Choline What it is Many scientists believe that choline should be classed as an accessory food substance and not as a vitamin. There are some marked differences between the occurrence and function of choline and the other vitamins. Firstly, whereas vitamins tend to function as catalysts or coenzyme factors in metabolism, choline forms part of the actual cell structure. Secondly, choline can be synthesised in the liver providing there is a sufficient supply of methionine; vitamins (with the possible exception of vitamin D) cannot be synthesised by animals. Thirdly, the requirement for choline is greater than that of other vitamins and is measured in g/kg feed compared to mg or even mcg/kg for the vitamins. Choline is a strong organic base which is widely distributed in nature. Since it forms part of cells it tends to be very firmly bound and therefore only partly digested and available for absorption. A relatively large amount is also converted by intestinal bacteria into trimethylamine which is of little value. Some European documents still refer to choline as vitamin B4 and others as vitamin B7. What it does Choline is an essential material for building and maintaining cell structure. It is a constituent of lecithins which are fatty substances (lipids) with one of the three fatty acid molecules replaced by choline which is joined to the glycerol part of the molecule through a phosphoric acid linkage. The remaining two fatty acid molecules can vary according to the availability of different fatty acids in the diet and in the body. This leads to a range of different lecithins depending on fatty acid inclusions. Lecithins are present in every plant and animal cell and play a fundamental role in cellular structure and activity. Choline plays an essential role in fat metabolism in the liver. It functions by preventing abnormal accumulations of fat (fatty livers) by converting excess fat into lecithin or by increasing the utilisation of fatty acids in the liver. A third function is as a constituent of acetylcholine which makes possible the transmission of nerve impulses. It is also a methyl group donator. The methyl groups used by folic acid or vitamin B12 during transmethylation are obtained from materials such as choline, betaine and methionine. Such materials tend to be mutually replaceable in this role. Since this is only one of the functions of choline a dietary supply is essential because these compounds cannot substitute for choline in its other functions. If too much is given There are no indications of any toxicity resulting from large inputs of choline. Since a large proportion of ingested choline is broken down during digestion excess supplies are simply not absorbed but rejected in the faeces. Dietary levels above 2 g/kg feed may slightly reduce the growth rate of foals. Biosynthesis Caecal micro-flora synthesise choline for their own lecithin requirements for cell production. These cells later die and are passed through the remainder of the gut. There is no evidence of any advantage to horses because there is very little digestion and absorption beyond the caecum. How it is measured One of the difficulties with assaying feeds for their choline content is the close bonding of choline with fatty acids in lecithin. It is almost impossible to ensure a complete separation for quantitative measurement. For this reason a microbiological assessment is usually made using Neurospora crassa. The amount of choline in a feed is usually measured in mg or g/kg. Assessment of status A horse’s choline status cannot be accurately estimated because of the universal nature of choline. Some information can be gained from assays of choline in blood plasma but these are very variable and probably meaningless. Relationships with other ingredients The dietary requirement for choline depends on the levels of methionine, betaine and other methyl donators in the ration. For similar reasons there is a relationship between choline as a methyl group donator and the vitamins responsible for transmethylation. Fats contain lecithin so there is also an interaction between fats and choline -- fatty products in feeds containing choline, and choline being required in greater proportion for animals laying down fat. Choline also has a physical action on materials with which it is mixed. Both pure choline and the more common supplement, choline chloride, are extremely hygroscopic. The attraction for water is so great that water-labile materials such as vitamins A, D and thiamine are adversely affected. Wherever possible choline chloride and its dilutions should be added to feeds separately from the remainder of the vitamins. Requirements and allowances An accurate assessment of a horse’s requirement for choline is very complex because it varies according to the available supply of other methyl donators such as betaine and methionine. The most reliable estimates are based on studies using diets containing an abundance of methionine and vitamin B12. High levels of fat also appear to increase the requirements for choline. In contrast, it is also found that a large proportion of the ”required” choline is not absorbed but is converted to trimethylamine and excreted. Also the organic binding of natural choline may make a considerable amount unavailable. So trials using crystalline choline or choline chloride may not produce the same results as the same amount of natural choline shown to be present in the feed ingredients. Any recommendation of requirement must therefore be assessed in relation to the specific method of calculation and the conditions of the estimation. In the absence of more detailed and accurate information, feeds should be designed to contain not less than 1 g choline per kg feed dry matter in the presence of adequate methionine and vitamin B12. Most feed ingredients contain reasonable amounts of choline but there is no information regarding its biological availability. In practice, therefore, most rations are supplemented with 25-50% of the known requirements to ensure a reasonable adequacy. The following supplements are recommended:     Choline 100 %   Choline Chloride 50 %   Choline Chloride 50 % (mg/ day)     (mg/ kg feed)   Adult performance horses in training   80   184   1840 Adult performance horses in light work   100   230 1380 Mares & stallions   100   230   1115 Ponies, hunters & hacks   80   184 920 Young horses and ponies 1 – 2 years   100   230   920 Foals and young horses less than 1 year   150   345 345-1035 Stability Natural choline, which is bound into cells as part of lecithin, is very stable. Crystalline choline is deliquescent so that it is generally available as a viscous liquid. Choline chloride is also hygroscopic but is less alkaline and easier to handle. It can be obtained as a 70% liquid product or adsorbed onto an absorbent carrier such as silica gel which then contains 50% choline chloride (43% choline). This prepared material is less hygroscopic but is still aggressive towards other vitamins. It is reasonably stable when mixed into meals but some destruction to trimethylamine may occur during pelleting or extrusion, resulting in a distinctive, unpleasant smell.