Humans realized the necessity for ascorbic acid (known commonly as vitamin C) by the mid to late 1700s when hundreds of sailors enlisted with the British Royal Navy became afflicted with a condition that caused spongy, bleeding gums and loosening of the teeth. For reasons inexplicable at the time, the consumption of citrus fruits such as oranges and lemons provided relief from the symptoms. It was not until 1928 that scientists identified the chemical component within the fruits that boosted the health of those mariners: ascorbic acid.

In contrast to humans, most horses do not require ascorbic acid in their diets because sufficient quantities are produced in the liver from the breakdown of glucose.

Within the body, ascorbic acid is vital to antioxidant defenses, which protect membranes and other structures from free-radical-induced oxidative damage. Aside from its antioxidant properties, though just as important, ascorbic acid is essential for proper calcification of bone and formation of dentin (the principal component of teeth).

Old, ill, or extremely stressed horses may have plasma ascorbic acid levels below normal. Weaning, often a time of great anxiety, caused stall-confined foals to have diminished plasma ascorbic acid concentrations. Deficiencies due to illness and stress are normally transient.

While rare, long-term deficiency of ascorbic acid has been documented. Signs include poor hair coat, depressed immunity, delayed wound healing, vertebral malformations such as scoliosis (curvature of the spine) and lordosis (sway back), and hemorrhaging.

Researchers have tested the effect of ascorbic acid supplementation on common problems in horses. Because ascorbic acid deficiency causes bleeding, supplementation with the vitamin has been tried on racehorses in an attempt to alleviate exercise-induced pulmonary hemorrhaging. Results of these trials, however, were inconclusive. In other studies, intake of ascorbic acid failed to improve fertility in mares or stallions.

Ascorbic acid supplementation may have a place in the management of aged horses, particularly those with Cushing’s disease. Scientists at RutgersUniversityfound that antibody response to vaccines was greater in old horses when they were supplemented with ascorbic acid. Aged horses with chronic infections may also benefit from ascorbic acid supplementation.