Vitamin K, which is classified as a fat soluble vitamin, has been present in racing rations for many years both in forages and as one of a range of vitamins and minerals added to concentrate feed. To date, vitamin K has not been a fashionable vitamin, unlike vitamin C or vitamin E that have received a lot of attention from the perspective of research. Vitamin K is probably best known for its function in blood clotting, but more recently its role in bone metabolism has been highlighted both in humans and horses. In this article, I will explain the role of dietary vitamin K in the context of racing and touch on some new research that has emerged in horses with respect to bone metabolism.
Many forms of vitamin K
Vitamin K is in fact not a single vitamin but a group of vitamins with a similar structure and resulting action. Two forms of vitamin K that you may be familiar with are phylloquinone or vitamin K1 and Menaquinone or vitamin K2. Vitamin K1 or phylloquinine is found in green leafy plants including grass both fresh and dried. Vitamin K2 is synthesised by bacteria present in the digestive tract particularly in the hindgut. There are also various synthetic forms of vitamin K that are available for use in horse feed. Each of these forms of vitamin K have a similar core structure, but with different side chains. Differences in the side chain configuration can affect the absorption of vitamin K from the digestive tract. Interestingly, menadione, which is the synthetic form of vitamin K often used in vitamin and mineral premixes and supplements, has no side chain. This limits its delivery around the body as it less able to bind to the plasma protein responsible for transport around the body it in blood.
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 whilst 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 synthesised 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 absorption 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 that has been used in the past to treat navicular disease. Dicoumarol is a vitamin K antagonist that can be formed from coumarin present in mouldy hay or haylage containing sweet clover. However, the affected forage would have to be fed for several weeks before a vitamin K deficiency arose.
Requirements for vitamin K
The level of vitamin K required in the diet of horses is poorly defined and is 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 synthesised in the large intestine in people. In horses, the general consensus seems to be that their requirement will lie somewhere in between, depending on the health and efficiency of the hindgut. However, traditionally 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, which is crucial to the function of the GLA protein. For example, when blood clots, there are many protein derived clotting factors that facilitate the clotting process. Some of these clotting factors for example, factor II (prothrombin), VII, IX, and X depend on adequate vitamin K to facilitate their ability to bind calcium and hence support the blood clotting process.
Exercise induced pulmonary haemorrhage (EIPH) or ‘bleeders’ are a considerable problem in racing and many supplements have emerged over the years claiming to alleviate this problem. 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 blood clotting. 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
Other vitamin K dependent GLA proteins have also been discovered in other areas of metabolism where the ability to bind calcium is important. As a result vitamin K is also 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-K dependent GLA proteins have been isolated in bone these are osteocalcin, matrix Gla protein and protein S. Bone forming cells called osteoblasts synthesise the GLA protein osteocalcin, which needs to be able to bind calcium in order to mineralise bone. In fact, the efficiency with which osteocalcin binds calcium is reduced if the glutamic acid residues have not been activated (carboxylated) in the presence of vitamin K. Matrix GLA protein has been found in bone, cartilage and blood vessels and it is thought that its calcium binding capacity helps to prevent calcium being deposited in these tissues where it is not required.
These bone related GLA proteins are thought to be more sensitive to reduced vitamin K status compared to the clotting factors involved in blood coagulation. This is probably because the clotting factors have ‘first call’ on dietary vitamin K via the liver and bone GLA proteins are lower down in the queue. In practical terms, this may mean that bone metabolism may be adversely affected before any measured change in vitamin K status is detected through analysis of blood coagulation. The proportion of blood osteocalcin that is present in the activated (carboxylated) form can potentially be used as a more sensitive measure of vitamin K status and one which is more relevant to bone metabolism.
Is vitamin K, the missing piece in the bone metabolism jigsaw?
There is no doubt that there are many factors, both dietary and non dietary, involved in the occurrence of skeletal problems such as fractures, OCD and sore shins in horses in training. Recent work carried out in Australia suggests that dietary vitamin K may be another factor to be addressed in order to minimise the likelihood of skeletal problems.
A recent study in yearlings suggested that vitamin K status was inadequate in horses with radiographically identifiable bone lesions. This research group observed that yearlings with no radiographic lesions showed a significantly higher measured bone mineral density compared to those with one or more lesion. These researchers then assessed vitamin K status by measuring the % of circulating osteocalcin that was activated or carboxylated and relating this to the prevalence of radiographic lesions. Interestingly, those yearlings with one or more visible lesions also had a significantly decreased % of circulating carboxylated osteocalcin compared to the yearlings with no lesions.
This research group has also shown that a group of 2 year old Thoroughbreds in training supplemented with a water soluble bioavailable form of vitamin K showed a suggested improvement in bone density compared to the non supplemented control group, although this result just missed statistical significance. However, the increase in bone density was above and beyond the increase observed in the control group as a consequence of training. Further work also showed an improved OCD lesion score in a small group of horses supplemented for 3-7 months with the same form of vitamin K.
Certainly the recent research on vitamin K in horses suggests that perhaps we should be looking at the status of this vitamin more closely. The use of more sensitive indicators of vitamin K status, such as % carboxylated osteocalcin may prove to be useful, especially in horses that have limited access to fresh pasture, such as in horses in training. Whilst much more research needs to be done to understand the significance of these findings on skeletal health, it potentially opens the door for more efficient forms of vitamin K supplementation, if it is clearly established that vitamin K status is sub-optimal.