Unlike guinea pigs and humans, horses synthesize their own vitamin C (ascorbic acid) in the liver, so it is not a dietary essential. However, endogenous production may be insufficient during periods of intense exercise, illness, stress, or in aged horses whose liver function has declined. Supplemental vitamin C from fresh vegetables and fruits provides additional antioxidant support and enhances iron absorption from plant sources.
No established daily requirement exists because horses produce their own vitamin C — typically 70 to 80mg per kilogram of body weight per day, or roughly 35 to 40 grams for a 500kg horse. Senior horses or those under heavy work stress may benefit from supplemental vitamin C at 5 to 10 grams per day, but for most healthy horses, no dietary supplementation is needed.
True deficiency is rare since horses make their own. Suboptimal levels may manifest as slow wound healing, increased susceptibility to infection, and poor recovery from intense exercise, particularly in senior or chronically stressed horses.
Excess vitamin C is water-soluble and excreted in urine. It is not a practical concern from dietary sources or moderate supplementation.
| Life Stage | Size | Min | Max | Unit | Notes |
|---|---|---|---|---|---|
| Adult | — | 0 | 0 | mg | Horses synthesize their own vitamin C at approximately 70-80mg per kg body weight per day. No dietary requirement established. Senior or stressed horses may benefit from 5-10g supplemental vitamin C daily. |
Source: NRC 2007, general veterinary consensus
Vitamin C regenerates vitamin E after it has neutralized a free radical, effectively recycling this fat-soluble antioxidant. Since horses synthesize their own vitamin C, this regenerative cycle operates continuously and amplifies the protective effect of dietary vitamin E.
What this means: This synergy happens naturally in your horse's body thanks to endogenous vitamin C production. Ensuring adequate vitamin E intake allows this cycle to function optimally. Senior horses with reduced vitamin C synthesis may benefit from supplemental vitamin C to support this partnership.
