Vitamins for Bone Health in HorsesBy Kentucky Equine Research Staff · June 22, 2017
Young horses require vitamins for growth. Some vitamins must be provided nutritionally, while others can be synthesized by the healthy individual. While vitamin D gets the lion’s share of attention for skeletal contributions, other vitamins are just as important, including vitamins A, C, and K.
Vitamin A has a distinct role in equine growth with both deficiency and toxicity of vitamin A adversely affecting growth, body weight, and rate of gain in young growing horses. “In the growing horse, vitamin A supports the proper functioning of osteoclasts, or bone-resorbing cells, during bone remodeling,” said Catherine Whitehouse, M.S., a nutrition advisor for Kentucky Equine Research (KER). “There is limited information on absolute vitamin A requirements for growing horses, but for horses grazing sufficient quantities of green pastures, vitamin A requirement can be met entirely by the carotenes in the forage.”
In some regions, vitamin A supplementation is particularly important because of the short growing season of grasses. In one study, weanling foals supplemented with 40,000 IU of vitamin A per day (~160 IU/kg body weight per day) along with hay and oats had improved serum levels during winter and spring, but supplementation had no effect during the summer when the horses were on pasture. Toxicity levels for vitamin A are estimated to be around 1000 IU/kg body weight per day, with excess vitamin A being implicated in developmental orthopedic disease (DOD) in growing horses.
Vitamin C is important in the growing horse because it plays an important role in collagen synthesis and repair. “Collagen is protein that provides strength and flexibility to connective tissues, including skeletal structures,” explained Whitehouse.
Vitamin C deficiency is not normally observed in horses because they can synthesize ascorbic acid from glucose in the liver. Production of vitamin C in the liver can be limited, however, and in some circumstances the supply may not be adequate to meet the requirement of the horse. Supplementation of vitamin C may be beneficial during dietary deficiencies of energy (in particular glucose and glucose substrates), protein, vitamin E, selenium, and iron, and in times of rapid growth.
Vitamin D is important in the maintenance of calcium homeostasis in the blood, which is vital for normal mineralization of bone as well as for a host of other body functions. In some horse-production systems, young horses are often kept out of the sunlight to prevent dulling of the coat and to allow for ease of management. For horses not exposed to sunlight or artificial light, the requirement for dietary vitamin D is 800 IU of vitamin D/kg of diet dry matter, which can be achieved with the use of a fortfied feed fed at the manufacturer's recommendation.
Vitamin D should not be given in an effort to treat DOD by increasing calcium and phosphorus absorption and bone mineralization. “DOD has not been shown to be caused by vitamin D deficiency and supplementation with vitamin D will not make up for diets that are not properly fortified with calcium and phosphorus. Oversupplementation of vitamin D is toxic and results in extensive mineralization of cardiovascular and other soft tissues,” said Whitehouse.
Vitamin K is necessary for the proper clotting of blood, a process that uses calcium. Another calcium-related function of vitamin K is its role in bone and cartilage metabolism. If vitamin K is in short supply in the horse’s body, it is first used to guarantee that the clotting factor is supplied. For this reason, it is thought that any deficiencies may shortchange use in bone metabolism. Vitamin K is found in high levels in forages, with cereal grains containing very little vitamin K. Vitamin K is also produced by intestinal bacteria in a healthy hindgut.
Equine dietary vitamin K requirements have not been determined (Nutrient Requirements of Horses, 2007), but there is some suggestion that vitamin K supplementation might be helpful for young horses that do not have access to fresh pasture or have less than adequate hindgut health.