Here's Eleanor's view (by the way, no matter how "scienftific", Plum's
newsletter has been found to be quite useful to a few folks--and I have never
found Eleanor at a loss for scientific documentation):
Horses and humans do
have different digestive tracts but there is no evidence they have
different metabolism of energy substrates in the muscle. There are some
very convincing arguments for keeping endurance horses well supplied
with hay but these have nothing to do with energy generation. The hay
serves to hold a reservoir of fluid and electrolytes sorely needed
during competition. I will check on volatile fatty acids as an energy
source for you but I have not seen anything published about how they are
used. Unless the horse is an aberrant species (which is highly
unlikely) volatile fatty acids from the colon are used by the intestinal
cells, converted to glutamate and used by intestinal cells or carried to
the liver for conversion. I've never seen anything about VFAs as an
energy source for muscle. Unless I find something published to the
contrary, I don't think they can be considered a significant energy
source during an endurance ride.
I have not seen one shred of proof
that fat is a superior endurance fuel in human or any animal (I'll check
that out again too now that you brought it up). Popular yes, proven
superior no. The horses may hold their weight better (not an
insignificant consideration with endurance animals but not really
directly related to energy production) and look better, may even tie-up
less often but I have not seen any evidence they perform better - win
more, place higher up more. I like the following reference:
Nutritional strategies for promoting fat utilization and delaying the
onset of
fatigue during prolonged exercise.
Lambert EV, Hawley JA, Goedecke J, Noakes TD, Dennis SC
J Sports Sci 1997 Jun 15:3 315-24
Abstract
Carbohydrate ingestion before and during endurance exercise delays the
onset of fatigue (reduced
power output). Therefore, endurance athletes are recommended to ingest
diets high in carbohydrate
(70% of total energy) during competition and training. However,
increasing the availability of plasma
free fatty acids has been shown to slow the rate of muscle and liver
glycogen depletion by
promoting the utilization of fat. Ingested fat, in the form of
long-chain (C16-22) triacylglycerols, is
largely unavailable during acute exercise, but medium-chain (C8-10)
triacylglycerols are rapidly
absorbed and oxidized. We have shown that the ingestion of medium-chain
triacylglycerols in
combination with carbohydrate spares muscle carbohydrate stores during 2
h of submaximal (<
70% VO2 peak) cycling exercise, and improves 40 km time-trial
performance. These data suggest
that by combining carbohydrate and medium-chain triacylglycerols as a
pre-exercise supplement
and as a nutritional supplement during exercise, fat oxidation will be
enhanced, and endogenous
carbohydrate will be spared. We have also examined the chronic metabolic
adaptations and effects
on substrate utilization and endurance performance when athletes ingest
a diet that is high in fat (>
70% by energy). Dietary fat adaptation for a period of at least 2-4
weeks has resulted in a nearly
two-fold increase in resistance to fatigue during prolonged, low- to
moderate-intensity cycling (<
70% VO2 peak). Moreover, preliminary studies suggest that mean cycling
20 km time-trial
performance following prolonged submaximal exercise is enhanced by 80 s
after dietary fat
adaptation and 3 days of carbohydrate loading. Thus the relative
contribution of fuel substrate to
prolonged endurance activity may be modified by training, pre-exercise
feeding, habitual diet, or by
artificially altering the hormonal milieu or the availability of
circulating fuels. The time course and
dose-response of these effects on maximizing the oxidative contribution
of fat for exercise
metabolism and in exercise performance have not been systematically
studied during moderate- to
high-intensity exercise in humans.
ti