First, a brief interlude...
> I apologise. Dr. Snow has a reputation, much of it good. The word "god" is
> yours, however.
Yes, and I only meant to convey my own respect for someone who has done
far more work in the field than I ever will. No advocation of group
adoration is implied or expected.
All humans are
> capable of error. This certainly includes me.
And me, as well, far more often than I would like (who wouldn't?) and
sometimes in truly spectacular fashion. Sometime let me tell you about
my first husband....<g>!
I will, though, reserve the
> right to determine what is appropriate in my own expression. I am sorry if I
> have offended you.
And I apologize that I often mix my own opinions with scientific fact
(sometimes also subjective) without clearly delineating which is which.
You're tuned in, looking for solid knowledge, and have a
> great deal of valuable knowledge already imbedded in your mind. But, to me,
> you are still a student, at least in the applied aspects of the things we
> discuss here.
Absolutely, and plan on being one for as long as I can still read a
book, process incoming knowledge and remember my own name without
dribbling porridge down my front. I do not consider myself an
"authority" on much of anything, I only try to pass on information from
other sources, sometimes not very well. I will try harder to label
opinions as such, although I retain the right to speak from the heart
regarding my own critters. I am admittingly a doting fool when it comes
to my own furry tribe.
If you and I were sitting in a room discussing these things, I
> would have said precisely what I've said here, and there would be far less
> acrimony between us.
Then I would be delighted to buy the first round if you are planning to
come to the convention. Or ever make it to L.A.
Meanwhile, on to the discussion re glycogen loading for endurance
horses:
First, please understand that I do not now or ever have made the
statement that glycogen-loading will positively kill or injure an
endurance horse. I only have concerns, based upon the below sources,
that glycogen-loading may not work as well for endurance horses as it
may in other venues, such as for humans and probably racehorses. There
is zero research as far as I know, on glycogen-loading specifically in
endurance horses, probably because funding for endurance research (at
least around here) is pretty hard to come by. Therefore, trying to
extract tidbits for endurance horse management must generally come from
exercise studies conducted on racehorses, which do not always
equate---sometimes, but not always. I do not claim to have read every
article on carbohydrate metabolism and would be delighted to find some
data which allays my concerns regarding endurance horses specifically.
My concerns and my opinion is based on the below sources. I could swear
that I have read similar statements in other research articles, however,
as I cannot lay my hands on them right this minute (they consist of a
comment here or there, not the main focus of the study, therefore I
can't easily find them through "keywords"), I will throw them out as
"evidence". I would not want to stake my life on any of this, in my
mind it is only enough to raise some question about this topic.
1) In a study by Pagan, et al published in 13th Equine Nutrition and
Physiology Symposium, "The effect of exercise and diet on muscle and
liver glycogen repletion...", the following statements are made:
Quote: "There is still some question whether glycogen loading is
beneficial at all for horses. In Topliff's (1985) study, the horses had
reduced work performance after glycogen loading and Pagan et al (1986)
reported higher heart rates and more anaerobic glycolysis during
strenuous work when the horses were fed a high CHO diet that resulted in
greater levels of muscle glycogen.
Acute rhabdomyolysis ("tying-up") has been associated with high
carbohydrate diets for many years. Carlstrom (as reported by McClean)
proposed that high CHO diets resulted in the increased storage of
glycogen and an abnormally high rate of lactic acid production which
caused degeneration of the muscle cells...(then a description of a horse
in this study on a high carbo diet which tied up badly)...obviously,
there are factors other than diet involved in the etiology of the
disease, but it appears likely that high muscle glycogen may be
detrimental to horses already predisposed to "tying-up". Muscle
glycogen "loading" appears to be most successful when high CHO diets are
fed to horses following intensive exercise bouts which deplete large
amounts of glycogen from the muscle. The usefulness of this practice,
however, is questionable." End of quote.
ME: From this, I derived (and maybe I'm wrong) that glycogen-loading may
be a problem in horses prone to tying-up, as many endurance horses are.
I have no idea how this would affect a horse trying to tie up DURING a
ride. I also gathered that glycogen-loading would be of most use
following a protocol of intense exercise (in this study, it was 10 m/sec
for 14 minutes) that largely depletes glycogen. In the practical sense,
I can't see many riders galloping at 22.37 mph for 14 minutes nonstop
immediately before an endurance race. I may be overly conservative, but
I would be more concerned about causing lameness, not to mention putting
the idea that Galloping is Fun into my horse's head when control is
tentative at the best of times <g>.
2) The discussion on glycogen loading in Equine Sports Medicine, by W.E.
Jones, DVM, pg. 28. Please excuse me if I quote most of the passage, it
also contains a decent explanation of what g-loading is for those that
don't really know (more bang for the buck, so to speak):
Quote: "The concept of "glycogen-loading is based on the principle that
if the resting glycogen of a muscle is increased, the time to fatigue at
a given workload should increase, delaying the manifestations of
fatigue. Glycogen-loaded humans have exhibited 37% increase in
anaerobic performance. Glycogen manipulations are applied to horses in
the attempt to deplete glycogen stores by combining several days of
low-carbohydrate, high-fat and high-protein feeding with strenuous
exercise. This depletion phase is then followed by 2 or 3 days of
reduced work and high carbohydrate feeding ("repletion") in hopes of
increasing the glycogen load to a level above that at the beginning of
the program by a rebound "overshoot". These programs have succesfully
increased resting intramuscular glycogen contents in Standardbreds, TBs
and Quarter Horses. Unfortunately, subsequent performance by either
unchanged by greater muscle glycogen content or was decreased (Kline,
1981; Topliff, 1983; Topliff, 1985). In addition, training itself can
increase intramyocyte glycogen contents. The magnitude of this effect
is not known, but the aforementioned increases in muscle glycogen
contents may have resulted from the exercise regimens employed, rather
than from the high-carbohydrate feedings.
Drawbacks to the use of glycogen loading include the increased
intracellular water retention that accompanies high intramuscular
glycogen concentrations, increasing the likelihood of both dehydration
and overestimation of an animal's fitness (Maughan, 1981). In addition,
because of the glycogen depletion-repletion cycle is only effective in
those animals with muscle fibers that are "trained" to both store and
metabolize glucose rapidly and efficiently, there is a risk of
triggering exertional myopathy after the high carbohydrate
feeding-resting-supercompensation phase. Muscle stiffness from lactic
acid accumulation was linked to rapid carbohydrate metabolism (McClean,
1983). Uncertainty persists, however, concerning the role of glycogen
reserves in the onset of myopathies." End of quote.
ME: What I derived from this supports what was stated in Pagan's study
above. Although I will be the first to admit that some of this research
is more than decade old, I think it has at least enough merit to be
worth considering. I also note that simply training will increase
glycogen storage in the muscle (this is also supported elsewhere), so I
personally would prefer to increase glycogen through conditioning rather
than glycogen-loading, although the two are also not mutually exclusive
concepts, either.
3) Conversation with Dr. Snow. I know, I know, this is simply his
subjective opinion, therefore I cannot supply citations to go along with
what he told me. However, I have included a passage from the lecture he
presented to the Equine Nutrition Society of Australia in 1991, as the
information was essentially identical, except that Dr. Snow also gave me
his OPINION that glycogen-loading may be more risky than are justified
for use in endurance horses. Please note that I said "risky", not
"deadly". For those of you unfamiliar with David Snow, he is a
veterinarian and equine exercise physiologist (his degrees are BSc(Vet),
BVSc, Ph.D. and MRCVS), and obviously one whom I respect very much (my
own personal opinion here).
>From "Energy and Carbohydrate Boosters", 1991, pg. 41-42:
Quote: "Following any type of exercise, there is a decline in the
glycogen stores within the muscle and liver. The extent of this decline
will depend on the intensity and duration of exercise. During endurance
rides of the order of 100 km (ME: that's about 62 miles), glycogen
within muscles can decline between 50-100%. In racing activites, the
decline is usually of the order of 30%. It appears that there is little
difference in the extent of this decline for races over 1000 or 2,500m.
In endurance activites, it has been shown that glycogen depletion
leads to fatigue. In the human athlete, to delay complete glycogen
depletion and therefore fatigue, dietary programmes leading to glycogen
loading of muscles have been undertaken. It has also been postulated
that similar strategies should be adopted for horses in endurance rides.
However, most evidence would indicate that such a strategy cannot be
aopted in the horse, and that a normal, high energy diet is all that is
required. The reason for this is that the horse muscle normally stores
glycogen in very high amounts, ie, concentrations are normally similar
to those seen in a glycogen-loaded human.
Recent work has also shown that the rate at which repletion of
glycogen can occur is only slightly influenced by diet. Very high
carbohydrate diets do not appear to be capable of speeding up the
repletion. It should also be noted that the general rate of glycogen
repletion is slower than in man. Following almost complete depletion,
it may take up to 92 hours fro complete repletion to occur. This may
explain why horses cannot compete as frequently as the human athlete
within a short period of time.
In some countries, a glucose polymer (Carboboost) used in human
athletes has been made available for the horse. It has claimed to be of
benefit in both endurance and racehorses. Some preliminary trials
carried out by the author of this paper has indicated that these claims
are unliikely to be justified. Two studies were carried out. In one
study in which blood glucose concentrations were compared following
either oral glucose or Carboboost administration, no elevation in blood
glucose following the Carboboost occurred. Furthermore, in a study in
which glycogen depletion of 40-50% occurred, the feeding of high doeses
of Carboboost over several days was found to have no effect on the rate
or extent of glycogen repletion." End of Quote.
ME: From this I derived that the documented benefits of glycogen
loading in horses is still a little lacking, although this very well may
have changed in the five years since this paper was presented. I also
at the time discussed all this with my research advisor, Dr. Steve
Wickler, DVM Ph.D., who is also a very respected researcher and is a
past president of Association for Equine Sports Medicine. In short, he
agreed with Dr. Snow's opinions.
Here is my bottom line personal position on glycogen-loading, based on
the above information and based on what I have been taught in assorted
exercise physiology courses. NOW HEAR THIS---THIS IS MY OPINION!!!!
1) I feel that there is a chance that glycogen-loading MAY increase the
risks of tying-up, especially in horses that are already prone to doing
so. How great that risk may be, I don't know. Maybe nothing, maybe
significant. My best horse has a significant dose of Quarter Horse
blood and I suspect he may be a touch prone, so I tend to be very
conservative in this area. The amount of "risk" probably differs
greatly from horse to horse as well.
2) It is pretty much agreed that endurance work is almost entirely
aerobic work. There is a method of determining the proportion of fats
vs. carbohydrates being utilized by measuring the amount of oxygen being
utilized and the amount of carbon dioxide being blown off. At
submaximal aerobic activity levels, significantly more fat is being used
for energy production than is carbohydrate (this is in any exercise
physiology text and is not a new concept). It seems logical to me
therefore that I should pay more attention to maximizing fat utilization
in my horse than to carbohydrate levels and utilization, as there is at
least some opinion (above) that my best efforts may or may not change
the glycogen picture all that much. Research indicates (please don't
make me cite all that, too, this is already a book) that both
training aerobically AND feeding a high fat diet increases the body's
ability to utilize fats as fuel for exercise. Also, that feeding fat
produces a glycogen-sparing effect. My own logic tells me that if I
cannot increase glycogen stores significantly by "loading", perhaps I am
better off training my horse to utilize fats as a fuel source as
much as possible and thereby partly bypass the use of glycogen at all.
The end result will still be LESS glycogen depletion and therefore
LESS fatigue, which is all I'm really looking for anyway. This is the
primary point I had to make regarding racehorses vs. endurance horses.
Racehorses exercise at close to (hopefully) maximum effort, certainly
well over the anaerobic threshold and therefore rely much more heavily
on carbohydrates for energy. Therefore I can see glycogen loading being
of much more value in TBs or Quarter Horses.
I hope I have explained my point of view adequately. Anyone having read
all this to the very end deserves a medal.
One last edit on my part. I earlier made the statement that slow twitch
muscle fibers store more glycogen. This was a mistake on my part (and
hardly the only one). I mixed up the fact that ST fibers have higher
concentrations of MYOGLOBIN, which is a different thing, and the fact
that muscle fibers (all types) store more glycogen in response to
training. That's what you get for trying to remember all this
fancy-shmancy stuff when you're way past tired.
Just cuz I can't resist, Tom, you made a comment that in your opinion,
it is the coaches and trainers who are the innovators that move
physiology forward, while research struggles along behind. With all due
respect, I politely disagree. I believe that applied and theoretical
science is a symbiotic relationship---one puts it into practice while
the other quantifies the benefits or LACK of benefits, thereby avoiding
wasted or useless effort. Trainers without proven scientific data to
back them up are quacks and researchers without trainers to implement it
and athletes to benefit from it are simply increasing the amount of
rhetoric available. One of my favorite quotes is from a Nobel Prize
winner, who said "If I have seen farther than most, it is because I have
stood on the shoulders of giants." I like to think this principle
applies equally well to both trainers and research scientists, don't
you?
Susan Evans
California State Polytechnic University