As horses – like most other mammals – can make their own vitamin C, its role in equine diets was at one time rarely considered. However, more recent research has shown that supplemental vitamin C may be beneficial for stabled horses, those suffering from respiratory challenges and sick horses.
Vitamin C is an important water soluble antioxidant in the horse’s body, which is manufactured from glucose in the liver. Rates of production have been identified at 72 g per day. It prevents free radical damage, regenerates other antioxidants including vitamin E and is involved in connective tissue structure. Concentrations in the lung fluid and blood plasma of horses affected with recurrent airway obstruction (RAO) are reduced, and the requirement of these horses is believed to be higher than that of healthy horses. Researchers have shown that supplementary vitamin C increases blood plasma concentrations and concentrations in the lung lining fluid.
The recommended daily serving for supplementary vitamin C is 10-30 mg per kg of bodyweight daily, for horses under stress, sick horses, and those with RAO. This is equivalent to 5-15 g daily for a 500 kg horse. All stabled horses are exposed to increased levels of airborne irritants and allergens, and even those without clinical disease such as RAO should have extra respiratory support, which includes vitamins C and E.
The source of supplementary vitamin C used is important because some types are more bioavailable to the horse than others. Ascorbic acid – the most commonly available form – has a low absorption in the horse, whereas ascorbyl monophosphate and ascorbyl palmitate have a higher bioavailability. Ascorbyl palmitate is believed to be the most bioavailable, but its use in supplements is prohibited by its high price. Ascorbyl monophosphate is a good compromise.
Vitamin C supplementation should never be stopped abruptly, but instead gradually decreased over 1-2 weeks. Stopping it abruptly can interrupt the body’s own synthesis and lead to suppressed levels and an increased risk of infection. Too much vitamin C is unhelpful because it acts as a pro-oxidant, increases acidity in the body and can cause gut disturbances. 20 g (ascorbic acid) has been given daily for 8 months without adverse effect, and this is probably the maximum long-term daily serving recommended.
Sick and stressed horses, those with RAO, and stabled horses may benefit from supplementary dietary vitamin C, ideally from a more bioavailable source such as ascorbyl palmitate or ascorbyl monophosphate.