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For horses · May 22, 2014

Horses make their own Vitamin C, until stress makes that impossible

Under normal conditions your horse’s liver handles vitamin C production just fine, but the moment stress, illness, or a long journey hits, those levels drop fast, right when his immune system needs them most.

Dapple grey horse at stable doorway receiving a supplement scoop as stress depletes its vitamin C

By Beryl Shuttleworth, BSc Hons Biochem

Last reviewed: 2026-06-12

Horses make their own vitamin C in the liver, so under normal circumstances, there’s no need to supplement. But stress, illness, epidemics or disease changes that picture quickly. Rising cortisol drives vitamin C out of the adrenal glands and flushes it through the kidneys, leaving plasma levels depleted and immune defences weaker for it. That’s when supplementation earns its place, supporting immune function, collagen synthesis, and antioxidant capacity. One thing worth knowing: oral bioavailability is limited in horses, so form and timing matter. And if you’ve been supplementing for a while, taper off gradually rather than stopping cold, abrupt cessation after prolonged use can trigger a rebound deficiency.

Horses make their own vitamin C, so supplementation is pointless

I hear this one a lot, and honestly, the first part is completely true. Horses do synthesise ascorbic acid from glucose via the glucuronic pathway in the liver, which is why no formal dietary requirement has ever been established for them [9]. Unlike us humans, or guinea pigs, they’re not dependent on getting it from their diet under normal conditions [9]. So I can see why people conclude that supplementing is just money down the drain.

The problem is that phrase “normal conditions” is carrying a lot of weight.

The moment a horse faces significant stress, a long journey, illness, infection, a heavy competition schedule, plasma vitamin C levels can drop sharply. The reason is fairly straightforward: elevated cortisol mobilises vitamin C from the adrenal glands and drives up renal excretion at the same time [2]. The liver’s natural synthesis simply can’t keep pace with that kind of demand, leaving you with a functionally depleted horse right at the moment its immune system needs the most support.

And we can see that play out in the evidence. In weanling horses under severe transport stress, unsupplemented animals developed clinical signs of shipping fever (pleuropneumonia) at four times the rate of those receiving 5 g of vitamin C twice daily for five days after transport [2]. In a separate controlled endotoxin challenge, plasma vitamin C dropped in 83% of horses within just one hour of endotoxin infusion, and supplementation was associated with attenuated neutrophil depletion, a meaningful marker of innate immune response [5].

There’s also the collagen connection, which matters a great deal for any horse in athletic work. Vitamin C is a direct cofactor for the enzymes responsible for hydroxylating proline and lysine during collagen synthesis and cross-linking [7]. Without adequate levels, you simply don’t build good collagen, and that has real implications for tendons, ligaments, and connective tissue repair [7][8].

The blanket claim that vitamin C supplementation is always unnecessary simply ignores the physiology of stress. For horses facing genuine physiological demands or genuine risk, it’s a reasonable, evidence-supported choice, not wishful thinking.

How Vitamin C works in the horse’s body

Your horse can actually make his own vitamin C (ascorbic acid). He converts glucose into it via the glucuronic acid pathway in the liver, using an enzyme called gulono-lactone oxidase, the same enzyme that humans and guinea pigs lack, which is why we have to get ours from food. Under ordinary circumstances, horses produce plenty, and no formal dietary requirement has ever been established as a result.

So why do we ever need to supplement it? The short answer is stress.

When your horse’s body mounts a stress response, cortisol rises, and one of cortisol’s effects is to mobilise vitamin C stores out of the adrenal gland while also ramping up how much the kidneys excrete. The result is a rapid drop in plasma vitamin C. In one controlled study, simply infusing horses with a sublethal dose of endotoxin was enough to send plasma vitamin C falling in 83 per cent of them within a single hour. That gives you a sense of how quickly reserves can be exhausted when the immune system is called into action.

What does vitamin C actually do once the body is under that kind of pressure? At the immune level, it appears to help maintain the innate response, in that same endotoxin study, supplementation was associated with less severe neutrophil depletion, meaning more circulating immune cells were preserved. In a small preliminary study of weanling horses transported for 50 hours or more, supplementation was linked to a lower incidence of shipping fever (pleuropneumonia), which fits the picture of vitamin C helping the immune system hold its ground under sustained physical and mental stress.

On the antioxidant side, high-dose intravenous vitamin C produced dose-dependent reductions in reactive oxygen metabolites and measurable increases in plasma antioxidant capacity within two to six hours in healthy horses. It’s worth knowing, though, that oral and intravenous routes are not interchangeable here: oral bioavailability in horses is poor, which means we woud need to compensate by feeding a higher dose by mouth.

For structural tissues, vitamin C plays a specific and non-negotiable role: it is a cofactor for prolyl hydroxylase and lysyl hydroxylase, the enzymes that prepare collagen precursors for cross-linking into stable fibres. Without adequate vitamin C, you simply cannot build proper collagen. In cell culture work using equine superficial digital flexor tendon progenitors, vitamin C supplementation increased collagen content and reduced inflammation markers associated with tendinopathy. Broader animal-model research has shown it can accelerate bone healing after fracture and increase type I collagen synthesis in tendon-healing models.

One thing I’d flag on the practical side: if your horse has been on vitamin C for an extended period, his own synthesis can quietly down-regulate in response. Stop supplementation abruptly and you risk a temporary deficiency while the body catches up. The sensible approach is to taper the dose over at least a week rather than stopping all at once.

When and how to supplement vitamin C in horses, what the evidence actually supports

When supplementation makes sense

Here’s the thing about horses and vitamin C: under normal conditions there’s no established dietary requirement for a healthy horse [9]. So if your horse is at rest, eating well, and doing light work, routine supplementation isn’t something I’d push for.

Where it gets genuinely interesting is during periods of real physiological demand. Think prolonged transport, illness or severe disease. In those situations, plasma vitamin C drops quickly, cortisol drives it out of the adrenal glands and the kidneys excrete more of it than usual [1, 2]. That’s the moment the body actually needs extra support.

Timing: give it when the stress is happening

This one is simple once you understand the mechanism. Cortisol depletes stores, so supplementation during the stress event is when you’re actually replenishing what’s being lost. Waiting until your horse has recovered means you’ve already missed the window [2].

The other factor to take into account is risk. For example, African Horse Sickness season. It absolutely makes sense to have higher levels of circulating Vitamin C in times like this.

Duration: short courses, not indefinite use

I’ll be honest, this is where I part ways with the idea of vitamin C as a permanent daily add-in for every horse. Prolonged supplementation in horses that aren’t under significant stress has a real downside: it can suppress the horse’s own vitamin C production. In one study, both supplemented groups showed decreased plasma vitamin C for one to three weeks after supplementation ended, because endogenous synthesis had been dialled back [3]. That’s not a minor footnote, it’s a meaningful consequence.

If you’re giving vitamin C after a significant stressor, a long journey, a bout of illness, or during AHS season, keep the course purposeful and time-limited, not open-ended.

Tapering off matters

Please don’t stop abruptly. The Merck Veterinary Manual is clear on this: vitamin C supplementation should be tapered over one week or more rather than cut off suddenly, because abrupt cessation after prolonged use risks a rebound deficiency while the horse’s own synthesis gets back up to speed [3].

Route of administration

Oral bioavailability in horses is genuinely poor [4]. Intravenous administration gives the most reliable and measurable increases in plasma concentrations [4, 6], but that’s a veterinary procedure, it’s not something you’re doing at home. For most horse owners, oral supplementation is the practical route, and I think that’s fine, as long as you go in knowing absorption is limited. For me, that actually strengthens the case for using it strategically during high-demand periods rather than as a low-dose daily top-up that may not shift anything meaningful.

Contraindications and cautions

– Avoid long-term supplementation in horses that aren’t under documented stress, suppression of endogenous synthesis is the real risk, and it’s a documented one [3].
– Always taper the dose on withdrawal; don’t stop cold [3].
– There’s no evidence of adverse effects from oral supplementation, or even from high-dose intravenous administration in healthy horses [6, 8], but suppressed self-synthesis following prolonged use is a real consequence, not a theoretical one [3].

What we’d recommend


Immune Mix

Supports immune function and antioxidant defences when stress, illness, or growth deplete your horse’s natural vitamin C reserves.

See the product

Frequently asked questions

Do horses need vitamin C supplementation if they can make their own?

Under normal conditions, horses synthesise perfectly adequate vitamin C from glucose in the liver, which is why no formal dietary requirement has ever been established for them. The picture changes, though, the moment your horse is under serious stress, fighting illness, or enduring a long haul. Plasma vitamin C drops sharply because cortisol drives both mobilisation from the adrenal glands and increased loss through the kidneys. That’s the worst possible time for immune support to falter. Supplementing during those specific windows makes very good biochemical sense.

Does vitamin C actually help horses during transport stress or illness?

The early evidence is genuinely encouraging. In one study, 4 out of 5 unsupplemented weanlings developed shipping fever (pleuropneumonia) after a long journey, compared with just 1 out of 5 given 5 g twice daily for 5 days. In a separate trial, 83% of horses showed a rapid fall in plasma vitamin C within an hour of endotoxin exposure, and those that were supplemented showed less neutrophil depletion, which is a meaningful sign that innate immune defences were better maintained.

How does vitamin C support tendon and joint health in horses?

Vitamin C is a required cofactor for prolyl and lysyl hydroxylase, the enzymes responsible for building and cross-linking collagen. Without enough of it, collagen structure simply can’t hold together properly. In equine tendon cell studies, vitamin C supplementation increased collagen content and reduced the inflammation markers associated with tendinopathy. Broader animal model evidence from a systematic review also points to accelerated bone healing and increased type I collagen synthesis after supplementation.

Is long-term vitamin C supplementation safe for horses?

High-dose intravenous vitamin C at 100 mg/kg produced no adverse effects or clinicopathological changes in healthy horses, so acute safety looks good. The one genuine caution with prolonged use is that it can suppress the horse’s own synthesis, and plasma vitamin C can stay depressed for one to three weeks after you stop. If you’ve been supplementing for an extended period, taper the dose gradually over at least a week rather than stopping all at once. That one step avoids a rebound deficiency.

Is oral vitamin C effective in horses, or does it need to be given intravenously?

Oral bioavailability in horses is genuinely poor, that’s worth being honest about. Intravenous administration reliably raises plasma concentrations and has shown both immune-supporting and antioxidant effects in controlled trials. That said, oral supplementation still has a rational place during high-demand periods like stress, transport, or recovery. When endogenous synthesis is struggling to keep pace, even a partial boost to plasma levels is worth having.

Can vitamin C help with oxidative stress and inflammation in horses?

Yes, and the mechanism is well established, vitamin C converts free radicals into stable, harmless forms before they can damage tissue. In a controlled equine trial, high-dose vitamin C produced dose-dependent reductions in reactive oxygen metabolites and raised plasma antioxidant capacity within just two to six hours. There’s also evidence that combining vitamins E, C, and beta-carotene in the diet reduces free radical production and associated tissue damage during inflammatory or stressful conditions.

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