Feeding the Endurance horse during a race?

[Doc-t]

This is a question that can have several answers depending on a lot of factors. But the principles should be the same: 1) eliminate gut inflammation, replace the lost glycogen and 3) supply the needed daily amino acids. However, when any animal (horses or humans for example) are being used in an athletic capacity beyond the basic needs, other factors apply.

When glucose is consumed from any glycogen storage (muscles and liver) it needs to be replaced ASAP. The only way to do this is to consume carbohydrates, specifically glucose. The most abundant sources of glucose for horses is starch. This is found in all forage included fresh as well as harvested and stored (all hay). It is also found in seeds such as oats. When the amount consumed is in excess of the needs to restore lost glycogen, the excess glucose is converted into body fat for future use or is burned within the mitochondria. However, body fat cannot be accessed and used when there is insulin present and that only occurs when glucose is being consumed in the diet.

To complicate matters, the mitochondria needs to be flexible in their choice of fuels. This is called metabolic flexibility. When glucose is consumed in excess and IR (insulin resistance) has not established itself, excess glucose will be driven into the muscle cell where it cannot be stored – the glycogen storage capacity is filled. This excess glucose must be burned by the mitochondria. Not only does this create free radicals and produce less energy per molecule compared to the burning of fat, the available fat within the muscle cell is not used. This excess intracellular fat is, in fact, the cause of insulin resistance through the blocking of the GLUT4 transport mechanism by the diglyceride molecule (DAG).

With all this in mind, how dose one feed a working athlete? The simple answer is to add more fresh forage. However a grass turn out is not usually available for a horse on a multi day ride. The next best thing is hay without regard to the starch content. The excess starch in hay in one or two days of feeding will not change the metabolism much especially if this is the hay the horse has been eating before the ride.

So the question is really this – do I need to add more carbohydrates (glucose) to a horse that has worked 50 miles in a day for 1 or 2 days? This is determined by the glucose deficit at the end of the 50 miles. We can assume for an endurance ride it is a full deficit. The question now is this – when did the full depletion occur? This will be obvious if half way through the ride your horse stops. But if your horse goes the complete 50 miles and has more miles in his “tank,” then he probably has not fully depleted the glycogen reserves. How this is achieved is based on two thoughts. 1) has the horse been trained well enough to have this reserve capacity (the more skeletal muscle there is the more glycogen storage there is. 2) How flexible are the mitochondria in their ability to use both fat and glucose? Further, is “fat flux” (the availability of stored body fat’s ability to be sent to the muscle cells) in free flow mode or has insulin blocked this ability?

If there is good metabolic fuel flexibility in the muscle mitochondria, then as the ride progresses, the horse will use fat as the primary fuel and dip into the glycogen for the boost needed to climb or run. This flexibility will conserve the glycogen but at the end of the 1 or 2 days, the horse will have consumed body fat. This is normal and your horse, if conditioned correctly, will have enough body fat for this reserve but not excess body fat that requires more energy to carry. Ah, the fine art of training horses!

From my point of view, adding de-hulled oats (aka “race horse oats” of high density oats due to “triple cleaning”) is adding extra glucose for those horses consuming all of their glycogen during the race. They are also getting glucose from the starch of forage. If the amount of glucose fed is more than what is required to replace the glycogen then the next day the horse will have an insulin spike which will adversely affect his ability to utilize fat during the second day.

Another thing to keep in mind is the very old disease of work horses called “Monday morning disease.” This is where the working plow horses worked 6 full days with grain being fed every day. Then on the 7th day (Sunday) they were not worked but fed their full ration of grain. This led to tying up (severe muscle cramps) on Monday with muscle damage so severe that the myoglobin was released from the damaged muscle into the urine damaging the kidneys. Because of this it was also called “Coffee colored urine disease.”

This emphasizes my opinion that there is no call for adding any grain to a horse diet especially when they are worked very hard. While 100 miles in 2 days is NO DOUBT very hard to do, it is still within the ability of a horse that is trained well and has metabolic flexibility. They WILL lose body fat and will need hydration, electrolytes and fuel (cellulose and starch), but adding excess glucose may or may not be needed in the form of grain when there is adequate glucose in the starch of forage.