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Energy expenditure on a ride

This ought to liven up the discussion for a while!
I've been having a side conversation with ti regarding fuels for 
endurance horses and, as usual, he stimulated me to do a bit of digging
into the literature. Came up with some estimates (these are
rough) that might be of interest to the list:

It has been estimated that a horse expends 12.4 kcal per hour of fast
trot/canter (and this does not include hill work or sand type footing) per
kg of weight.
Assuming the elite horses (500 kg combined horse/rider/tack weight)
are covering a relatively flat 100 mile course at the 
fast trot/canter mode (a little over 12 mph) in about 8 hours of travel
time, they would expend 
about 50 Mcal of energy. This is in addition to the 16 Mcal required for
"maintenance". 8 ounces of pure carbohydrate provides 0.9 Mcal. 
Total caloric content of muscle glycogen in a 1000 lb horse has been 
estimated to be 15 Mcal, liver glycogen 0.6 Mcal, intramuscular
triglycerides (IMTGs-
fat stored in the muscle) is 18 Mcal.
If you rely exclusively on glycogen, glucose from carbos and IMTG
you would have to give over 16 eight ounce carbo doses during those 8 hours
(1 lb per hour!) just to cover the exercise induced caloric expenditure.
You probably do not
want to totally deplete the glycogen stores, so the caloric deficit would
be even greater.
So adipose (fat) tissue calories (366 Mcal available in the average 1000 lb
which could not be used in the glucose driven environment of the carbo
charged horse, 
or protein calories from muscle breakdown, which would not be inhibited by
the high glucose/insulin
but would not be very desirable, or VFA (from digestion of fiber in the
large intestine)
utilization has to kick in somewhere. The VFAs are utilized via the 
Emden-Meyerhoff and TCA cycles (the pathways used by glucose and glycogen
during aerobic work), 
which means their utilization would appear as carbo use in your RER (this
is a measurement
which reflects the type of "fuel" being used-fats have low RER,
Carbohydrates have high).
So, as Susan has so often said, both a good store (not excessive, however)
of body
fat and a gut full of good hay, grass or beet pulp may help carry
our partners through those long rides-even carbo charged front runners.
Food for thought.

Just for fun I estimated Fling's energy expenditure
in our 20 hr 52 min completion of the OD 100. We averaged a slow trot/walk
speed (4.8 mph-about 6.6 kcal/kg/hr). Not taking into account the effort of
climbing those endless mountains, she burned about 62 Mcal! Maybe I should
speed up!

Sarah and Fling (yes-faster, Mom, faster!)

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