Fat metabolism in the exercising thoroughbred horse.
The thoroughbred horse has been selectively bred for speed and has a high capacity for
carbohydrate metabolism. The following series of studies investigated the relative
contribution of fat and carbohydrate to energy production during exercise of varying
intensity. Furthermore the work assessed the capacity of the horse to increase the
contribution of fat to energy production as the result of either an acute increase in the
availability of plasma free fatty acids (FFA) or as the result of chronic fat supplementation.
Finally an adaptational response to feeding a fat supplemented diet was described.
The variation in plasma long chain FFA over a 24 hour period was described. The early
hours of the morning represented the period of greatest variability in plasma FFA
concentration. This period was characterised by a significant increase in total and individual
FFA concentration, which was unrelated to feed intake. As a result of the reported circadian
rhythm in plasma FFA all subsequent exercise studies were performed during the period of
least variability in plasma FFA concentration.
A model for the pre-exercise elevation of plasma FFA, using a combination of a triglyceride
emulsion and the heparinoid type substance pentosan polysulphate, was used to investigate
the effect of increased FFA availability on fat utilisation during prolonged low intensity
exercise. Pentosan polysulphate was used in preference to heparin following an investigation
of their relative lipolytic and anticoagulative properties. Pentosan polysulphate when
administered at 3 times the dose of heparin resulted in a comparable increase in plasma total
lipase activity. When co-administered with a triglyceride emulsion, pentosan polysulphate
resulted in a similar increase in plasma FFA concentration relative to that produced with the
same triglyceride emulsion and heparin. The anticoagulative effect of pentosan polysulphate,
however, was approximately 9 times less than that of heparin, as measured by activated
partial thromboplastinti me.
The contribution of fat and carbohydrate to energy production during exercise was influenced
by both the intensity and duration of exercise, as indicated by measurements of respiratory
exchange ratio (RER). The inter-horse variability in RER was greatest during low intensity
exercise. An increase in the contribution of carbohydrate to energy production occurred at
the onset and during the early stages of prolonged exercise and as the result of an increase in
exercise intensity. A proportion of horses exhibited an increase in the utilisation of fat during
low intensity prolonged exercise as a result of a pre-exercise elevation in plasma FFA concentration. RER was consistently lower during exercise in 5 out of the 7 horses studied
following a pre-exercise elevation of plasma FFA. Furthermore, plasma glucose
concentration was elevated above that observed during the control session in 4 of these 5
horses for at least the first 15 minutes of exercise.
A prolonged period of fat supplementation resulted in an improved management of the fat
load. Following 10 weeks of dietary treatment a significant increase in plasma cholesterol
concentration and a significant decrease in plasma triglyceride concentration was reported.
The decrease in plasma triglyceride concentration was associated with a mean 50% increase
in post pentosan polysulphate plasma total lipase activity. It is suggested that the increase in
the post pentosan polysulphate plasma total lipase activity may have reflected an increase in
muscle lipoprotein lipase activity. A significant increase in the activity of muscle citrate
synthase was observed during the period of fat supplementation. No significant change
occurred in muscle ß-hydroxyacyl CoA dehydrogenase activity or in the concentration of
resting muscle glycogen and triglyceride as a result of fat supplementation. RER was
significantly lower in the latter stages of prolonged low intensity exercise, during the period
of fat supplementation, relative to the same exercise performed before the introduction and
following 5 weeks of withdrawal of the fat supplemented diet. The reduction in RER during
the period of fat supplementation was associated with a greater exercise induced increase in
plasma FFA concentration. The above differences were also apparent during moderate
intensity exercise, although, examination of the individual horse data revealed that the effect
was not as clear as that observed during low intensity exercise. No significant differences
were reported in either RER or plasma FFA concentration in response to moderate/high
intensity exercise during the period of fat supplementation. Neither were any significant
differences observed in either RER or plasma FFA concentration in the control group at any
exercise intensity. An increased availability of plasma FFA and an increase in the oxidative
capacity of muscle, as well as an enhanced ability to utilise plasma triglycerides may have
contributed to the increase in fat utilisation, observed during low and moderate intensity
exercise, in response to fat supplementation .
The effect of differences in the hormonal
response to a fat supplemented diet as a precipitant of the observed adaptational responses in
these studies requires further investigation.