Ascorbic Acid-Vitamin C What it is Centuries ago sailors discovered that the disease scurvy could be prevented by sucking a lime or a lemon. The substance responsible for preventing scurvy was identified as ascorbic acid in the early 1930s. It is a fairly simple chemical, similar to sugars such as glucose, with a chain of six carbon atoms. Most animal species, including horses, are able to manufacture ascorbic acid in kidneys from glucose and certain other substances containing chains of six carbon atoms. A few mammals and birds, notably man and guinea pigs, lack the enzyme L-gulonolactone oxidoreductase which catalyses the last stage in this synthesis. These species must have an exogenous, dietary supply of ascorbic acid. What it does Ascorbic acid is transported to all living cells for use in important oxidation/reduction reactions in cell metabolism. It is essential for the formation and maintenance of function of the intercellular substances of skeletal tissues. It also exerts a stimulating action on immune response mechanisms. According to recent research, it plays an essential role in transporting iron ions from plasma to storage sites. If too much is given Very large amounts of ascorbic acid given to horses might induce the development of kidney stones but this condition has never been reported. Since ascorbic acid is very soluble in water any excess intake is rapidly excreted via the kidneys. How it is measured Vitamin C is usually measured in weight units of pure crystalline l-ascorbic acid. There are still occasional references to the International Unit, which is the activity of 50 mcg of l-ascorbic acid. Carbon atoms 2 and 3 on the 6-carbon chain are readily oxidised to produce dehydroascorbic acid which has the same vitamin activity as the original l-ascorbic acid. Sodium ascorbate has 89% of the vitamin value of pure l-ascorbic acid. Erythorbic acid, which is the structural mirror-image of ascorbic acid, (it is sometimes called d-ascorbic acid), has less than 5% of the vitamin C activity of l-ascorbic acid. Assessment of status The amount of ascorbic acid in blood plasma does not reflect the total body supplies of vitamin C except when supplies are limiting. There is a maximum saturation level of 18-20 mcg/ml. The amount in the kidneys only reflects the immediate production since it is rapidly transported to other tissues such as the liver. The concentrations in the adrenals, liver, muscles and duodenum tend to be better indicators of total vitamin C status. Requirements and allowances Very young foals produce very little ascorbic acid and benefit from additional supplies. Mares’ milk contains adequate supplies but foals reared artificially need supplements of 200 mg ascorbic acid per kg feed dry matter (2mg ascorbic acid per ml milk or milk substitute) to produce the maximum economic response. Performance horses under stress may also have a dietary requirement but the efficiency of absorption from the gut is very limited. Up to 20 g/day may have to be given to active horses to ensure that adequate amounts are absorbed. Blood plasma ascorbate levels reduce markedly as the housing environment becomes less favourable. A direct relationship between energy intake and serum ascorbate has also been demonstrated. Horses may require a vitamin C supplement to make up the shortfall in endogenous production under inclement conditions. Levels of 150-250 mg/kg feed are usually sufficient. Similarly, if energy intake is likely to be severely restricted, ascorbic acid should be added to the feed at 150-300 mg/kg. Some research work suggests that excitable horses are over-stressed by training and racing and cease to produce any ascorbic acid. Tests have shown that they may benefit from daily supplies of up to 20 g ascorbic acid which appears to replace the normal endogenous supplies. The amount added to feeds should reflect the anticipated losses during processing and storage and should be increased by a factor of five for pelleting or extrusion, or by 50% for mixing as dry meals, unless ascorbyl 2-phosphate is used as the vitamin C source when no overage is required. The suggested supplements under normal environmental conditions are as follows:     Ascorbic acid supplement     mg / kg   g / day Adult performance horses in training   150   1,5 Adult performance horses in light work   -   - Ponies, hacks & hunters   -   - Mares & stallions during season   150   0,6 Young horses 1-2 years   -   - Foals & yearlings less than 1 year   200   0,2-0,6 Contents of feed ingredients Ascorbic acid is present in fresh vegetables and fruit. The feed ingredients used for the manufacture of compound feeds are unlikely to contain any measurable amounts. Stability Crystalline ascorbic acid is relatively stable in air if moisture is completely absent. In the presence of even small amounts of moisture there is rapid oxidation, first to dehydroascorbic acid and then to other, non-vitamin-active pro- ducts. This irreversible oxidation is accelerated by alkalis and by the presence of metal ions (such as copper). Some oxidative losses occur even during mixing into dry feeds; these are usually between 10-30%. The amount lost during pelleting or cubing is directly related to the amount of steam used and the temperature of the pelleting chamber. Such losses may amount to 85-90% of the ascorbic acid added. There may be less loss if the ascorbic acid is coated with ethyl cellulose. Extruded feeds lose ascorbic acid during manufacture at a similar rate to pelleted feeds. Ascorbyl salts such as ascorbyl 2-phosphate are much more stable during feed pelleting operations than ascorbic acid. Additional ascorbic acid is lost during storage after manufacture. The amount varies according to the temperature and humidity of the store but is usually about 10% per month. Livestock conditions suggesting further needs Since it has been shown that parasites and infectious diseases seriously affect plasma ascorbate levels, additional exogenous supplies are needed to rebuild the normal body pool. A lethargic thoroughbred in otherwise good condition might benefit from up to 20 g ascorbic acid. Poor, draughty stables reduce blood levels to an extent that supplements should be given to horses kept under these conditions during winter months. There are no known clinical conditions in horses which require supplementary ascorbic acid.