Back to basic nutrition this week with a post about the essential nutrient, vitamin A. Interestingly, despite vitamin A being commonly added to supplements, its source in the horse’s natural diet of green forage is a completely different compound to that which is used in supplements.
Vitamin A is used in the body for healthy vision, growth and bone development, immune function and reproduction. The recommended daily amount for an adult horse is 22-30 000 IU. Deficiency of vitamin A in the horse causes weight loss, anaemia, respiratory and gut dysfunction, weepy eyes and poor vision. Toxicity is possible with over-supply, and causes bone fragility, reduced blood clotting and internal haemorrhages, and skin lesions. The upper safe limit for vitamin A intake is about 160 000IU (for a 500 kg horse).
There is no vitamin A in green forages and browse, but instead these are rich in compounds called carotenes, which are used by the plant for photosynthesis (the process whereby plants use light to turn carbon dioxide to oxygen whilst it makes carbohydrates as an energy source).
Some carotenes are converted in the horse’s body to vitamin A, in a form called retinol, so they are sometimes called pro-vitamin A. The carotene with the highest vitamin A activity is beta-carotene. Horses actually absorb some carotenes intact as well as converting some to vitamin A, so both are available in the body. Carotenes are non toxic and it appears there is no upper limit to safe intake. They are powerful antioxidants in the body and can enable immune cells to function more efficiently.
Vitamin A and beta-carotenes are classed as fat soluble vitamins because they are digested with fat and require the action of bile to become available in the gut.
Preserved forages lose their beta-carotene content during processing and then further during storage. After two years of storage, hay will lose about 90% of its original content of beta-carotenes.
Horses without access to green forages must have their diet supplemented with vitamin A. Although beta-carotenes are not considered essential and are not routinely added to horse feeds and supplements, some scientists believe that supplementation to growing and breeding horses, and performance horses who do not have access to pasture is beneficial. However, not all sources of beta-carotene are bioavailable to the horse, so the right type has to be selected for supplementation. Dehydrated grass meal or grass pellets that retain their green colour are a source of beta-carotenes. Synthetic sources that are ‘micellized’ (processed in such a way as to increase solubility) appear to be more bioavailable than other types.