're talking specifically about elytes, how concentrated the solution
is depends on:
1) How large the dose was (keeping in mind that formulations differ widely);
2) How much water was ingested concurrently or within the recent past to
dilute the salt;
3) Any feed in the stomach with the capacity to affect the overall
concentration of the stomach contents (which is a whole 'nother discussion,
as the effects of feed vary widely)
4) To some extent, the chemical composition of the elytes---are they
administered in a powdered form (ie an ounce of EnduraMax), or are the
elytes those inherently contained with a flake of hay that will require
additional processing to make available?

So the basic physiology is that if the concentration of salt is higher in
the stomach than it is in the bloodstream, the normal body response is to
dilute the stomach contents before it'll release it into the small
intestine.  Fluids originating from the bloodstream are secreted into the
lumen of the stomach until the salt is adequately diluted, after which it
all goes on down the road and gets absorbed into the bloodstream.

So here are just some of the issues to consider---while the salt is sitting
in the stomach getting diluted by body fluids, it can be irritating the
lining of the stomach, potentially contributing to formation or maintenance
of gastric ulcers.  Conclusion, we don't want (to use Heidi's term) "a big
wad" of salt sitting in the stomach delaying emptying time---we want the
stuff to ideally arrive already diluted as much as possible, and not in
overly large doses, so that it will sail right on through towards absorption
in the small intestine.

Thus, my recommendation not to dose elytes in "big wads".  My preference for
a dose is an ounce or two per hour, *depending on conditions and intensity
of exercise*, preferably while/after the horse is having a drink.  Keep in
mind that that ounce or two is also well-cut in most cases with other
substances, such as sugar of some sort, applesauce, Maalox, Tums, whatever.
IMO, a far cry short of a big wad.

Another issue to consider is pH within the stomach, which also affects
gastric emptying.  If stomach contents are too alkaline (and offhand, no, I
don't know the optimum pH), then gastric emptying is slowed until gastric
secretions bring pH down.  A lot of riders add buffering agents like Maalox
or Tums to their mix against potential ulcer problems.  Something to
consider is whether the negatives of buffers outweigh the benefits.  Would
it be better to not add buffers at all and get those elytes shipped on
through and into the small intestine as quickly as possible, minimizing
their irritating effects on existing ulcers?  Frankly, I have some doubts
that a few cc's of Maalox make a doggone bit of difference in soothing an
ulcer.  In any case, we don't know what the optimum solution here is, in
part because we don't really know why endurance horses are developing
gastric ulcers in the first place.  In the meantime, my recommendations
based on all these factors remain that I suggest dosing appropriate to the
conditions and in small and frequent intervals to facilitate gastric
emptying and rapid absorptions without excessive negative effects on the
gastric environment.

Could impaired gastric emptying have been a significant factor in Darolyn's
horse dying?  If the stomach ruptured, then it damn sure was a factor
somehow and somewhere.  Is an electrolyting protocol alone sufficient to
totally impair outflow?  I sincerely doubt it, but that's just my opinion.
Was it a *contributing* factor?  Maybe.  No way to even comment without
knowing exactly what the protocol was, and probably not even then.

Okay, back to our regularly scheduled program---so now the salts are
adequately diluted according to the body's requirements and have moved into
the small intestine for absorption.  Assuming glucose is available
concurrently, then sodium, chloride, glucose and water all get pulled from
the gut into the bloodstream through transport cells.  The e'lytes are
circulating through the bloodstream to be used in whatever chores those
elytes are slated for.  Via kidneys, baroreceptors, adrenals, etc the body
controls how concentrated the blood is within a fairly narrow range----above
or below that, and you potentially have problems, sometimes life-threatening
problems.

So let's say that a dose of electrolytes has just moved from the gut into
the bloodstream.  More than is currently needed for neurologic functions of
muscle contractions and so on.  The are two other functions to be considered
in distance horses---1) cooling the body via sweat production, and 2)
maintaining adequate water intake via thirst responses.

Regarding sweat production, evaporation of water is what cools the body, but
salt is how the water is actually transported to the body surface.  Salt is
actively transported externally, the water follows because that's what water
does, <g> and voila, sweat and cooling.  If there isn't enough salt in the
circulation for transport, than how hydrated the horse is is
irrelevant---the horse is simply not going to sweat, and thus core body
temperatures may rise to lethal levels.  This is where the vast majority of
elytes go during an endurance ride---how much depending on how hard the
horse has to sweat to try to cool himself adequately.  Truman has pointed
out on many occasions that at high humidities, horse will sweat buckets all
day, but often can't come close to evaporating enough sweat to remove heat.
They just keep sweating without deriving any benefit from it.  Thus a horse
in humid conditions working even at moderate to slow speeds may easily have
elyte requirements that FAR exceed a horse in dry conditions, even one
working at much faster speeds.  If those requirements aren't met, then that
horse is at far greater metabolic risk of a crash.

Lots more to talk about, but I promised myself a nice ride this afternoon,
so I'll post more tomorrow.

Susan G, DVM



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