0

Active horses · Oct 28, 2014

Lysine matters most, but your horse’s muscles need the whole picture

Lysine is the first limiting amino acid in most equine diets, but protein synthesis quietly stalls the moment any single essential amino acid falls short of what muscles need.

Dapple grey sport horse standing in a barn aisle where lysine and essential amino acids support muscle development

By Beryl Shuttleworth, BSc Hons Biochem

Last reviewed: 2026-06-12

Lysine is the most common limiting amino acid in equine diets, but supplementing it on its own isn’t enough for meaningful muscle development, protein synthesis stalls whenever any single essential amino acid runs short. Research found that raising lysine intake alone, without addressing other limiting amino acids, didn’t improve whole-body protein synthesis. Where it gets more interesting is when lysine and threonine are balanced together: horses receiving both showed measurable improvements in muscle mass and lower markers of muscle breakdown. The NRC itself acknowledges that individual amino acid requirements beyond lysine remain poorly defined for horses.

Supplementing lysine alone is enough to support muscle development in horses

It’s easy to see why this belief took hold. Lysine is the most commonly limiting essential amino acid in equine diets, and the NRC has historically focused its guidance on lysine above all others, so naturally, the supplement industry followed suit. If lysine is the bottleneck, fix lysine, job done. The trouble is, it’s rarely that straightforward.

Protein synthesis doesn’t work like topping up a single low tank. Every protein your horse builds has a precise amino acid sequence encoded in its DNA. If any one of those amino acids is missing when the ribosome needs it, the whole synthesis process stalls. Correcting lysine simply exposes the next limiting amino acid, most likely threonine.

The research reflects this quite clearly. A controlled trial in yearling horses found that increasing lysine intake alone, on a diet already limiting in other amino acids, failed to produce a meaningful improvement in protein synthesis, the diet was limiting in something beyond lysine, and multiple amino acid interactions were at play [3]. Meanwhile, horses supplemented with both lysine (20 g/day) and threonine (15 g/day) showed measurably improved muscle mass scores, higher plasma creatinine (a marker of muscle mass), and lower 3-methylhistidine (a marker of muscle breakdown), even when baseline diets appeared to meet crude protein requirements [5].

In exercising horses, things become more involved still. Branched-chain amino acids, isoleucine, leucine, and valine, are oxidised during exercise and can take up to 48 hours to return to pre-exercise levels [6]. Modelling of amino acid turnover in training and racing horses also suggests that serine, glutamine, histidine, and others are lost in sweat at disproportionately high rates and may become conditionally essential during intense work [7], despite not appearing on the standard essential amino acid list at all.

Feeding balanced protein, with an amino acid profile matched to muscle tissue composition, produces greater muscle fibre size and higher concentrations of key free amino acids compared to a standard protein diet, even when total essential amino acid intake is similar [4]. That finding says something worth sitting with: it’s the profile that matters, not the headline lysine figure.

So while lysine-only supplementation is certainly better than nothing, it’s an incomplete answer to an amino acid supply problem that spans the full essential range, and in hard-working horses, well beyond it.

How amino acids build, and limit, muscle

Muscle protein is assembled from amino acids in a fixed sequence determined by the horse’s DNA. If any single amino acid in that sequence is missing, synthesis stops, no partial protein is made. It’s this requirement for balance that really matters, not just the total amount of protein in the diet.

Some amino acids the horse can synthesise himself; others have to come from feed. These are the essential amino acids. The one most commonly in short supply is lysine, which is why it tends to be described as the first-limiting amino acid in equine diets.

Lysine alone, though, doesn’t tell the whole story. Threonine is the next most commonly limiting amino acid, and the evidence suggests its requirement in working Thoroughbred horses may be higher than the NRC models predict, field measurements put it at 67-80% of lysine intake, against the NRC’s estimated 53-62%.

And even getting lysine and threonine right may not be enough. A study in yearling horses found that increasing lysine intake alone didn’t improve protein synthesis, suggesting a different amino acid was the real bottleneck, the diet was limiting in something else entirely. Protein synthesis appears to depend on several amino acid interactions at once, rather than a single rate-limiter.

The specific amino acids that matter also shift with workload. During endurance exercise, serum branched-chain amino acids (isoleucine, leucine, valine) fall and can take up to 48 hours to return to baseline, because they’re being oxidised as fuel. On top of that, modelling of amino acid turnover in training and racing horses shows that serine, glutamine/glutamic acid, histidine, phenylalanine, and aspartic acid are lost in sweat at disproportionately high rates, enough that they may become conditionally essential during hard work and recovery, even though they’re normally classified as non-essential.

In practice, the research bears this out. Exercising horses fed an amino acid profile matched to muscle tissue composition had higher concentrations of free methionine, leucine, phenylalanine, and arginine in muscle tissue, and developed greater type II muscle fibre areas and diameters compared to horses on a standard protein diet, despite similar total essential amino acid intake between the two groups. And horses supplemented with 20 g lysine and 15 g threonine daily over 14 weeks showed improved muscle mass scores, higher plasma creatinine (a marker of muscle mass), and lower 3-methylhistidine (a marker of muscle breakdown).

Feed source plays a role here too. Legumes like lucerne (alfalfa) are rich in lysine but low in methionine and cysteine; cereal grains tend to show the opposite pattern. Plant-based protein isolates broadly tend to fall short of leucine, lysine, and methionine relative to what muscle tissue needs, which limits their muscle-building contribution when fed alone. Antinutritional factors in legumes, including phytic acid and trypsin inhibitors, can reduce digestibility and amino acid bioavailability further still.

How to use amino acid supplementation effectively in horses

Who needs it and when

Mature horses at maintenance, horses in active training, and aged horses under light exercise all show measurable responses to amino acid support, but the triggers and priorities differ, so it’s worth thinking about where your horse sits before reaching for a supplement.

What the research actually supports dosing-wise

Diets containing balanced protein with an amino acid profile matched to that of equine muscle tissue showed greater muscle free amino acid concentrations and larger type II muscle fibre areas and diameters compared to a standard equine diet. That’s a meaningful difference, and it came down to protein quality rather than simply feeding more of it, something worth bearing in mind if you’re wondering why your horse isn’t responding the way you’d expect.

Duration

The protein requirement is there every day, so this isn’t an occasional intervention. What matters most, over time, is the quality of the protein your horse is consistently receiving.

Timing around exercise

Branched-chain amino acids (isoleucine, leucine, valine) fall during endurance work and take up to 48 hours to return to baseline. Separately, modelling of amino acid turnover in training and racing horses identifies serine, glutamine/glutamic acid, histidine, phenylalanine, and aspartic acid as disproportionately lost in sweat during high-intensity exercise, making them conditionally essential during hard work and the recovery window, even though they’re classed as non-essential at rest. The recovery period is a window worth paying attention to, though the research doesn’t yet specify an exact post-exercise timing protocol.

Protein source matters as much as dose

Plant-based proteins, legumes included, tend to be short on methionine and cysteine, and antinutritional factors such as phytic acid and trypsin inhibitors can reduce bioavailability further. Legumes and cereals do complement each other reasonably well on paper, since legumes are lysine-rich while cereals supply the sulphur amino acids, but this balance can shift depending on what your horse is actually eating day to day. High-quality sources such as soybean offer a broader, more useful amino acid spread compared to cereal grains or grass hay alone. If your base diet is already well-balanced, a targeted supplement filling specific gaps tends to be a more sensible approach than simply adding more crude protein.

What we’d recommend


Herbal Horse Ultimate mix

Ultimate mix supplies the full range of essential amino acids a working horse needs to build and hold on to muscle.

See the product

Frequently asked questions

Why isn’t lysine supplementation alone enough to build muscle in horses?

Think of protein synthesis as an assembly line, if any single amino acid is missing, the whole process stalls. Lysine is the most commonly limiting amino acid, but supplementing it alone won’t address the second limiting amino acid, threonine, or others further down the queue. Research in yearling horses fed diets limiting in amino acids beyond lysine found that increasing lysine intake alone didn’t improve protein synthesis, which points to multiple amino acid interactions at work.

How much lysine does a horse actually need per day?

For mature horses at maintenance, nitrogen retention studies put the minimum lysine requirement at 36 mg/kg body weight per day, with an optimum of 54 mg/kg per day by broken-line analysis. Horses in work need more, exercise increases both crude protein and lysine requirements, and that increased need often calls for targeted supplementation rather than simply relying on a higher total feed intake.

Does threonine matter as much as lysine for horses?

It does, and the NRC figures may well underestimate it. The NRC places threonine at 53-62% of lysine intake, but field studies on mature Thoroughbreds in actual work suggested requirements of 67-80% of lysine intake based on plasma amino acid responses, noticeably higher. If you’re supplementing lysine without giving thought to threonine, you’re addressing the first bottleneck and leaving the second one largely untouched.

Do horses in hard work need different amino acids than horses at rest?

Yes, in two fairly distinct ways. Branched-chain amino acids, isoleucine, leucine, and valine, are oxidised during endurance exercise and can take up to 48 hours to return to baseline, creating a conditional demand during intense training. Separately, modelling of amino acid turnover in training and racing horses shows that serine, glutamine, histidine, phenylalanine, and aspartic acid are lost in sweat at disproportionately high rates and may become temporarily conditionally essential, even though they’re ordinarily classified as non-essential.

Is soybean or legume-based protein a good way to meet a horse’s amino acid needs?

Legumes, soya included, are genuinely rich in lysine, which is one reason soybean meal has long been valued as a horse feed ingredient. That said, legumes tend to be low in methionine and cysteine, and plant-based proteins generally fall short on leucine and methionine relative to muscle tissue composition. Antinutritional factors such as phytic acid and trypsin inhibitors can also reduce digestibility. A legume-based protein source works well when it’s complemented, typically by cereals or a targeted amino acid supplement.

Sources

We use cookies to keep the shop working, and with your permission to understand how the site is used and to measure our advertising. You can change this at any time.