vitamin k ISSUE 19.qxd:Jerkins feature.qxd
28/1/11
22:28
Page 2
VITAMIN K
limits its delivery around the body as it is less able to bind to the plasma protein responsible for transport. Forage can be a variable source of vitamin K The level of vitamin K1 found in grass can be extremely variable and is degraded rapidly by the action of ultraviolet light. Work carried out in Australia reported that the level of vitamin K in Couch, Prairie, Phalaris, and Rhodes grass was reduced by between 30-70% following just 7 hours of drying in sunlight with a UV index between 2 and 7. So while access to fresh pasture will contribute to vitamin K intake, the reliance on hay or haylage in stabled horses in training may limit dietary vitamin K intake from this source. Vitamin K2 or menaquinone can be synthesized in the hindgut of horses by the resident microflora, but the contribution to overall vitamin K status is unknown as the degree of absorption of vitamin K from the hindgut has not been established. Assuming that vitamin K absorption from the hindgut is good as for ruminants, it is still likely to depend on digestive health and the presence of a good balance of resident bacteria. Bacterial vitamin K synthesis and absorption will be reduced if there has been extensive damage to the gut, or if the normal balance of microflora has been disrupted e.g. by antibiotics, or as a result of hindgut acidosis. Hindgut acidosis can occur in horses in training when large high-starch meals are fed chronically, especially where the cereal starch has a low digestibility. Dietary vitamin K is absorbed primarily in the small intestine, and the process can be impaired when the absorption of fat from the diet is decreased, or as the result of the intake of certain substances known as antagonists. Antagonists of vitamin K absorption can be drugs such as warfarin, which has been used in the past to treat navicular disease. Dicoumarol is a vitamin K antagonist that can be formed from coumarin present in moldy hay or haylage containing sweet clover. However, the affected forage would have to be fed for several weeks before a vitamin K deficiency arises. Requirements for vitamin K The level of vitamin K required in the diet of horses is poorly defined and complicated by the difficulty in assessing the contribution from bacteria in the hindgut. Ruminants have a low dietary requirement for vitamin K, as bacterial synthesis in the rumen is high, but in contrast there is very little vitamin K synthesized in the large intestine in people. In horses, the general consensus seems to be that their requirement will lie somewhere in between,
Supplements that include vitamin K are claimed to help prevent bleeding
depending on the health and efficiency of the hindgut. However, traditional measurements of clotting function have been used to assess vitamin K status but as we will discuss below this may not be the best reflection of optimum status for bone metabolism. Vitamin K, a pivotal role in blood coagulation The crucial role of vitamin K is linked to its requirement as an essential co-factor for an enzyme that modifies the structure of an amino acid known as glutamic acid. Glutamic acid is found in many body proteins with crucial functions in both blood coagulation and bone metabolism. These functional proteins are known as GLA proteins. The glutamic acid residues in these GLA proteins are activated or carboxylated in the presence of vitamin K, which enables them to bind calcium, a crucial element to the function of the GLA protein. For example, when blood clots, there are many protein-derived factors that facilitate the clotting process. Some of these, such as factor II (prothrombin), VII, IX, and X, depend on adequate vitamin K to facilitate their ability to bind calcium and hence support the bloodclotting process.
Exercise induced pulmonary hemorrhage (EIPH) or ‘bleeding’ is a problem in racing and many supplements have emerged over the years claiming to alleviate it. However, as the exact cause and mechanisms of EIPH have not as yet been determined, it is difficult to give any credence to most of these products given the lack of scientific studies to support their purported beneficial effects. Vitamin K has often featured in such products, presumably in the belief that its effects on coagulation will deliver improved or speedier bloodclotting. EIPH, however, is not associated with a lengthened time for blood to clot and so supplementation with vitamin K in this respect is probably futile. A role for vitamin K in bone metabolism Additional vitamin K dependent GLA proteins have been discovered in other areas of metabolism where the ability to bind calcium is important. As a result vitamin K is seen as essential for some aspects of bone and cartilage metabolism and also for cardiovascular health. The involvement of vitamin K in bone metabolism has received some recent attention in equine nutrition. Three vitamin
ISSUE 19 TRAINERMAGAZINE.com 41