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It's important to know that lactic acid is the result of anaerobic
glycolysis---short-term, emergency energy production without the
presence of oxygen---therefore in a normal, healthy resting horse, the
lactic acid levels should be close to zero. There are instances in
which lactic acid levels will be present in a non-exercising horse, but
generally these are going to be in an animal that is somehow otherwise
being stressed, such as impeded blood flow to a muscle group,
post-surgical stress, etc. or if there's some sort of metabolic
disturbance, such as in horses that chronically tie-up.
Exercise physiologists will often refer to the term Onset of Blood
Lactate Accumulation (OBLA), which is 4 mmol/liter. Another useful term
is VLA4, which is the velocity at which a horse can travel at the point
at which his blood lactate levels hit 4 mmol/liter. It used to be that
physiologists thought that a heart rate of 158 bpm marked the threshold
at which a horse really turned on the anaerobic glycolysis pathways and
at which blood lactate levels began to quickly accumulate. However, as
Bob Morris pointed out to me some months ago, this "anaerobic threshold"
varies between horses and in response to conditioning, and he's right,
it is now accepted that the heartbeat associated with OBLA can vary
pretty widely. That's what you get for not keeping current with new
research coming out. Live and learn.
The levels of lactic acid measured in the blood will continue to
accumulate as intensity and duration of exercise increases. Generally,
the musculoskeletal system will be unable to tolerate high levels of LA
for long and will be forced to slow down or stop. This is because the
LA is produced within the muscle cells and as LA accumulates, the
environment within the muscle cell becomes more acidic. The enzymes
associated with energy production only operate well within a fairly
narrow pH range and efficiency decreases as the pH falls. I posted
something about this just a few days ago, it should be in the archives.
When measuring blood lactate levels, the LA will continue to rise for
about ten to twelve minutes after the cessation of exercise. This is
because LA leaves the muscle cell by diffusion, meaning it just leaches
out on it's own, it is not actively pumped out, so it takes a few
minutes for the majority of the LA to leave the muscle cell and enter
the blood stream. What will help clear the blood lactate is to continue
mild muscle contractions, which squeeze and release the muscle cells to
indirectly pump LA out. Also keeping the heart pumping at mild levels
keeps the blood flowing through the systems, which also helps clear
lactic acid. As the LA leaves the muscle cells, about 18-20% of it
circulates back to the liver, where it is removed and recycled through a
process called gluconeogenesis back into glucose and glycogen for more
energy production. This is a relatively slow process, so this is
glucose for future energy production, not immediately. A good reason to
not exhaust your horse's energy stores during extended exercise. About
8-10% of the lactic acid is converted to proteins, and a very small
percentage is going to cleared through the kidneys and sweat. The
majority of lactic acid (about 65%) is going to be oxidized to water and
carbon dioxide while being used as a metabolic fuel by skeletal muscle
as well as brain, kidney, liver and heart muscle. Once oxygen is
available again, the LA is converted to pyruvic acid which is utilized
in the TCA cycle for energy production. This is why horses actually
recover resting LA levels faster if they're being walked than if they're
just allowed to stand---not only does muscle contraction help remove LA
from the muscle cells, it also maintains the metabolic demand of the
body to help recycle the lactic acid and thereby remove it from the
blood. Any good university text on exercise physiology (The
Physiological Basis for Exercise and Sport by Edward Fox, Richard Bowers
and Merle Foss is very good) will have a compete description of the
biochemistry and fate of lactate removal post-exercise. Since the basic
concepts are the same in both horses and humans, the human text will do
you just fine. The above book also cites specific, recent research.
The maximum recorded LA levels in intensively exercising horses has been
measured under lab conditions at around 30 mmol/liter and in a few
instances at over 40 mmol/liter after repeated bouts of maximal
treadmill sprinting. In the latter case, these are truly screaming
levels and I wouldn't be surprised if these caused some permanant damage
to the horses involved. Maximum tolerable lactate levels vary widely
among species, for example sea turtles commonly tolerate levels of over
100 mmol/liter. Blood lactate levels of 20-25 mmol/liter are commonly
measured in Thoroughbreds after the longer races or in steeplchasing, or
in harness racing. Blood lactate levels in show jumpers tend to be much
lower, only around 2 to 3 mmol/liter even in international competition
and the average levels for most endurance horses is around the same 2-3
mmol/liter. The exception to this would be the horse being ridden too
hard, too fast, too long, greatly contributing to the onset of fatigue.
A very good reason to know where your horse's general anaerobic
threshold is and try to stay below it most of the time.
I will rummage through my library and try to find the specific
references that quote the observed lactate levels in jumpers and
endurance horses. In the meantime, for literature citations, you will
need to use KSU's AGRICOLA database system in the library. Wait for the
SEARCH prompt, then type in (without quotes) "lactic acid AND horse? AND
exercise". Just like that. That should give you a TON of potential
references to print out and then go ferret out in the library's
references. There are hundreds and hundreds of studies done on equine
lactate levels on every aspect, much more than I can possibly even start
to cover here. A book I would also HIGHLY recommend, especially if
you're planning on making equine exercise physiology part of your future
career, is "The Athletic Horse---Principles and Practices of Equine
Sports Medicine" by David R. Hodgson, Reuben J. Rose. It isn't cheap
(about $80-90) and is not light reading---if you don't already have a
good understanding of basic biology/physiology, it may be hard to
understand, but then you can always keep a basic bio book around and go
look up whatever you don't understand. Anyway, it is an outstanding and
up-to-date book, very technical and detailed, based on thousands of
cited references all listed in the backs of each chapter, so an
excellent place to find specific information with specific references.
KSU's library may have it, or someone else in the vet med department, or
you can probably do an inter-library loan.
like to
> know if there is anything in the literature regarding the effect of lactic acid
> on exercise...good or bad.
See above brief discussion of pH level's effect on exercise, plus the
recent post "Re: anaerobic conditioning" last week. Lactate levels are
directly related to fatigue, along with glycogen depletion and other
factors.
I did a database search on the subject and
> found some articles that Susan wrote back in California, but I haven't been
> able to locate them in the library.
It's nice of you to consider me as any sort of a source for anything,
but for a graded paper, don't use anything other than references from
refereed professional journals, preferably ICEEP (International
Conference on Equine Exercise Physiology), Journal of Animal Science,
Journal of Experimental Biology, Journal of Nutrition, Journal of
Applied Physiology. Proceedings from the Equine Nutrition and
Physiology Society are okay, but not quite as good, sometimes the
statistics are a little shakier than the above listed journals. For the
most part, try not to use veterinary journals such as Veterinary Record
or Equine Veterinary Journal---this is absolutely not a flame to DVMs,
but DVMs are of course primarily trained in the technical aspects of
diagnosis of disease, treatment, etc, and are usually not formally
trained in research methodology and statistics unless they also have a
Ph.D or at least an MS, which ARE research degrees. So alot of the
articles in veterinary journals are reports of specific cases studies,
not always research in the sense of statistical models and methodology.
This isn't always the case, you just have to be careful if you're
quoting facts based on one DVM in Outer Mongolia that happened to find
one horse that tolerates LA levels of 245 mmol and outruns fighter jets.
And never, never, never quote lay publications like Equus of Western
Horseman. It's not that the information is necessarily wrong, but
always hunt down the published research that the article is based on and
use that. "Lay publications" would also include anything written by
anybody on this list, certainly including me. Just use it as a starting
point and track down the original source. I once made the mistake of
citing a lay publication for a class paper and got well and truly
creamed over it. There's too much empirical research available for you
to EVER use lay publications as a source, unless you're simply quoting
someone's opinion.
> Additionally, since there is an increase in the amount of lactic acid
> secreted in the stomach after a meal is consumed would this have any
> effect at all on exercise and performance and thus affect the time at which
> horses should be fed prior to exercise.
I think you might be confusing lactic acid with gastric acid. Lactic
acid is only produced within muscle cells as a result of anaerobic
glycolysis. Gastric acid (hydrochloric acid) is secreted by parietal
cells the stomach mucosa in response to the hormone gastrin (which also
stimulates gastric motility), and histamine, which in turn are
stimulated by vagus nerve activity and the presence of peptides and
proteins in the stomach. The purpose of gastric secretions are to help
break peptide bonds in the food bolus, activate some of the gastric
enzymes such as pepsin and kill of some of the bacteria that enter with
the food. There are numerous factors that can affect acid production in
the stomach, including stress. It's very common in sick foals to have
stomach ulcers (they'll starting grinding their teeth from the pain) due
to the stress of being sick, in pain, being treated, being separated
from Mom (who's also upset) or whatever. You would definitely have to
do an AGRICOLA database search on this subject---articles published in
Proceedings from Equine Nutrition and Physiology Society would be a
likely place to look, also Journal of Nutrition. Try the search
parameters "horse? AND gastri?" By using an incomplete word along with
a question mark, the results will include articles that involve any word
with the root gastri- such as gastrin, gastric enzymes, gastric
secretions, etc.
Another good general reference is "The Horse" by J. Warren Evans (no
relation), Anthony Borton, Harold Hintz, L. Dale Van Vleck. Good
general information on nutrition and nutritional physiology with
specific references cited. Don't use the book itself as your citation,
use the original research cited.
Good luck with your paper and nutrition class.
Susan Evans Garlinghouse
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