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Vitamin C, also known as ascorbic acid, is an important water-soluble vitamin for horses. It works together with vitamin E and selenium to provide antioxidant protection against cell damage. [1][2]
In addition to its antioxidant properties, Vitamin C plays other roles in the horse’s body including involvement in collagen production, hormone synthesis, and bone calcification. It also supports the absorption of iron from the gut. [3]
Horses do not have a nutritional requirement for vitamin C because they can typically synthesize enough of this vitamin in their liver. However, older horses, those with compromised liver function, or other health conditions such as chronic infections and respiratory illness may benefit from supplemental vitamin C. [3][4]
When choosing a Vitamin C product for your horse, keep in mind that ascorbic acid is relatively unstable and degrades quickly when mixed into a feed or supplement. Choose vitamin C supplements that provide this ingredient in a stable form.
Vitamin C for Horses
Vitamin C’s primary role in the horse’s body is as a powerful antioxidant that protect cells from oxidative stress and damage caused by harmful free radicals. By neutralizing these free radicals, vitamin C contributes to the overall health and function of cells, supporting various physiological processes. [1]
This essential nutrient also supports joint health, immune function and hormone synthesis in its role as an enzyme co-factor.
Roles of Vitamin C
Vitamin C is necessary for several physiological functions in horses, including: [6]
- Antioxidant Protection: Free radicals, which are reactive molecules produced during metabolism, can cause oxidative damage to DNA, fats, and proteins. Antioxidants play a crucial role in neutralizing these free radicals, thereby providing protection against such oxidative damage.
- Immune Function: Vitamin C accumulates in cells of the immune system and enhances their ability to kill microbes. [19]
- Collagen Production: Vitamin C is a cofactor for enzymes involved in making collagen, which is an important protein found connective tissues such as cartilage, ligaments, and tendons.
- Vitamin D Conversion: Vitamin C is needed for the conversion of vitamin D3 to its active form calcitriol.
- Hormone Production: Vitamin C is involved in making norepinephrine (noradrenaline), an important stress hormone and neurotransmitter.
Sources of Vitamin C for Horses
Endogenous Production
Like most mammals, horses can make ascorbic acid from glucose in their liver. A healthy horse can produce an estimated 72 grams of ascorbic acid per day. [4]
Humans and guinea pigs are the only mammals that cannot synthesize vitamin C internally. Unlike humans and guinea pigs, horses have the enzyme L-gulonolactone oxidase in their liver, which enables endogenous Vitamin C production. [4]
The adrenal gland also stores a limited reserve of ascorbic acid. However, prolonged exposure to stressors can deplete vitamin C reserves. In this case, endogenous production may not be sufficient to maintain adequate levels of this vitamin in the body. [4]
Dietary Sources
Horses can also obtain vitamin C through dietary intake by consuming plants that produce vitamin C for their own antioxidant protection. While the vitamin C content has not been measured in forages typically consumed by horses, in other plants the levels range from 25 – 800 mg of ascorbate per 100 grams of fresh weight. [18]
It is hypothesized that vitamin C content is lower in hay compared to fresh pasture. This is because ascorbic acid is oxidized when exposed to heat and light, both of which are required to make hay.
Horses can also obtain vitamin C from certain commercial dietary supplements formulated to provide this nutrient either as part of a premix or on its own.
Veterinarians may also recommend subcutaneous, intramuscular, or intravenous injections of vitamin C for horses that require a higher dose due to a medical condition. [15]
Vitamin C Requirements for Horses
The National Research Council (NRC) currently does not list a specific vitamin C requirement for horses. [7] Researchers believe that healthy horses can synthesize enough vitamin C in their liver to meet their needs.
Recommended Intake Level
Although there is currently no dietary requirement for vitamin C in horses, there may be cases in which supplemental consumption of this nutrient can benefit equine health. Dietary requirements specifically determine the minimum amount of a nutrient necessary to prevent deficiency, rather than addressing the optimal intake of nutrients.
Some researchers recommend the following vitamin C intake levels to support health: [8]
- Horses in work: 1500 mg per day
- Gestating mares: 600 mg per day
- Growing foals: 400 mg per day
These daily intake recommendations are based on improving antioxidant status to support fertility, promote exercise performance recovery, and maintan optimal muscle health in foals.. [8]
Nutritional Deficiency
The classical sign of vitamin C deficiency in mammals is scurvy, a condition involving impaired collagen synthesis and weak connective tissues. No cases of scurvy have ever been documented in horses.
However, several studies have identified lower ascorbic acid levels in the blood of horses suffering from a range of health conditions, such as: [7][12]
- Poor wound healing
- Poor performance
- Poor coat quality
- Depressed immune system
- Recurrent airway obstruction (ROA)
- Epistaxis – bleeding from the nose
- Strangles – an infectious disease that causes abscesses in the upper respiratory tract
- Acute rhinopneumonia – a respiratory infection caused by equine herpesvirus
Although these conditions are associated with lower blood levels of ascorbic acid it remains uncertain whether supplementing affected horses with vitamin C would lead to improved outcomes. The effectiveness of supplemental vitamin C in addressing these conditions has not been extensively studied in horses.
Excess Intake
There is not a significant concern for vitamin C toxicity in horses because this nutrient is water-soluble and not stored well in the body. Excess amounts are easily excreted in urine.
Administering doses of 20 g per day (equivalent to approximately 44 mg of ascorbic acid per kilogram of body weight) to horses over the span of eight months has not demonstrated any adverse effects. [7]
Vitamin C Status
A blood test can be done to measure plasma ascorbate levels in horses. A study of 488 horses reported average values of 5.9 mg/L, with a range of plasma concentrations between 0.8 – 5.9 mg/L. [7]
Healthy foals have higher concentrations of ascorbic acid in their blood plasma shortly after birth. The concentration of this nutrient declines during the initial month of life, indicating its potential significance as an antioxidant during the postnatal period. [9]
There are currently no established reference ranges for Vitamin C in horses, making it difficult to interpret plasma ascorbate levels. However, when combined with measures of vitamin E, selenium, and antioxidant enzymes, plasma ascorbate concentrations could offer valuable insights into the overall antioxidant status of the animal.
Vitamin C Supplements for Horses
As an ingredient in feed products and supplements, ascorbic acid can be obtained from plants or synthesized in a laboratory. Both natural and synthetic sources are chemically identical.
Ascorbic acid is highly sensitive to damage from heat and light and easily oxidized into an inactive form. [17] For this reason, Vitamin C is typically not included in supplements and feed formulations in the form of pure ascorbic acid.
Instead, various protected forms of ascorbic acid are used to improve its stability and bioavailability. [5][17]
Types of Vitamin C Supplements
Stable forms of ascorbic acid should always be used when supplementing this vitamin in your horses diet. Available commercial options include:
- Ethyl-cellulose coated ascorbic acid: The ascorbic acid is coated with cellulose – a fibre that is naturally abundant in the horse’s diet. This coating improves the stability of vitamin C, especially when it is mixed with minerals that can promote its breakdown.
- Ascorbyl palmitate: A fat-soluble form of ascorbic acid with greater oral bioavailability in horses than ascorbic acid. [5]
- Ascorbyl phosphates and sulfates: Mineral preparations such as calcium-ascorbyl-2-monophosphate or disodium-L-ascorbat-2-sulfate.
- Ester-C: This trademarked preparation contains ascorbic acid that is chemically attached to calcium (calcium ascorbate). Ester-C also contains vitamin C metabolites such as dehydroascorbic acid and calcium threonate. [5]
Mad Barn’s Omneity® is a comprehensive equine vitamin and mineral supplement that provides 360 mg of ascorbic acid per serving in the form of stable ethyl-cellulose coated ascorbic acid. This ingredient is stable when exposed to heat and over a long shelf-life.
