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Beating the Metabolic Pull
Part 1 - Hydration
Reprinted from Endurance News, June 2000
Susan Garlinghouse, MS
My belief has always been that if a rider understands some of the
why in nutrition and physiology, then it is much easier to understand
the how in making well-informed decisions during and between rides.
This article is the first in a three-part series explaining a little about
the way things work in an exercising horse, along with suggestions on how to
apply this knowledge for better metabolic integrity and performance.
Whether your goal is to race at FEI levels, top ten or just get back
into base camp before the barbecue is all gone, the common denominator is
that first you have to finish with a horse that is fit to continue. You do
not need an advanced degree to recognize the metabolically fit horse---he
has good gut sounds, is eating, drinking, is well hydrated, bright and
alert. Most of us have also seen the other end of the scale---the deflated
horse with an IV running into his neck, that the treatment vet is hovering
over, that is on his way to a clinic. The difference between the fit to
continue horse and the treated horse depends largely on three primary
metabolic factors---hydration, gut motility and energy balance. The first
two are so closely related as to be almost the same issue and are by far the
most critical factors in maintaining metabolic integrity. The third factor,
energy balance, has become a hot topic and can certainly make the difference
between a win and a middle of the pack finish. However, the amount of
rocket fuel on board is not going to help if your horse is dehydrated,
colicking and already in trouble. If you remember anything from these
articles, remember the order of priorities---hydration and motility first,
and then energy balance. Assuming your horse is conditioned for the
job at hand, and you have paid attention to maintaining hydration and
motility during every stage of the ride, you will find your horse has better
performance, recoveries and stamina, long before you start considering, "how
do I increase his energy?"
This first article covers hydration, which is much more involved than
just letting the horse drink at every water stop, and remembering to carry a
sponge. What exactly does water do in the body, anyway? For the endurance
horse, one of the most critical roles is the removal of excess heat during
exercise. During a fifty-mile ride in ambient temperatures, the average
horse will produce enough heat to melt a 150-pound block of ice, and then
bring that water to a boil. If that heat is not removed, the internal
body temperature will quickly rise high enough to literally cook the entire
body.
Evaporative cooling via sweat production and respiration accounts for the
majority of heat dissipation during exercise. Horses that are dehydrated
progressively lose their ability to produce sweat, a condition called
anhidrosis, resulting in loss of cooling and a concurrent rise in
body temperatures. As the body dehydrates and blood loses plasma volume and
fluidity, the cardiovascular system becomes less efficient at transporting
oxygen and other resources throughout the body. The heart rate increases to
compensate, so that a horse that canters easily at 130 beats per minute when
fully hydrated may have a heart rate of 20-30 beats higher when dehydrated,
simply due to the extra work of pumping less fluid blood. Not only does
this result in slower recoveries, but it also has a significant effect on
the efficiency of muscle function. To maintain the same intensity of work,
the horse will rely more and more heavily on anaerobic metabolism,
contributing to faster fatigue and greater incidence of metabolic disease,
such as colic or tying-up. As effort increases and efficiency decreases,
the body responds as though to an emergency (which, in fact, it is), and
begins to shunt blood flow away from less-vital organs, such as the gastroin
testinal tract, in order to maintain maximum circulation to heart, lungs,
muscles and central nervous system. As blood flow decreases to the
digestive tract, gut motility slows and may stop entirely, leading to colic
until blood flow and motility are restored.
Progressive dehydration also affects the normal functioning of the
"thirst center" in the central nervous system. Thus, dehydrated horses
badly in need of fluids may entirely lose interest in drinking voluntarily.
If you know your horse has been working and sweating hard throughout the
day, and yet is not drinking, do not assume he doesn't need water.
In fact, he may be approaching a metabolic crisis if not resolved quickly.
Don't make the mistake of thinking, "he knows best what he needs"---use your
head to make the right decisions on his behalf.
During a hot and strenuous ride, horses can lose from 1.5 - 4 gallons of
water per hour in the form of sweat. Over the course of a 50-mile
ride, this can often add up to ten (or more) gallons of fluid lost solely
through sweat production. Research conducted by Gary Carlson at UC Davis
indicates that the average Tevis horse experiences a net loss of almost five
gallons of fluid between the start and finish (equivalent to approximately
4% of body weight in a 900 - 1000 pound horse). Losses of over 12 gallons
have been measured, representing 10% of the body weight. Keep in mind these
numbers represent the fluids that remain unreplenished in the equine body,
after the horse has presumably had ample opportunity to drink
throughout the day. These results indicate that even under ideal
circumstances, horses may not be able to drink enough water to replenish the
fluids lost through sweat production, resulting in progressive dehydration.
It has been estimated that dehydration losses of as little as 3-4% (that
is, 3-4% of body weight has been lost in the form of fluid) have an adverse
effect on performance, even though outward clinical signs may not be readily
apparent. Horses experiencing an 8% dehydration have a capillary refill
time of 2-3 seconds, poor skin tenting, dry mucous membranes, dry feces
(and, therefore, are at greater risk of colic) and generally a high, hanging
heart rate. A horse at 10% dehydration is in serious trouble, requiring
extreme veterinary intervention, and at 12%, the horse is close to imminent
death. Skin tenting alone is a relatively inaccurate method of determining
extent of dehydration, and often lags behind changes in true hydration
status. Therefore, along with the ride veterinarian, you must consider
all metabolic factors in evaluating your horse, including mucous
membranes, gut motility, heart rate, capillary refill time, attitude and way
of going.
What is the difference between a clinically normal horse with 4%
dehydration and one in metabolic distress at 8% dehydration? Less than
five gallons of fluid in the body can make the difference between completion
and a metabolic crisis. So--- your horse is already drinking at every
puddle and bucket, you have finally mastered that flying sponge trick, your
crew is waiting with plenty of cool water for washing, and you dutifully
clip his winter hair every year. What else can you do to improve his
hydration status?
One of the easiest ways to prepare for good hydration on Saturday is to
maximize forage intake the week before. Forages take several days to reach
the hindgut, so that Thursday's hay is in the cecum and large colon on
Saturday. For reference, the foregut consists of (in order) the stomach and
small intestine, while the hindgut consists of the cecum, colon and rectum.
Fiber both encourages water intake and absorbs and holds water as it moves
through the digestive tract. Although 90% of the water will have already
been absorbed prior to reaching the hindgut, several gallons are still
present and available as the hay moves through the system on Saturday. This
provides a significant extra reservoir of fluid and electrolytes to draw
upon during exercise-induced dehydration. Recent research has indicated
that feeding one of the soluble "super-fibers", such as soaked beet pulp,
along with hay, further increases this fluid reservoir. This extra water
alone may make all the difference between Completion and Trouble. Make sure
that the horse has hay available during the trailer ride to base camp, as
well as immediately upon arriving and unloading. Adequate fiber intake the
night before, as well as a dose of electrolytes, will trigger thirst
responses and drinking throughout the night to ensure the horse starts fully
hydrated.
The timing of meals fed before and during a ride also has an effect on
hydration. Many horses are still provided with a large "breakfast" before
the start, little or nothing until the lunch stop when another large meal is
provided, and then little or nothing again until the finish. Studies have
demonstrated that such feeding practices (more than 4-5 pounds of any type
of feed, spaced more than 2-3 hours apart) results in a large fluid shift
from the plasma volume (the fluid portion of blood) into the digestive
tract. These fluids are used to provide saliva and other gastric juices
needed to process the large meal. In a 1000-pound horse, these fluid shifts
may equal 4-5 gallons of fluid, resulting in a 15-24% decrease in total
plasma volume. Don't worry about the exact numbers, just think which is
easier for the heart to circulate---thin, fluid blood, or thick "sludge"?
While this fluid moves back into the plasma volume within a few hours, the
net result is a transient dehydration that can significantly affect
performance until the condition corrects itself. In a backyard horse
standing around doing nothing, the effect is relatively unimportant---to an
endurance horse that covers many miles in those few hours, the effect can
make a significant difference.
To avoid this fluid shift, simply avoid feeding large meals only at vet
checks---help your horse be a "nibbler" instead of a "feast-eater" during
endurance rides. The same amount of food, fed in small, frequent meals
every hour or two---instead of intermittent feasts--- avoids these fluidshifts entirely, and yet still provides the same total nutrition. Make an
effort to provide small amounts of food in between vet checks---a baggie of
hay or grain in a cantle bag, or a few minutes of grazing along the trail.
If you know you will be doing some footwork in the next few miles out of a
vet check, carry along a thin flake of hay and hand it out as you jog along.
Practicing eating along the trail at home will make it easier for your horse
to do so during a ride---and there are few tricks your horse will learn
faster than that you want him to eat along the trail! Although opportunity
differs for every rider depending on the goals for the day, the point is to
examine your riding plan and make an effort to provide small, frequent meals
whenever possible, avoiding the intermittent feast. Those few extra minutes
spent along the trail will be worth the effort in metabolic health and
performance.
The rule of "small and frequent" also applies to anything provided in an
oral syringe. While fluid shifts are not as large or dramatic, any
concentrated source of salt or sugar draws fluid inward until the diluted
solution is reabsorbed into the bloodstream a relatively short time later.
To minimize the effect, any oral syringing should be broken up into smaller
doses---better eight 2-ounce doses than two eight-ounce doses! Make every
effort to only syringe after the horse has already had a drink (preferably
immediately afterwards), as the less dilution required from plasma volume,
the better. Not only will plasma volume be spared, but also absorption of
the electrolytes into the system will be more efficient and thus more
available during exercise.
Pre-loading electrolytes several hours before the start and throughout the
day not only avoids progressive electrolyte depletion, but also triggers a
complex endocrine response in the kidneys and central nervous system to
encourage early drinking. Once absorbed, the body does not store excess
electrolytes, so pre-loading should be limited to the night before and
several hours before the start. Pre-loading for days and days before a ride
does no harm, but is simply a waste, as the kidneys have long since flushed
the excess out in the urine as soon as current needs have been met. While
salt does trigger a thirst response, and can be used to encourage drinking
during a ride, the response is not an immediate one. Use this as an early
strategy to maintain a metabolic edge throughout the day---if you wait until
the horse is already dehydrated and in a crisis state, the best you can hope
for is damage control. Recognize the difference between a horse that is not
drinking because he doesn't like what is being offered, and one in a
metabolic emergency. In many instances, all the horse may need is a few
extra minutes to recover, eat some green grass, hay or mash, and then will
drink normally. If the horse is not drinking when you know he should be, is
uninterested in food, recovering poorly, acts dull or colicky, or is
otherwise exhibiting signs of exhausted horse syndrome, do not attempt to
magically fix the situation with a large oral dose of electrolytes alone.
At this point, it's entirely possible to make the situation worse instead of
better.
Realize that the horse is in a crisis and seek veterinary help
immediately---although correcting the electrolyte imbalance is an immediate
priority, administration with fluids via intravenous or nasogastric tube
into the stomach, rather than oral syringing, may be required to prevent
further deterioration of the situation.
Although not as prevalent as in past years, it is still common to see
endurance horses being fed rations which are well in excess of protein
requirements, especially in the West, where good alfalfa is cheap and
plentiful. While many horses have and do compete successfully on
high-alfalfa rations, this too has an effect on hydration status and should
be a consideration in your metabolic strategy plan. Horses that compete
well on high-alfalfa rations are most likely doing well in spite of
the high dietary protein, not because of it---undoubtedly a testament
to the many other management, conditioning and riding factors that a smart
owner puts into a successful ride. For every horse that wins a ride while
consuming a high-alfalfa ration, there are undoubtedly many others that
could have finished, placed higher, or earned better vet scores by simply
decreasing the dietary protein consumed. This conclusion is supported by
Dr. Sarah Ralston's work at Rutgers University, which suggests the incidence
of metabolic pulls increase as dietary protein levels significantly exceed
requirements.
Mature performance horses only require 8-10% crude protein in their
diet, and these needs do not significantly increase with the demands of
endurance conditioning. Good-quality grass hay or pasture easily provides
these protein requirements regardless of the level of performance. If you
are in doubt about the quality of forage, a few pounds of a 12-14% grain mix
from a reputable company ensures adequate protein without supplying excess.
Supplying "extra" in the form of alfalfa or high-protein supplements, such
as Calf-Manna, to "support muscle development", is neither required nor
beneficial.
A high protein ration's effect on hydration is based upon its inherent
nitrogen content. Once protein requirements have been met, the body
utilizes excess protein for energy production. The amino acid molecule is
snipped apart and the carbon backbone sent into energy-producing pathways,
while the remaining nitrogen atom is discarded. Nitrogen is first degraded
to ammonia and then to urea, which is subsequently filtered out by the
kidneys and excreted in the urine. Both ammonia and urea are toxic
substances, therefore urine production to remove them from circulation takes
priority over water conserving responses during exercise. The net effect is
that horses consuming high-protein rations have increased urine production
and higher water requirements simply to clear the body of an avoidable waste
product. In horses living in box stalls (not uncommon in highly developed
urban areas), the increased ammonia and urine production can lead to greater
incidence of upper respiratory irritation, as well as poorer hoof wall and
sole quality. During a ride, when water intake may already not be enough to
keep up with loss, the additional loss of water through increased urination
is an added contribution to potential dehydration.
While excess protein does contribute to energy production, the pathway
is a relatively inefficient one, as protein metabolism produces 3-6 times
more waste heat than does the utilization of an equivalent amount of
carbohydrates or fat. In cold climates, this heat production from excess
protein can be used to help maintain body temperature, especially during the
off-season. However, during hot weather and prolonged exercise, this excess
heat must be removed from the body via the same cooling mechanisms as heat
from exercising muscles---sweat production and respiration. During intense
exercise in hot or humid conditions, the net effect is a greater heat load
to dissipate, increased fluid and electrolyte losses, and yet another
contributing factor to potential dehydration.
Does this mean you should not feed alfalfa at all during endurance rides?
Not necessarily. Alfalfa contains high levels of both calcium and
potassium, and small amounts throughout a ride can help offset electrolyte
deficits. However, a few pounds at vet checks are sufficient, especially
if you are otherwise providing electrolyte supplementation, and more alfalfa
is not necessarily better! If your horse is being picky at a stop, and
refuses anything but alfalfa at vet checks, better to let him eat more
alfalfa than he really needs than to not eat anything at all. Ideally,
however, provide limited amounts of alfalfa, while offering other lower
protein feeds such as grass hay, beet pulp or grain-based mash. At home (if
alfalfa is fed at all), limit intake to 25% of the forage ration, and never
more than 50%. Again, while many horses continue to compete successfully on
high-alfalfa rations, its effects on hydration status should be a
consideration in your management plan.
To summarize the main strategies included in this article:
- Maximize forage intake for several days before the ride, including the
use of "super fibers", such as beet pulp.
- Pre-load with electrolytes the night before and several hours prior to
the start.
- Provide small, frequent meals every hour or two along the trail by
carrying along a few pounds of feed, or by intermittent grazing.
- Anything provided in a syringe should be provided in small doses at
frequent intervals, preferably after a drink.
- Provide a ration adequate, but not excessive, in dietary protein by
limiting alfalfa and other high protein feeds.
Next month's article will address strategies to maintain gut motility.
References:
- Lewis, LD. Equine Clinical Nutrition: Feeding and Care. Williams &
Wilkins, Baltimore. 1995.
- Hodgson, DR, RJ Rose. The Athletic Horse, Principles and Practices of
Equine Sports Medicine. WB Saunders Company, Philadelphia. 1994.
- Carlson, GP. Thermoregulation, fluid and electrolyte balance. In Snow
DH, Persson SGB, Rose RJ (eds): Equine Exercise Physiology. Cambridge,
Granta Editions, 1983, pg 291.
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