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For horses · Jun 6, 2017

Glycogen depletion, not lactic acid, is what stops your endurance horse

Understanding exactly how your horse produces and exhausts energy during a long ride is the only rational basis for building a supplement programme that actually works.

Dapple grey Arabian endurance horse trotting a red-dust desert trail as glycogen reserves deplete over distance

By Beryl Shuttleworth, BSc Hons Biochem

Last reviewed: 2026-06-12

Fatigue in endurance horses is driven primarily by glycogen depletion, not lactic acid accumulation, aerobic metabolism dominates at moderate pace, with the anaerobic threshold reached around 140-150 beats per minute. Muscle glycogen supplies roughly 50% of energy in the first 30 minutes but falls below 20% after an hour. Fat supplementation produces a glycogen-sparing effect, preserving carbohydrate stores for longer into the ride, while endurance training itself increases muscle mitochondrial density and oxidative capacity to further delay fatigue onset.

Loading up on starch is the best way to maximise muscle glycogen in endurance horses

I understand why people believe this. Glycogen is made from glucose, glucose comes from starch, so more starch equals more glycogen, it seems logical. Plenty of feeding programmes for endurance horses are still built around high-starch hard feeds for exactly this reason.

The problem is that the evidence doesn’t support the conclusion. High-starch diets provide only modest increases in muscle glycogen stores and have shown no demonstrated benefit to endurance performance. Worse, they may actually increase heart rate and blood lactate during intense exercise compared to moderate starch feeding, the opposite of what you want going into a long ride.

Fat supplementation tells a different story. When free fatty acid concentrations are elevated before exercise, the horse draws more heavily on fat as fuel, which preserves muscle glycogen and blood glucose for longer. That glycogen-sparing effect is the mechanism that actually extends aerobic performance. Controlled feeding trials using dietary fat at levels up to 16% of diet (as soybean oil) showed safe, efficient energy provision alongside greater resting muscle glycogen, and conditioning combined with fat supplementation produced 37% increases in resting muscle glycogen compared to training alone.

The metabolomic evidence reinforces this: the best-performing endurance horses show superior lipid utilisation profiles compared to poor performers, suggesting that fat metabolism, not carbohydrate loading, is the key adaptive response to sustained aerobic exercise.

So if you are pushing starch to top up glycogen, you may be doing it at the cost of the very metabolic efficiency your horse needs. A cup of vegetable oil added to the ration each day is a more evidence-backed approach than chasing glycogen through the grain bucket.

How an endurance horse fuels muscle, and where fatigue actually begins

At moderate pace, an endurance horse runs almost exclusively on aerobic metabolism: oxygen carried to the muscles by the blood converts glycogen, glucose, and free fatty acids into ATP, the molecule muscles need to contract. The by-products are benign and cleared through sweat and respiration.

Once the horse pushes past roughly 140-150 beats per minute, climbing a hill, surging ahead, demand outstrips what aerobic pathways can supply. Anaerobic metabolism takes over, producing lactate as a by-product. In a well-conditioned endurance horse, anaerobic work should be brief; the best performers show elevated lactate only at the finish line, not throughout the race.

Glycogen depletion, not lactate accumulation, is the primary driver of fatigue in prolonged endurance work. In the first 30 minutes, intramuscular glycogen supplies roughly 50% of energy; after one hour, that contribution falls below 20%. Blood glucose contributes only around 10% of total energy used, and although fat stores are large, fatigue sets in before they are exhausted.

This is precisely why fat supplementation is worth paying attention to. When horses arrive at exercise with higher circulating free fatty acids, a greater proportion of energy comes from fat oxidation. That glycogen-sparing effect preserves limited carbohydrate stores for the high-intensity efforts that come later in competition. Controlled trials using soybean oil at up to 16% of diet confirm the approach is safe and efficient, with training combined with fat supplementation producing 37% increases in resting muscle glycogen alongside more stable serum enzymes and electrolytes.

Glycogen repletion after exercise is slow, at least 48 hours are needed to return levels to normal. High-starch diets offer only modest increases in stored glycogen and, at high levels, can actually increase heart rate and blood lactate during intense work compared to moderate starch feeding.

Endurance training itself drives meaningful muscular adaptations: increased mitochondrial density, greater oxidative capacity of muscle fibres, higher resting glycogen stores, and upregulation of enzymes for cellular energy production, all of which push the anaerobic threshold higher and delay fatigue onset.

How to use these ingredients effectively in an endurance horse’s programme

Dietary fat supplementation
The glycogen-sparing benefit of fat relies on building elevated free fatty acid concentrations *before* competition, not on the day. Feed fat as part of the daily ration consistently, research has used soybean oil at levels ranging from 4% to 16% of total diet, with the combination of fat supplementation and conditioning producing a 37% increase in resting muscle glycogen compared to training alone [6, 14]. One mainstream equine nutrition source suggests vegetable oil at 100-120 g per kg of body weight as an appropriate supplementation level [3]; confirm the arithmetic against your horse’s total ration before applying that figure. Fat supplementation at these levels has been shown to be safe and did not adversely affect health markers [6, 14].

Allowing enough time for glycogen repletion
This is the one rule most endurance riders break: muscle glycogen after a hard effort takes *at least 48 hours* to return to normal [4]. Schedule recovery days accordingly. High-starch diets in the lead-up to competition offer only modest increases in glycogen stores and, importantly, have been shown to *increase* heart rate and blood lactate during intense exercise compared to moderate-starch feeding, so piling on the hard feed before a ride is counterproductive [4].

FOS (fructooligosaccharides) for hindgut support
FOS works as a prebiotic, selectively feeding beneficial bacteria, particularly *Bifidobacterium* species, that produce short-chain fatty acids supporting hindgut function [22, 23]. In yearling horses, doses of 8 g/day and 24 g/day both reduced fecal pH in a linear, dose-dependent manner, supporting a microbial environment that favours beneficial species, with no adverse effects on fecal consistency at either dose [25]. FOS also reduced disruption to colonic microbial populations following abrupt diet changes [23], useful if competition travel forces feed changes. The research pool does not support a single definitive daily dose beyond what the yearling study tested; formulation and dose both influence outcome [24].

MSM for exercise recovery
MSM’s anti-inflammatory effect operates through NF-kB suppression and reduction of IL-1, IL-6, and TNF-α [18]. Efficacy requires consistent, repeated dosing, human data indicates at least one week of supplementation is needed to reach therapeutic levels due to slower excretion despite rapid absorption [19]. A horse-specific study confirmed MSM supports improved skeletal muscle inflammatory gene expression following exercise [21]. No equine-specific dose is established in the pool; do not extrapolate human dosing figures without veterinary guidance.

**Echinacea (*Echinacea angustifolia*) for immune support**
A 42-day course in horses increased neutrophil phagocytic ability, peripheral lymphocyte counts, neutrophil tissue migration, red blood cell concentration, and haemoglobin levels [15].

What we’d recommend


Herbal Horse Ultimate mix

All round support for endurance and other sport horses.

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Frequently asked questions

Why do endurance horses fatigue, is it lactic acid or glycogen running out?

Both matter, but they operate at different points. Lactic acid becomes a problem when a horse exceeds roughly 140-150 beats per minute and tips into anaerobic metabolism. For most endurance horses travelling at moderate pace, that threshold is rarely sustained, so the more common cause of deep fatigue is glycogen depletion. Intramuscular glycogen contributes around 50% of energy in the first 30 minutes but drops to under 20% after an hour, and once those stores are spent, the horse has very little left to draw on.

Does feeding more fat actually help an endurance horse go further?

Yes, and there is decent evidence behind it. When free fatty acids are elevated before exercise, the horse uses proportionally more fat as fuel, which preserves muscle glycogen and blood glucose for harder efforts later in the ride. Controlled trials using soybean oil at up to 16% of diet showed safe fat supplementation, increased resting muscle glycogen with training, and more stable serum electrolytes, all relevant for a long-distance horse.

How long does it take for a horse’s muscle glycogen to recover after a hard ride?

At least 48 hours. That is not a conservative estimate, it is the finding from Merck’s review of equine exercise physiology. High-starch diets offer only modest increases in glycogen stores and have not been shown to improve endurance performance; they can actually raise heart rate and blood lactate during intense work compared to moderate starch feeding. Post-exercise glucose infusion does accelerate glycogen repletion in research settings, but practical dietary management over 48-plus hours remains the realistic recovery window.

Can MSM help with recovery and muscle inflammation in working horses?

There is now horse-specific evidence for this, not just extrapolation from human studies. A published trial found MSM supplementation in adult horses improved skeletal muscle inflammatory gene expression following exercise. The mechanism is well characterised: MSM suppresses NF-kB signalling, which reduces inflammatory cytokines including IL-1, IL-6, and TNF-α, and it reduces oxidative stress. It is also a meaningful source of sulphur, which incorporates into cartilage and connective tissue.

Does adding a prebiotic like FOS to an endurance horse’s feed actually change hindgut function?

FOS is not a gimmick for horses. Supplementation has been shown to improve dry matter and crude protein digestibility, buffer disruption to colonic microbial populations during diet changes, and shift fermentation toward beneficial short-chain fatty acids. Dose-dependent reductions in faecal pH indicate a more acidic environment that favours beneficial bacteria over pathogenic ones. For an endurance horse whose hindgut fibre fermentation is a sustained energy source mid-ride, maintaining that microbial population matters.

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