Equine metabolic syndrome (EMS)

By Dr Vicki Glasgow, Harbro Nutritionist

Metabolic diseases of the horse are becoming more prevalent, with the major focus on Equine Metabolic Syndrome (EMS), because of its link with laminitis and because more and more horses seem to be affected. In a grazing season that’s had a higher than average rate of laminitis cases, here’s a reminder of how to manage these nutritionally.


Physical appearance is one of the best methods of diagnosing EMS. They tend to have unusual fat deposits; cresty neck, over the shoulders and the rump (including the tail-head). Geldings may have sheaths which appear swollen due to an accumulation of fat. These horses are generally described as good doers, and owners find it virtually impossible to reduce their weight by calorie restriction alone. Many are not fed any hard feed at all. The most common scenario is an obese horse and/or a horse with regional adiposity that is presented to the vet with pasture induced laminitis. Expansion of adipose tissues in the neck area is proving to be the best indicator of EMS in horses. There is a scoring system which looks purely at scoring “cresty neck”. This increased neck circumference has been shown to be associated with insulin resistance in both horses and ponies.

The leaner EMS horse

EMS is less commonly detected in leaner horses – in fact, there may even be a slightly different condition affecting these types, but as yet the research is inconclusive. They key features for this type is the regional adiposity (these horses may be thin enough to have ribs showing and yet still have a cresty neck), and increased laminitis risk, despite being fit and with a generally low BCS. These horses were either previously obese and are now being maintained in a leaner condition through good management or they are in the early stages of Cushing’s disease (as yet undetectable through testing). This second group must be clarified through further research and if you are faced with this type of horse it is probably worth testing for both Cushing’s disease and insulin resistance.

Insulin Resistance

Insulin resistance is the factor associated with EMS that is most likely to predispose the horse to laminitis. The primary energy source in the body is glucose (sugar) and it is used to fuel many metabolic processes. When glucose levels in the blood are elevated after a meal, insulin is normally produced to regulate glucose levels in the blood and its utilisation. The job of insulin in the body is to facilitate glucose uptake by the cells in the body (muscle and fat cells) and promote the formation of glycogen or fat as energy stores. Insulin resistance has been defined as “a failure of tissues to respond appropriately to insulin”. In other words, the cells in the body become resistant to the glucose uptake action of insulin. This means that more insulin is needed to keep blood glucose levels at normal levels after a starchy meal. This limits energy availability for the horse and results in high levels of insulin in the blood that can be detrimental to the horse if these are higher than normal for prolonged periods of time. This situation can be compared to type 2 diabetes in humans and there is some indication that the circulation problems that may be the cause of laminitis in horses (as yet research has not completely confirmed this fact), are closely linked to the circulation problems that human diabetics face.



There is still little known about what causes a horse to become insulin resistant, but research is ongoing. There seems, however, little doubt that obesity is the main trigger to the onset of EMS. Research suggests that this is due to the fact that fat releases its own hormones, one of which is cortisol. This interferes with the ability of insulin to move glucose into the cells. Obese horses have more fat cells, so more cortisol is produced and this means more interference with the actions of insulin. Not all fat horses are insulin resistant, and this is thought to be related to the type of fat that is laid down and the different hormones released by these different types. Omental fat (stored in the abdomen) is thought to be the main culprit for insulin resistance.


There is still no definitive, practical method for detecting insulin resistance in the horse. It is, in fact, a real challenge. Many factors can affect the test results such as, stress, time of day, time of year/season and when and what the horse was last fed. The “gold standard” tests involve administering glucose and insulin to the horse and monitoring the outcome. This is clearly not simple (would need done on vet site) or cheap and there is some indication that the test itself may present a laminitis risk. With cost and horse’s stress and convenience in mind most vets will take a single blood sample to measure glucose and insulin levels. There is one school of thought that says that the horse should be fasted for 6 hours before this test is done. Recent research has shown however that this is not necessary and can in fact be counter-productive due to the fact that it is likely to cause stress in the animal and therefore affect the test results. Hard feed should be withheld but horses should be allowed access to soaked hay. Research in the USA has shown this to have no effect on the test. All this should be discussed with your vet if you feel your horse needs testing.


If the animal is obese then weight reduction is a must, all grain or feeds containing grain must be withdrawn from the horse, the other key to success is limiting or eliminating access to pasture. This is only a short term exercise until the horse is down to an appropriate BCS. Pasture represents uncontrollable calorific intake and presents the biggest danger to the EMS horse. Exercise must also be increased – this is key to losing weight and aiding insulin resistance. Grass intake can be reduced by using a grazing muzzle or placing the horse in a “starvation” paddock, where the horse should be fed on soaked hay. A sometimes useful tool for those horses that have lost the excess weight and now need to be maintained is to set up a race track. This set up means that not only can you restrict the amount of grass the horse gets but it also has to walk and exercise to get the grass. Exercise and restriction in one action.  Dieting should be carried out slowly with hay (soaked) being the main source of calories to the horse. It should be fed at 2.5% Body Weight (BW) to start with, alongside a vitamin and trace element supplement (Look at Harbro One Scoop). This is gradually reduced to 1.5% of current BW and then further reduced to 1.5% of ideal BW. It may take up to 6 months for the horse to lose weight, and may be a painful exercise for both the horse and the owner but worth it in the long term.

The leaner horse with EMS, is a harder animal to feed, especially if it is still working at a reasonable level. These horses should never be fed grain or coarse mixes, due to the high starch and sugar content which makes maintaining weight and providing energy for work more problematic. Make use of unmolassed sugar beet, and oil to provide any extra energy required, also alfalfa chaffs, especially those without molasses or with a very low level. These are a very good source of both protein and energy. Each animal should be treated as an individual and it is best if you have this type of animal to speak to a nutritionist.

Remember that prevention is always better than cure and if you suspect your horse may have EMS or be insulin resistant then take action now, rather than later. Also bear in mind that as horses age, the more insulin resistant they become. Feeds high in starch and sugar cause significant spikes in blood glucose and insulin after a meal and this kind of feeding management over a prolonged period of time (years) could potentially lead to insulin resistance. In fact, one scientist is of the belief that the damage to the EMS horse’s metabolism is done within the first 10 years of its life. He advocates feeding youngsters on low starch/sugar diets as it is likely that over-feeding at this stage leads to a middle aged horse with EMS. This fact is worth remembering when deciding what to feed your good-doer type youngsters.

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