Vitamin A What it is In the early years of this century carotene, a yellow pigment found in many fats, oils, fruit and vegetables, was shown to be an essential part of the diet for life and health of all animals. Other related carotenoids were found to have a similar, but reduced, dietary effect. Fish liver oils produced the same nutritional benefit due to their contents of a carotene-related alcohol, which was given the name vitamin A. Later research found that animals, including horses, converted carotene into vitamin A. What it does Vitamin A is involved in various essential body processes which are not all fully understood. It is associated with vision. The visual pigment rhodopsin, which actually receives light rays in the eye, contains vitamin A. Vitamin A is also involved in the formation and preservation of epithelial tissues (surface skin layers) and mucous membranes. Vitamin A influences intra-cellular structures and some of the enzymes responsible for cell metabolism. If too much is given Tests have shown that horses can tolerate more than 1 million I.U. vitamin A per kg feed. Should a horse consume grossly excessive levels there is loss of weight accompanied by bone decalcification, internal and petechial haemorrhage, poor coat or skin condition and increased heart rate. These symptoms are often accompanied by poor muscle tone and depression. Chronic hypervitaminosis A generally interferes with the absorption and use of other fat soluble vitamins. How it is measured Vitamin A activity is measured in International Units (I.U.) and different vitamin A compounds have different activities: 1 I.U. = 0.3 mcg vitamin A alcohol (retinol) = 0.344 mcg vitamin A acetate (retinyl acetate)   = 0.55 mcg vitamin A palmitate (retinyl palmitate) or, measuring the activity of 1 mg of each compound: 1 mg retinol = 3,333 I.U. vitamin A 1 mg retinyl acetate = 2,907 I.U. vitamin A 1 mg retinyl palmitate = 1,818 I.U. vitamin A What about ß-carotene equivalence? The efficiency of conversion of ß-carotene to vitamin A varies according to circumstances. The molecule of ß-carotene is structurally similar to a double molecule of vitamin A. Theoretically conversion should involve no waste and 1mg carotene should convert to 1 mg vitamin A. This does not happen because the carotene molecule is not cleaved centrally to form two molecules of vitamin A but is progressively reduced by the successive removals of carbon groups from one end. The maximum yield is 0.5 mg vitamin A, or 1,667 I.U. Rats have been shown to convert at this rate but horses are less efficient. The efficiency of conversion is adversely affected by excessive liver stores of vitamin A or by flooding the blood system with preformed vitamin A. Any comparative data relate to specific research results and may not be generally applicable. As a broad guide 1 mg carotene produces about 400 I.U. vitamin A in horses. Assessment of status Vitamin A is stored mainly in the liver with smaller amounts present in kidneys and other organs. These stores are not static but are continuously mobilised and replaced through the blood plasma. The amount of vitamin A in circulation in the blood plasma at any specific time is not a good measure of the total body reserves because it is influenced by recent absorption from food and by the variable needs of different parts of the body. If the total body-pool is critically low then the amount in the blood is also extremely low - which is characteristic of a prolonged deficiency. A sample of liver taken by biopsy can be a better indicator of vitamin A status but still cannot give a true measure of total body reserves because various areas of the liver hold different amounts and the total liver weight would be needed for estimation. Requirements and allowances There are many tables of reference listing optimum allowances for horses of varying sizes and ages but the following can be used as a guide:     I.U. / kg   I.U. / day Adult performance horses in training   6000   60000 Adult performance horses in light work   8000   50000 Ponies, hacks & hunters   6000or   18000 Mares & stallions   10000   40000 Young horses 1-2 years   10000   36000 Foals & yearlings less than 1 year 12000 12000 - 36000 Stability of carotene and vitamin A The long, partially unsaturated, hydrocarbon chains in carotene and vitamin A are oxidised relatively easily, producing materials of no vitamin value. Oxidation is accelerated by moisture and heat, and catalysed by small quantities of metallic ions such as copper. Grinding and pelleting mixed feed or individual ingredients also leads to losses of unprotected carotene or vitamin A. Commercial sources of vitamin A are protected within a gelatine matrix. Even so, losses in complete premixes containing vitamins, trace minerals and choline chloride can be rapid and it has been known for more than 50% of the vitamin A to be lost in 2 or 3 weeks. A more normal loss in such premixes is 10% per month whereas losses in premixes of vitamins without trace minerals are much lower. Losses during feed mixing and pelleting can be 6-8% and additional losses on storage may be 4-10% per month depending on the container and the temperature and humidity of the store. The addition of free water into the mixer (humidification) is extremely aggressive and can accelerate vitamin A losses. It is generally recommended that the vitamin A supplement added to pelleted feeds should be increased by 30% to allow for losses during manufacture and up to 3 months’ storage.