Answers to three common questions about equine nutrition
A: Veterinarians have identified an increased incidence of laminitis resulting from the use of feeds with a high glycemic index. The glycemic index of a feed is the relative increase in blood glucose that results after ingestion relative to other feeds or a standard such as oats or glucose. For example, the glycemic index for corn (or increase in blood glucose) is 63 percent of that for glucose. A high glycemic feed has high nonstructural carbohydrate content (low fiber) and causes the largest and fastest increases in blood glucose and insulin concentrations. Examples of feeds with a high glycemic index are oats, corn, barley and molasses. Ingestion of these common ingredients of sweet feed cause peak concentrations of blood glucose in the range of 130 to 200 mg/dl within 15 minutes.
Processing grains also affects their digestibility. For example, cracking corn and hulling oats increases their digestibility, which results in more rapid and dramatic increases in blood glucose and insulin concentrations than their unprocessed whole grain counterparts do. Pelleted and extruded feeds generally have blunted increases in glucose and insulin concentrations, possibly because of heat-induced changes that lower digestibility. Oat starch is more digestible than corn starch, and barley has the lowest glycemic index of these grains.
Grass hays generally have the lowest glycemic index; however, there is a wide variation among grass hays depending on the type of grass, stage of maturity, and curing processes. It was previously thought that feeding grass hay had an insignificant effect on blood glucose and insulin concentrations. But hays with more than 20 percent nonstructural carbohydrate content can have a marked effect on these values.
Fructan is the most common nonstructural carbohydrate that is rapidly fermented in the colon. In addition, grass and hay can contain soluble sugars and starches that are digested in the small intestine, thereby resulting in glucose absorption. The nonstructural carbohydrate content of grasses is highest in young spring grass and in the fall when sunny days are followed by overnight freezing temperatures. Therefore, pasture grass nonstructural carbohydrate content can be up to 40 percent during the spring and fall if environmental conditions are favorable. Soaking hay in warm water can lower the water-soluble carbohydrate content when the nonstructural carbohydrate content is unknown. However, sufficient alteration of the carbohydrate content of most hays can require soaking periods that are not practical or desirable under most management situations.
Q: How does diet relate to the development of equine metabolic syndrome?
A: Obesity is an important factor in equine metabolic syndrome (EMS) development. Regional adiposity with regional accumulation of crest neck adipose tissue with or without generalized obesity is especially associated with the development of EMS and laminitis. It is generally assumed that obesity results from an imbalance between energy intake and expenditure.
Many factors influence this imbalance, including the number of hours of turn out, a horse’s activity during turn out, the amount and type of exercise, genetic factors and a horse’s stage in life. Obesity is a significant contributing factor to the development of insulin resistance in EMS, but non-obese horses also develop EMS. Obese horses have higher circulating inflammatory cytokine concentrations—a finding pivotal to the pathogenesis of metabolic syndrome in people.
Evidence is mounting to suggest that insulin resistance is the key factor associated with EMS and laminitis. In England, ponies with higher insulin concentrations have the greatest risk for developing laminitis when body condition scores are similar. Insulin infusion causes laminitis in horses. Laminitis-prone ponies develop higher concentrations of circulating insulin when fed a fructan.
Horses adapted to high-glycemic-index feeds have reduced insulin sensitivity and higher insulin responses to feeding carbohydrates compared with horses fed higher fiber and fat diets with similar caloric contents. Although not definitively proved, this evidence taken together suggests that feeding high glycemic feeds can increase the risk of developing laminitis by increasing insulin concentrations. On the other hand, horses adapted to a predominantly forage ration develop a lower plasma insulin concentration even when fed a carbohydrate challenge. Therefore, feeding low glycemic feeds results in lower plasma insulin concentrations and improved insulin sensitivity, reducing the risk of laminitis.
Q: What do you recommend feeding obese horses with EMS?
A: Feeding horses with EMS that have laminitis should focus on reducing the nonstructural carbohydrate to lower plasma insulin concentrations and increase insulin sensitivity. This feeding regimen must include eliminating grain and sweet feed (absolutely no corn, oats or molasses) and greatly limiting access to pasture. Horses with active laminitis should be denied any access to pasture. Grass hay should be the main component of the ration.
Providing hay with a nonstructural carbohydrate content below 12 percent will increase insulin sensitivity in nearly all of these patients. The best results can be obtained by measuring the hay’s nonstructural carbohydrate content, which is usually done by commercial or state laboratories.
Caloric restriction should be combined with increased exercise activity to produce slow weight loss. Grass hay fed at 1.5 percent body weight (0.015 X 1,000 lb = 15 lb hay daily) should provide sufficient caloric intake unless high levels of exercise are expected. When grass hay alone does not maintain an adequate body condition score, feeds rich in highly digestible fibers can be fed. Beet pulp (soaked in water before feeding) and rice bran are suitable options. Vegetable oil can be added to these feeds to further increase the caloric intake. Feeds based on beet pulp and rice bran that have a nonstructural carbohydrate content below 20 percent are offered by most feed companies. Optimally, the serum insulin concentration is measured seven to 10 days after the feed is changed to assess an animal’s response. The goal is to return the blood insulin concentration to normal (< 60 micro international units/ml).