Over the past 20 years there has been a major shift towards using high fibre based feeds and feeds with a high oil content or oil added as a supplement in the nutrition of the endurance horse.
This is supported by an online survey of feeding amongst endurance riders in the UK last year (Marlin and Sadler, 2015), which found only 15% of riders fed cereal-based feeds whilst 68% fed highly digestible fibre and 63% fed oil.
This type of feeding has many desirable aspects. For example, energy provided by oil as opposed to starch is less likely to cause hindgut disturbance and colic, less likely to result in adverse behaviour and reduces the risk of tying-up, particularly in horses with a predisposition to tying-up.
Whilst all oils provide a safe form of energy, not all oils are equally beneficial. Oils such as linseed are high in omega-3 and low in omega-6. Omega-3 oils have an anti-inflammatory action and have been shown to help horses affected by Sweet Itch. They are also less likely to irritate the gastro-intestinal tract compared with oils such as soya or corn, which are very low in Omega-3 but high in Omega-6. In the above survey two-thirds of riders fed oil either occasionally or daily. Feeding oil daily for several months has also been shown, perhaps not surprisingly, to increase the horses’ capacity to use oil during endurance exercise and in turn this decreases the rate at which the muscle glycogen stores are used up; this is important as it is glycogen (carbohydrate) stores that limit endurance capacity. Even relatively thin horses would have sufficient fat stores to be able to complete several 160km rides.
Highly digestible fibre also contributes to a healthy gastro-intestinal tract and may also increase water content of the hindgut although the extent to which this water is available to the horse during a race has not been demonstrated scientifically.
So feeds high in carbohydrates, either in the form of highly available sugars or starch, have tended to become less popular. We all know the risk of colic and laminitis is increased when starch reaches the hindgut without being fully digested. So in daily training, the high-fibre high-oil feeding regimen is ideal for endurance horses, and combined with the fact that most endurance horses also get virtually unrestricted forage probably contributes to the relatively low rates of colic and laminitis in this population.
But are we missing something when it comes to competition?
Horses use glycogen in each training session. The harder and longer they work, the more glycogen is used up. For the muscles on average the glycogen never gets to zero, but it may get to zero in some individual muscle fibres, especially in longer distance rides and races. The first question is how well do horses restore their muscle glycogen after a training session? Interestingly, horses restore muscle glycogen fairly slowly compared with humans. It may take 24 to 48 hours to fully restore following a training session and even longer following a race. We know that the horses who rehydrate faster after exercise restore glycogen quicker and there is some evidence that a source of carbohydrate is required. So what if your horse is on a high-fat low-starch diet? A paper presented at the Equine Science Society meeting in Florida by Dr Joe Pagan (Pagan et al. 2015) showed that horses on high oil and low starch diets did not restore their muscle glycogen fully even by 72 hours following exercise compared with horses on a moderate or high starch intake. In fact by 72 hours the high and moderate starch diet horses had recovered 94% whilst the low starch (high fibre/high oil) horses had recovered only 63%. The implication here is that on a high fibre/high oil diet your horse may well start a race with his energy stores below optimal if you have been exercising in the three days before the race.
The second question relates to what energy your horse gets during an actual competition. During the competition the large intestine produces volatile fatty acids from the microbial digestion of fibre in the hindgut. This fibre that is digested during the ride is primarily the fibre that you fed your horse the day before the competition.
However, the muscles are using a combination of energy from locally stored fat within the muscle cells and fat stored elsewhere that is broken down and reaches the muscle from the circulation as well as glycogen within the muscle cells and glucose in the circulation. The level of glucose in the blood plays a very important role as the brain uses only glucose and nothing else as an energy source. If the level of glucose in the blood falls, then you or your horse start to feel tired.
The level of glucose in the blood is normally maintained by the breakdown of glycogen stored in the liver which releases glucose into the blood. As muscle glycogen is broken down this also releases glucose, some of which goes into the bloodstream.
However, with prolonged exercise both the liver and muscle glycogen stores decrease with the consequence that the blood glucose level starts to fall. This in turn starts to cause tiredness and fatigue. At the same time, as well as the effect on the brain, the lowered muscle glycogen levels can bring about the “hitting the wall” effect. The horse now becomes more even paced and may struggle to change paces. This effect is well recognised by marathon runners.
This falling blood sugar effect is not just theory. We have measured blood glucose in horses during rides and have seen this fall and in a 160km race run at a moderate pace in cool conditions, 40% of the horses that finished has a blood glucose less than 4 mmol/l (normal is 6-7mmol/l) and 25% finished with a blood glucose less than 3 mmol/l. Therefore, two-thirds of the finishers had a blood glucose level that was low enough to reduce performance. In the case of horses eliminated for metabolic reasons, blood glucose values are also commonly below normal.
So the key points here are that low blood glucose is a component of fatigue, digestion of fibre during the ride cannot maintain blood glucose levels and blood glucose levels are maintained by liver and muscle glycogen stores but these become depleted through the course of a ride. However, blood glucose levels can also be boosted by feeding during the ride. But not if the feed is high fibre, high oil and low starch. To boost blood sugar levels requires a ready source of carbohydrate.
How can this be achieved?
Many people would understandably be reluctant to use a moderate or high starch feed if they were not feeding it regularly during training. Introducing a different feed could present a risk for colic if the starch reached the large intestine without being fully digested and certainly dehydration and electrolyte imbalances can already compromise gastro-intestinal function during endurance competition.
That really leaves two options: simple sugars (monosaccharides) such as glucose or fructose, or highly digestible polysaccharides such as maltodextrin. The downside of using glucose or fructose is that whilst the effect on blood sugar is rapid as these do not need to be digested and are quickly absorbed, the rapid rise in blood glucose can trigger an insulin response that then sends the blood glucose crashing so they must be used carefully.
Molasses, for example would produce the same effect as it is a mixture of sucrose (glucose and fructose joined together chemically), glucose and fructose. The same problem can occur with maltodextrin which is made up of chains of glucose units joined together, or honey which contains both fructose and glucose. However, there are some polysaccharides used in horse energy supplements which are fully digested before reaching the large intestine and therefore cannot result in disturbance to the large intestine and which increase blood glucose but without large increases in insulin. The latter is important as high insulin levels in the blood can reduce the usage of fat as an energy source, which is vital for endurance exercise.
In summary, low muscle glycogen and blood glucose levels play an important role in the onset of fatigue in endurance exercise. Horses that are on high fibre and or high oil diets may take longer to recover their muscle and liver glycogen after training and may fatigue earlier during competition if they are not receiving any source of available carbohydrate in their feed. One way to counteract this is to feed a source of easily digested carbohydrate both after training and during competition.
Marlin, D.J. & Sadler, S. (2015) An online survey of feed management and electrolyte use in endurance horses in the United Kingdom. J Equine Vet Sci 35, abstract 27, page 394.
Pagan, J.D. et al. (2015) Effect of non-structural carbohydrate intake on glycogen repletion following intense exercise. J Equine Vet Sci 35, abstract 58, page 408.
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